Visible light photo-oxidations in the presence of alpha-Bi2O3

alpha-Bismuth oxides of specific surface areas of 1-3 m(2) g(-1) were prepared by three different methods and their visible light activity was tested in the photodegradation (lambda >= 420 nm) of 4-chlorophenol. In method A, which led to powders of poor to moderate photoactivity, the starting materials BiONO3, Bi(NO3)(3)center dot 5H(2)O, (BiO)(2)CO3, and BiOCl were annealed at 500 degrees C without any pretreatment. In method B the salt (BiO)(2)CO3 was washed with water and subsequently calcined at 450 degrees C affording a very active powder. In method C the salts BiONO3, Bi(NO3)(3)center dot 5H(2)O and (BiO)(2)CO3 were dissolved in nitric acid and Bi(OH)(3) was precipitated by addition of sodium hydroxide. After annealing at 500 degrees C the resulting oxides exhibited moderate activity in the case of the (BiO)(2)CO3 precursor whereas highly active powders were obtained from BiONO3 and Bi(NO3)(3)center dot 5H(2)O inducing almost complete photomineralization of 4-chlorophenol. XRD analysis indicated the presence of 40-140 nm large crystallites of alpha-Bi2O3. From diffuse reflectance spectroscopy bandgaps of 2.80 +/- 0.05 eV and 2.93 +/- 0.05 eV were obtained, assuming an indirect or direct semiconductor, respectively. The quasi-Fermi potential of electrons at pH 7 was determined as -0.08 +/- 0.05 V (vs. NHE) through pH dependent photovoltage measurements. Repeated use of the presumed catalyst powder revealed that the mineralization is not a catalytic but a bismuth oxide assisted photo-oxidation. This result shed a critical light on previous reports on the photocatalytic action of binary and ternary bismuth oxides.