The present study demonstrated that the visible light irradiation promoted the electroless deposition of iron oxyhydroxide thin films on a bare Pt substrate in FeSO4 aqueous solution. Fe~(2+) is oxidized to higher valence under the presence of dissolved oxygen and then is precipitated as iron(III) oxyhydroxide on the Pt substrate. The light irradiation on the Pt substrate induced the formation of photocarriers (i.e., electrons and holes) in the preformed iron oxyhydroxide with a narrow band gap. The resultant holes in the valence band photocatalytically oxidized Fe~(2+) at the oxyhydroxide/solution interface as well as the Pt substrate. That is, the oxyhydroxide nuclei themselves acted as a sensitive layer for photoelectroless deposition. The photoactivation effect significantly depended on photon energy. Although the deposition rate was accelerated only under visible light irradiation, as stated above, the irradiation of UV light caused the reductive photocorrosion of the preformed deposits rather than the photocatalytic oxidation of Fe~(2+). The crystal structure of the as-deposited thin films was a metastable phase of iron oxyhydroxide containing sulfate ions, which is known as "schwertmannite". The films were composed of nanosized whiskers, and such nanostructures with a large surface area remained unchanged even after the phase transition to hematite by postannealing at 873 K for 1 h. In this paper, the photoelectroless deposition mechanism of iron oxyhydroxide was discussed in detail, and the positive effects of visible light irradiation on film quality were demonstrated.
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