Modulation of photocatalytic activity of SrBi2Ta2O9 nanosheets in NO removal by tuning facets exposure

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.