A method of preparing semiconductor nano crystal anatase TiO.sub.2 solution uses titanium alkoxide Ti(OR).sub.4 as a main component in combination with chelating agents in aqueous solution. A fluorescent lamp tube is coated with the semiconductor nano crystal anatase TiO.sub.2 solution to form a photocatalytic coating fluorescent lamp capable of cleaning air. Then a baking step is carried out at a low temperature about 100-250.degree. C. By doped anatase TiO2 with small amount about 0-1.0 wt % of precious metals complex or transition metals oxides as nano-particle on or in the anatase TiO2 nano-particle surface, the visible light photocatalysis efficiency is increased for air cleaning. By doped with small amount Eu.sup.+3 or rare earth metal ion on or in the anatase TiO2 nano-particle surface, which is a photocatalytic material acting as fluorescent agent, the fluorescent lamp has increasing brightness of when it is turned on.