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-lightactive (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 + BE bg and E VB = a + bE bg . However, contrary to earlier symmetric relationships of some metal oxides for which A = a = 1.23 eV, B = À 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.

Phenomenological Rule from Correlations of Conduction/Valence Band Energies and Bandgap Energies in Semiconductor Photocatalysts: Calcium Bismuthates versus Strontium Bismuthates

“…Positions of the conduction band energies (E CB ) and of the valence band energies (E VB ) as a function of the bandgap energy (E bg ) for the set of calcium and strontium bismuthates. The data reported as filled red squares and circles correspond to the calcium bismuthates, whereas the data illustrated as filled green squares and circles correspond to the strontium bismuthates reported earlier by Shtarev et al [1] The red and green solid lines correspond to the linear correlations E CB (E bg ) and E VB (E bg ) for calcium and strontium bismuthate, respectively. The red and green dashed lines show the extrapolation of the resulting correlations to the intersection points at E bg = 0 eV.…”

“…In an earlier study, [1] we demonstrated a novel trend in the empirical dependence of the conduction and valence band positions on the bandgap energy of semiconductors for alkali earth metal bismuthate photocatalysts. [2] The dependences E CB = f(E bg ) and E VB = f(E bg ) for the bismuthates could be approximated by the linear functions E CB = A + B E bg and E VB = a + b E bg , respectively, thereby illustrating that bismuthates of alkaline earth metals formed a new trend in the field of semiconductor photocatalysts.…”

“…The energies of the respective valence bands of the four bismuthates were determined from the low‐energy band edge of the O 2p XPS spectra (Figure ) in accordance with the method proposed by Chambers and coworkers; binding energies are given versus the NHE scale as were those reported earlier for the strontium bismuthates …”

“…Accordingly, a number of calcium bismuthates were synthesized (25 to 50 mol% in Ca) in this study and then characterized by XRD, SEM, EDX, XPS and DRS techniques, following which the linear correlations E CB = A + B E bg and E VB = a + b E bg were compared to the data on strontium bismuthates reported previously …”

“…The novel trend was characterized by a distinct relationship between the energy position of the valence band and the conduction band on the one hand, and the bandgap energy on the other. In the aforementioned study, [1] conclusions were reached on the basis of an analysis of the properties of three strontium bismuthates examined earlier in some detail by Shtarev and coworkers, [2] and one calcium bismuthate investigated by Wang et al [3] It seemed of interest, therefore, to discover whether different alkaline earth metals (Sr versus Ca) in bismuthates might affect such observed linear dependence, and if so how.…”