Mineralization of Phenol and 4-Chlorophenol Induced by Visible Light and Assisted by Semiconducting β-Bi2O3

Intensely yellow a-bismuth trioxide in the form of about 41 nm large crystallites has been prepared by thermal decomposition of (BiO)(2)CO3 at 400 degrees C. From the band gap energy of 2.3 eV obtained under the assumption of an indirect band-to-band transition and the quasi-Fermi level of -0.28 +/- 0.02 V (vs. NHE) a valence band edge position of 2.02 V is calculated. This semiconducting oxide assists a Vis light (lambda >= 455 nm) aerial mineralization of the aqueous model pollutants phenol and 4-chlorophenol exhibiting maximum rates at a bismuth oxide concentration of 2.0 g L-1. In the absence of oxygen no degradation occurs indicating that beta-Bi2O3 does not act as oxidizing agent. The mineralization suffers from photocorrosion of beta-Bi2O3 generating a mixture of' alpha-Bi2O3 and various bismuth carbonates as observed upon multiple use of the oxide in the mineralization of phenol.