Considerations of Trends in Heterogeneous Photocatalysis. Correlations between Conduction and Valence Band Energies with Bandgap Energies of Various Photocatalysts

Abstract A principal objective of our work was to illustrate the existence of unknown trends in the E CB = f (E bg ) and the E VB = f (E bg ) behavior for certain classes of compounds that also included a set of bismuthate photocatalysts; new trends may easily be hidden within more general trends. Indeed, when the data for the bismuthates are added to data related to novel visible‐light‐active (VLA) photocatalysts, the trend remains the same with high significance levels for the E VB = f (E bg ) linear correlation. Using well‐known data for complex (ternary) oxides, the search for new trends in the behavior of the E CB = f (E bg ) and the E VB = f (E bg ) dependences opens up the potential to develop new VLA photocatalysts. The feasibility of new desired trends can be estimated a priori using a modern theoretical analysis of the electronic structure of targeted semiconductors; note that new trends in the behavior of E CB and E VB versus bandgap energy E BG have been established for CaBi 6 O 10 , Sr 2 Bi 2 O 5 , Sr 3 Bi 2 O 6 , and Sr 6 Bi 2 O 11 . We also re‐examined earlier symmetric‐type plots for a set consisting of a large number of simple (binary) and a set consisting of complex (ternary) metal oxides and find some significant differences. We herein report a novel trend in the empirical dependence of the energy positions of conduction bands (E CB ) and valence bands (E VB ) on bandgap energies (E bg ) for alkali earth metal bismuthate semiconductor photocatalysts for which we show that their dependences follow the linear functions E CB = A + B E bg and E VB = a + b E bg . However, contrary to earlier symmetric relationships of some metal oxides for which A=a =1.23 eV, B =−0.5, and b =0.5 toward water splitting, the relationships found for the bismuthates show them to be asymmetric with negative slopes ( B <0, b <0) and a converging point A=a= 4.5 eV. Also re‐examined are the earlier symmetric‐type plots for a set consisting of a large number of simple (binary) and a set consisting of complex (ternary) metal oxides and find some significant differences. Variations in both slopes ( B <0 and b >0) and the converging point (1.4 eV< A=a <2.0 eV) is demonstrated. The photoactivity of some strontium bismuthates is predicted toward the photoelectrochemical reduction of CO 2 from the novel trend in behavior of their bands’ energy positions relative to their bandgap energies.