Mechanism insights into the enhanced activity and stability of hierarchical bismuth oxyiodide microspheres with selectively exposed (0 0 1) or (1 1 0) facets for photocatalytic oxidation of gaseous mercury

Abstract

Selectively exposing different facets of the photocatalysts to enhance the photocatalytic activity is a promising method. However, the role of exposed specific facets in a photocatalytic process is not fully understood. In order to clarify this mechanism, we have systemically explored the relationships between selectively exposed facets and photocatalytic activity of the nanoplate bismuth oxyiodide (BiOI). The BiOI was synthesized by pH-induced transformation method, and the photocatalytic activity was evaluated by photocatalytic oxidation of gas-phase mercury. The exposed specific facet ratios can be adjusted, and depend on the pH of the precursors. Our results showed that the BiOI with exposed (1 1 0) facets possesses an excellent photocatalytic oxidation activity on gas-phase mercury than the BiOI with exposed (0 0 1) facets. Characterization analysis indicated that the BiOI with exposed (1 1 0) facets can generate more superoxide radical (·O2-) than the BiOI with exposed (0 0 1) facets. Furthermore, both the BiOI with exposed (1 1 0) and (0 0 1) facets can not produce free hydroxyl groups (·OH) directly, but can generate it by transformation of the surface absorbance oxygen process. The BiOI with exposed (1 1 0) facets can generate more ·O2-, which accumulate on the conduction band of the BiOI and transform to ·OH by process of O2 → ·O2- → H2O2 → ·OH. Thus, the BiOI with exposed (1 1 0) facets can produce more ·OH than the BiOI with exposed (0 0 1) facets as well. This finding may enhance the comprehension of the mechanism between the selectively exposed facets and the photocatalytic activity of the as-prepared samples.