It has been mentioned that titanium oxide photocatalyst makes air purification possible using sunlight and fluorescent lamps. The range of light wavelength where the current photocatalyst can be used is below 400nm and no more than 4% of sunlight and 0.1% of fluorescent lamps can be used as energy. If long-wavelength light which is longer than 400nm is used, efficiency of photocatalyst could be improved greatly. Many researchers attempted the synthesis of photocatalyst using visible light in sunlight and fluorescent lamps, based on the idea mentioned in the above. A finding bringing the attempt to fruition has been recently done in Japan, attracting remarkable attention.

It has been so far aware that long- wavelength light which is longer than 400nm can be used by injecting metal ion into titanium oxide crystal but what has been recently confirmed is that further improvement of photocatalyst function under visible light can be seen by injecting nitrogen atom, not metal atom. The National Institute of Advanced Industrial Science and Technology (AIST) has succeeded in developing a new method to inject nitrogen into titanium oxide as shown in the Chart 5 and the future achievement of its research is being hoped.