The invention is directed to a catalyst having activity under the irradiation
of a visible light, the catalyst being an oxide semiconductor such as an anatase
type titanium dioxide, having stable oxygen defects. A method for producing a catalyst
having activity under the irradiation of a visible light which comprises treating
an oxide semiconductor with hydrogen plasma or with a plasma of a rare gas element,
comprising performing the treatment in a state substantially free from the intrusion
of air into the treatment system is also provided. An article comprising a base
material having the catalyst above provided on the surface thereof and a method
for decomposing a substance, comprising bringing an object to be decomposed into
contact with the catalyst above under the irradiation of a light containing at
least a visible radiation are disclosed. A novel photocatalyst which enables use
of a visible radiation is provided, as well as a method utilizing the photocatalyst
for removing various substances containing an organic matter or bacteria by photodecomposition.
