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Posted: Nov 8th, 2007
Simplifying the fabrication of nitrogen-doped titania coatings could benefit solar cell industry
(Nanowerk Spotlight) Titanium oxide (TiO2, titania) - due to its versatile optical, electrical and photochemical properties, its relative abundance and low cost, and its non-toxicity - is an important ceramic material with numerous applications as pigments; powders for catalytic or photocatalytic applications; colloids and thin films for photovoltaic, electrochromic, photochromic, electroluminescence devices and sensors; components for antireflecting coatings; or porous membranes for ultrafiltration. Nanocrystalline titania has become a prominent material for dye-sensitized solar cells (DSSCs, also known as 'Grätzel cells' after their inventor), which are photoelectrochemical cells that use photo-sensitization of wide-band-gap mesoporous oxide semiconductors. One major problem with the use of titania in solar cells is that its bandgap does not match that of visible light and titania therefore can only absorb 3-4% of the energy from sunlight. Grätzel cells decrease the bandgap of titania by using dye-absorbed TiO2 nanocrystals as one of the electrodes, resulting in a higher solar energy conversion of 10% or more. Other methods use doping and indeed the application of nitrogen-doped titania as photocatalyst has received increasing attention over the last years because N-doping is found to be particularly effective in decreasing the bandgap of anatase (many of the properties of titania depend on the structure of the TiO2 phase - mainly anatase, brookite and rutile). In order for photocatalysis-based applications to become commercially viable, it will be critical to design low-cost, reproducible, synthetic methods that yield controlled, reproducible, and easy-to-handle nanomaterials processed as coatings with high surface area and high porosity. Researchers in France and Spain now describe for the first time nanostructured coatings that fulfill all these requirements.
"The best previously reported results of titania photocatalysts were based on the use of nitrogen as dopant, using numerous approaches and reactants" Dr. Clément Sanchez explains to Nanowerk. "The incorporation of nitrogen into the titania network has been achieved trough different methods using a variety of nitrogen sources. But it should be noted that the visible-light photocatalytic activity of the resulting materials is highly sensitive to the synthetic route used in their preparation. To facilitate a low-cost, reproducible and fully controllable synthesis method, we have prepared mesoporous photoactive nitrogen-doped nanocrystalline titania films with controlled and ordered structure through a simple and reproducible sol–gel method."
The researchers prepared their N-doped titania films by using a simple approximation of sol gel chemistry combined with the Evaporation Induced Self Assembly (EISA) method and an appropriate ammonia treatment at different temperatures in order to obtain materials that are reproducible and effective visible-active photocatalysts. The results are mesoporous photoactive nitrogen-doped nanocristalline titania coatings with a controlled and ordered structure.
"We prepared several mesoporous N-doped titania films by annealing mesoporous nanocrystalline anatase under ammonia flow at different temperatures" says Sanchez. "Optical characterization shows that the discrete introduction of nitrogen is able to shift the titania absorption edge. When we performed photocatalytic tests we found the best results for the film nitrided at 500 °C. At this temperature the concentration of nitrogen in the structure is optimal as the oxygen vacancies do not play an important enough role yet in the recombination of the photogenerated electrons and holes."
Sanchez points out that their titania films are effective catalysts that can work under visible light. Since titania-based nanomaterials exhibit a wide range of environment-related photocatalysts applications – such as sensors, solar cells, fog proof and self cleaning glass, anti-bacterial, anti-viral, fungicidal, anti-soiling, self cleaning, deodorizing, air purification, water treatment and water purification – these applications can now be used in a wider range of the visible light spectrum.