Silica nanofiber/metal oxide nanocrystal composite and method for producing the same

A method for producing a silica nanofiber/metal oxide nanocrystal composite in which a metal oxide nanocrystal (A) is bonded to a silica nanofiber (B) constituting a disc-shaped network structure, the method comprising:

• (I) a step of causing association of a polymer having a linear polyethyleneimine skeleton by mixing a hot water solution and ice with each other in a mass ratio of 10/90 to 90/10, the hot water solution being obtained by dissolving, in advance, the polymer in hot water having a temperature of 70 to 100° C. in an amount of 0.5 to 10% by mass, so that a water medium has a temperature of 3 to 15° C. during the mixing;
• (II) a step of adding alkoxysilane to a water-based medium that contains an associate and is obtained in the step (I) to forma composite nanofiber including the associate as a core and silica that covers the associate, while the composite nanofiber spontaneously forms a disc-shaped network structure;
• (III) a step of depositing a metal oxide (A′) on a surface of the composite nanofiber constituting the disc-shaped structure obtained in the step (II) by mixing the disc-shaped structure with a hydrolyzable metal compound (C) in the water-based medium;
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• (IV) a step of calcining, at 400 to 1250° C., the disc-shaped structure in which the metal oxide (A′) is deposited on the surface of the composite nanofiber and that is obtained in the step (III) to form a silica nanofiber (B) through removal of the polymer in the composite nanofiber, to convert the metal oxide (A′) into a metal oxide nanocrystal (A), and to bond the metal oxide nanocrystal (A) to the silica nanofiber (B).