| Abstract
| - The visible-light-induced degradation reaction of 4-chlorophenol (4-CP) was investigated in aqueous suspensionof pure TiO2. Contrary to common expectations, 4-CP could be degraded under visible illumination (λ > 420nm), generating chlorides and CO2 concomitantly. The observed visible reactivity was not due to the presenceof trace UV light since the visible-light-induced reactions exhibited behaviors distinguished from those ofUV-induced reactions. Dichloroacetate could not be degraded under visible light, whereas it degraded witha much faster rate than 4-CP under UV irradiation. The addition of tert-butyl alcohol, a common OH radicalscavenger, did not affect the visible reactivity of 4-CP, which indicates that OH radicals are not involved.Other phenolic compounds such as phenol and 2,4-dichlorophenol were similarly degraded under visiblelight. The surface complexation between phenolic compounds and TiO2 appears to be responsible for thevisible light reactivity. Diffuse reflectance UV−vis spectra showed that 4-CP adsorbed on TiO2 powder inducedvisible light absorption. The visible light reactivity among several TiO2 samples was apparently correlatedwith the surface area of TiO2. The visible-light-induced photocurrents on a TiO2 electrode could be obtainedonly in the presence of 4-CP. It is proposed that a direct electron transfer from surface-complexed phenol tothe conduction band of TiO2 upon absorbing visible light (through ligand-to-metal charge transfer) initiatesthe oxidative degradation of phenolic compounds. When the surface complex formation was hindered bysurface fluorination, surface platinization, and high pH, the visible-light-induced degradation of 4-CP wasinhibited. The evidence of visible-light-induced reactions and the experimental conditions affecting the visiblereactivity were discussed in detail.
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