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

A number of calcium bismuthates were synthesized (25 to 50 mol% in Ca) and characterized by XRD, SEM, EDX, XPS and DRS techniques; the latter provided an estimate of the bandgap energies (E-bg=2.41 to 3.29 eV) via Tauc plots for indirect transitions, whereas XPS established the potentials (vs NHE) of their respective valence bands (and thus the conduction bands). Linear correlations existed between E-VB/E-CB and E-bg that when compared with those of strontium bismuthates (reported earlier) showed that differences in energies at E-bg=0 eV are related to the difference in the absolute electronegativities of Ca and Sr, from which the following empirical phenomenological rule is postulated: replacing one alkaline earth metal in bismuthates by another causes the points of intersection of the linear correlations E-CB(E-bg) and E-VB(E-bg) to be displaced by an amount equal to twice the difference in absolute electronegativities of these metals.