Comparison of importance between separation efficiency and valence band position: The case of heterostructured Bi3O4Br/alpha-Bi2O3 photocatalysts

Heterostructured Bi3O4Br/alpha-Bi2O3 nanocomposites are prepared via in situ one-step self-combustion of ionic liquids. Tetrabutylammonium bromide (TBAB) is employed to be fuel, complexing agent for ionic liquids, as well as the reactant supplying Br for the objective material. The ratio of Bi3O4Br/alpha-Bi2O3 can be easily adjusted by controlling the amount of TBAB. The heterojunctions show higher photocatalytic ability towards both azo dye methyl orange (MO) and colorless pollutant phenol. The electron spin resonance (ESR) test and p-nitro blue tetrazolium chloride (NBT) degradation result indicate that generation amounts of superoxide anion radicals (center dot O-2(-)) over heterojunctions are less than that over pure Bi3O4Br. Photoelectrochemical measurements show that separation efficiencies of photo-generated electrons and holes are decreased after the combination of Bi3O4Br and alpha-Bi2O3. Work function test and scavenger experiments display that holes play key role for pollutant degradation and the position of holes on alpha-Bi2O3 is lowered via hybridization. Thus, the enhanced photocatalytic activity over composites can be attributed to the position decline of alpha-Bi2O3 valence band, thus improving the reactivity of holes in direct oxidation of pollutants. In this case, valence band position is confirmed to be more important than separation efficiency of charge carriers in affecting photocatalytic performance.