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

Abstract

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.

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

“…The second band shifted toward higher energies to 531.9–532.1 eV, corresponds to oxygen with an oxidation state of −1. A large number of oxygen in an oxidation state of −1 seems likely characteristic of bismuthates of alkaline earth metals – similar results were shown previously for a number of strontium bismuthates, for which oxygen with an oxidation state of −1 was associated with the displacement of oxygen from normal lattice sites to interstitial positions.…”

“…The second band with maximum shifted to 350–351 eV also corresponds to calcium with an oxidation state of +2 albeit it is associated with a carbonate structure, as alkaline earth metals tend to absorb carbon dioxide from the air. A similar situation was observed on the surface of samples with chemically sorbed carbon dioxide in a number of strontium bismuthates …”

“…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, and one calcium bismuthate investigated by Wang et al . 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.…”

“…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 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.…”

“…A similar situation was observed on the surface of samples with chemically sorbed carbon dioxide in a number of strontium bismuthates. [2] The Bi 4f XPS spectra of the calcium bismuthates consist of two well resolved bands corresponding to Bi 4f7/2 and Bi 4f5/2 ( Figure S13). In all calcium bismuthates, bismuth exhibits an oxidation state of + 3.…”