This article reports on two novel visible-light-active strontium bismuthate photocatalysts (Sr 6 Bi 2 O 11 and Sr 3 Bi 2 O 6) prepared by solid-state synthesis for which the number of strontium atoms exceeded the number of bismuth atoms in the cation sublattice; for comparison, the bismuthate Sr 2 Bi 2 O 5 was also reexamined. All three bismuthates were characterized by a variety of spectroscopic techniques (XRD, XPS, EDX, DR, Raman, SEM, and EIS). Both newly as-synthesized bismuthates displayed photocatalytic activity toward the photodegradation of substrates in the gas phase (acetaldehyde) and in aqueous media (phenol), with the Sr 6 Bi 2 O 11 phase exhibiting a significantly greater photoactivity than the Sr 3 Bi 2 O 6 phase; by comparison, the bismuthate Sr 2 Bi 2 O 5 was photocatalytically inactive. Detailed photocatalytic mechanisms have been proposed to explain how composition and structure of the three bismuthates affect their photocatalytic activity. The role of point defects in their crystal lattice is described for processes in which the photocatalytic activity was inhibited. Inferences made were aided by examining the UV-induced coloration of these ternary metal oxides that provided information regarding defect levels within their respective bandgaps; such defects acted as electron traps and thus affected the photocatalytic performance of the bismuthates.

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Calcium bismuthate nanoparticles with orthorhombic and rhombohedral crystal lattices were synthesized and characterized by various techniques. The syntheses caused the formation of oxygen and cationic vacancies in bismuth‐deficient samples. Heat treatment of samples with anionic vacancies led to thermal annealing of oxygen vacancies in ambient air; the annealing process was reversible. Oxygen vacancies in Bi‐deficient CaBi6O10 governed the optical absorption beyond the fundamental absorption edge at E(hν) < Ebg ≈ 2.5 eV. Bandgap energies of the bismuthates with orthorhombic and rhombohedral lattices were 2.51 eV prior to annealing and 2.47 eV after annealing at 350 °C. The trend in photoactivity toward the degradation of methylene blue dye under visible light showed that the photoactivity of the orthorhombic samples (1‐3) increased with a decrease in the [Ca]:[Bi] ratio (1.0:6.3 to 1.0:4.3); the sample with lowest ratio (1.0:4.3) displayed highest photoactivity. By contrast, the rhombohedral samples (4‐6) showed increased photoactivity with increase in the number of Bi atoms; sample with highest ratio (1.0:6.3) displayed highest photoactivity.

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