In the formation of an oxide film on an aluminum fiber, it has been
difficult to form a thick alumina coating film on the aluminum fiber
which already has a natural oxide film formed thereon. To overcome this
problem, there is provided a method for forming an alumina coating film
which enables the deep penetration of oxygen into an aluminum fiber by
employing a three-stage heating treatment and an alumina fiber formed by
the method. Also provided is a system for producing water by
photocatalytic reaction, in which a photocatalyst comprising the alumina
fiber coated with titania is irradiated with light from a light source to
generate an active oxygen species, diffusing the active oxygen species in
water to impart the function of the active oxygen species to water. The
system can perform washing by utilizing an oxidation reaction with the
resulting water. Further, provided is a gas treatment system which
comprises the alumina fiber coated with titania to impart a
photocatalytic function to the aluminum fiber.