Titanium dioxide is a semi - conductor with wide band gap , and its photocatalysis operates only when it is irradiated under the ultraviolet radiation of sunlight , and also , the electron hole of tio _ 2 has very high reunited rate 但由于tio _ 2為寬禁帶半導體,對太陽光的利用僅局限于紫外部分,并且tio _ 2的光生電子空穴復合幾率很高。