In recent years, attention has been directed to (i) a photocatalyst which, by solar light or indoor illumination, absorbs environmental pollutants to decompose and remove the pollutants and (ii) a photocatalyst which, by solar light or indoor illumination, exhibits self-cleaning action against dirt adhering to a surface of the photocatalyst.  Active research into such photocatalyst materials has been carried out. 
Among photocatalyst materials, a titanium oxide shows high photocatalytic effect and has been widely studied.  However, because a titanium oxide has a large bandgap, a titanium oxide can absorb ultraviolet light but cannot absorb visible light.  Therefore, although the titanium oxide shows photocatalytic activity in response to ultraviolet light, the titanium oxide does not show photocatalytic activity in response to visible light.  This prevents a photocatalyst material made of a titanium oxide from producing photocatalytic effect in an indoor environment where there exists an extremely little amount of ultraviolet light. 
Meanwhile, research and development have been carried out into photocatalytic effect of photocatalyst materials made of materials other than a titanium oxide.  An example of such materials is a tungsten oxide.  Patent Literature 1 discloses that a tungsten oxide has a smaller bandgap compared with a titanium oxide and can therefore absorb visible light, and that it is therefore possible to use a tungsten oxide as a visible light response type photocatalyst material by causing the tungsten oxide to support a proper co-catalyst on a surface of the tungsten oxide. 
Furthermore, Patent Literature 2 discloses that a tungsten oxide, which serves as a photocatalyst and has a surface(s) partially covered with a titanium oxide, has higher photocatalytic effect than a tungsten oxide which is not covered with a titanium oxide, and can therefore quickly decompose volatile aromatic compounds in a gas phase.