Abstract:

Photocatalysts with exposure of different crystal facets often show great differences in their photocatalytic activities due to differences in surface atomic arrangement and coordination. Thus, the actual photoreaction mechanism of a specific crystal facet in photocatalysis deserves to be explored. In this paper, as a case study, SrBi₂Ta₂O₉ photocatalyst with preferential facet exposure was explored for the photocatalytic removal of NO at a ppb level. The efficiency of NO removal was remarkably improved by tuning the crystal exposure facet with high (2 0 0) facet exposure ratio. Optimized exposure of (2 0 0) crystal facet in SrBi₂Ta₂O₉ (SBT) by thermal calcination at 800 °C (SBT-800) leads to the highest NO removal activity of 51% under a 300 W Xe lamp for 20 min; under visible light, SBT 800 achieves a 5-fold enhancement in NO removal efficiency compared to its counterpart, SBT-900. Active species capture experiments prove that the superoxide radical ·O₂^- is the main active species for the photocatalytic removal of NO, and surface selective deposition experiments conclude that (2 0 0) is the main electron-rich crystal plane, based on which the results of density functional theory (DFT) computation reveal the BiO terminated nature of (0 0 1) crystal plane, where the models with both BiO and TaO terminated (0 0 1) planes were created and computated. Mechanistic study reveals that SrBi₂Ta₂O₉ with a larger exposure of (2 0 0) facet provides more active reduction sites, thereby reducing more O₂ to ·O₂^-, which further oxidizes the adsorbed NO to NO₂^-/NO₃^-. The present work underlines the role of facet tuning in the photoactivity modulation for NO removal photocatalytically.

Key words: SrBi₂Ta₂O₉, NO removal, Facet exposure, Photocatalysis, DFT computation