Solid-state synthesis, characterization, UV-induced coloration and photocatalytic activity - The Sr6Bi2O11, Sr3Bi2O6 and Sr2Bi2O5 bismuthates

This article reports on two novel strontium bismuthate photocatalysts (Sr6Bi2O11 and Sr3Bi2O6) prepared by a solid-state synthesis for which the number of strontium atoms exceeds the number of bismuth atoms in the cation sublattice; for comparison, the bismuthate Sr2Bi2O5 was also re-examined. All three bismuthates were characterized by a variety of spectroscopic techniques (XRD, XPS, EDX, DR, Raman, SEM, and EIS). Direct bandgap energies for the three bismuthates were assessed from diffuse reflectance spectra: 2.61 eV for Sr6Bi2O11; 3.40 eV for Sr3Bi2O6; 3.17 eV for Sr2Bi2O5, while the flatband potentials (versus NHE) of the corresponding valence bands were estimated from XPS spectra: +2.22 eV for Sr6Bi2O11; +1.71 eV for Sr3Bi2O6; +1.61 eV for Sr2Bi2O5. The two novel bismuthates displayed photocatalytic activity toward the photodegradation of acetaldehyde in the gas phase and phenol in aqueous media, with the Sr6Bi2O11 system exhibiting significantly greater photoactivity vis-a-vis the Sr3Bi2O6 bismuthate; by comparison, Sr2Bi2O5 was photocatalytically inactive in this case; their photoactivity was also assessed from the photodegradation of phenol in aqueous media (in all cases using UV-Vis irradiation from a Xe light source). Detailed photocatalytic mechanisms are proposed based on UV-induced coloration studies (carried out using a high-pressure Hg lamp; 365 nm) and on the photodegradation in the presence of radical scavengers to explain how composition and structure of the three bismuthates affect their photocatalytic activity. The role of point defects (oxygen vacancies) in their crystal lattice is described as they affect photocatalytic activity by acting as electron traps and potentially as electron/hole recombination centers.