AbstractA generation of reactive oxygen species (ROS) from TiO₂ under solar light has been long sought since the ROS can disinfect organic pollutants. We found that newly developed crystalline/amorphous reduced TiO₂ (rTiO₂) that has low energy bandgap can effectively generate ROS under solar light and successfully remove a bloom of algae. The preparation of rTiO₂ is a one-pot and mass productive solution-process reduction using lithium-ethylene diamine (Li-EDA) at room temperature. Interestingly only the rutile phase of TiO₂ crystal was reduced, while the anatase phase even in case of both anatase/rutile phased TiO₂ was not reduced. Only reduced TiO₂ materials can generate ROS under solar light, which was confirmed by electron spin resonance. Among the three different types of Li-EDA treated TiO₂ (anatase, rutile and both phased TiO₂), the both phased rTiO₂ showed the best performance to produce ROS. The generated ROS effectively removed the common green algae Chlamydomonas. This is the first report on algae degradation under solar light, proving the feasibility of commercially available products for disinfection.