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

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Abstract
α-Bismuth oxides of specific surface areas of 1–3 m2 g−1 were prepared by three different methods and their visible light activity was tested in the photodegradation (λ≥ 420 nm) of 4-chlorophenol. In method A, which led to powders of poor to moderate photoactivity, the starting materials BiONO3, Bi(NO3)3·5H2O, (BiO)2CO3, and BiOCl were annealed at 500 °C without any pretreatment. In method B the salt (BiO)2CO3 was washed with water and subsequently calcined at 450 °C affording a very active powder. In method C the salts BiONO3, Bi(NO3)3·5H2O and (BiO)2CO3 were dissolved in nitric acid and Bi(OH)3 was precipitated by addition of sodium hydroxide. After annealing at 500 °C the resulting oxides exhibited moderate activity in the case of the (BiO)2CO3 precursor whereas highly active powders were obtained from BiONO3 and Bi(NO3)3·5H2O inducing almost complete photomineralization of 4-chlorophenol. XRD analysis indicated the presence of 40–140 nm large crystallites of α-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.