Rutile TiO 2 layers were formed on substrates of Ti–(0–10)at%Au alloys by a simple process of air oxidation, and their antibacterial activities were evaluated under visible-light irradiation (λ ≥ 400 nm). Au was introduced into the TiO 2 layers on Ti–(1–10)at%Au alloys and existed as both metallic Au nanoparticles and dissolved Au 3+ ions. The TiO 2 layers that formed on Ti–5at%Au and Ti–10at%Au alloys exhibited visible-light photocatalytic activity, that is, degradation of stearic acid and antibacterial activity against Escherichia coli. These visible-light activities were attributed to the surface plasmon resonance of metallic Au nanoparticles and the decrease in bandgap energy caused by dissolved Au 3+ ions. The formation of hydroxyl radicals observed under visible-light irradiation is attributable to antibacterial activity. From a cost perspective, a Ti–5at%Au alloy is more suitable as a substrate for the formation of a TiO 2 layer with antibacterial properties than a Ti–10at%Au alloy.
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