Calcium Bismuthate Nanoparticulates with Orthorhombic and Rhombohedral Crystalline Lattices: Effects of Composition and Structure on Photoactivity

Abstract

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.

Effect of Composition on the Optical and Photocatalytic Properties of Visible Light Responsive Materials Bi26–xMgxO40

“…The samples Bi 26−x Mg x O 40 were prepared by the pyrolytic method, which has been proven to be successful in the synthesis of calcium bismuthates. 3 The chemical purity of the utilized reactants Bi(NO 3 ) 3 •5H 2 O, Mg(NO 3 ) 2 •6H 2 O, and sorbitol C 6 H 14 O 6 exceeded 99.0%. First, the reactants were ground, resulting in the formation of a viscous transparent solution that was due to water in the crystalline hydrates.…”

“…According to the earlier reports, the photocatalytic activity of bismuthates depends on the relative concentration of their constituent metal cations. 3 Here, we address this issue with respect to the monophase Bi 26−x Mg x O 40 samples.…”

“…One factor that enables such behavior is a recently established trend in the empirical dependence of valence and conduction band energy positions on the materials’ bandgaps . In contrast to calcium, − strontium, − and barium − bismuthates, which have been widely studied, little is known about magnesium bismuthates . The primary goal of our work is to elaborate a preparation route of monophase Bi 26– x Mg x O 40 samples that is followed by the study of their optical and photocatalytic properties.…”

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

“…The strontium bismuthates were synthesized and their photocatalytic activity investigated in earlier studies by Shtarev et al . in such reactions as the degradation of phenol and the oxidation of acetaldehyde as part of systematic studies into the photocatalytic properties of bismuth‐containing solids . That said, however, neither their application toward the photocatalytic water splitting process nor the photoreduction of CO 2 was the objective of the present study, although future studies should examine the behavior of these bismuthates in such processes.…”

Optical Properties of Various Strontium Bismuthates: Luminescence and UV‐induced Photocoloration

“…Bismuthates of various alkaline earth metals, primarily those of calcium, [1][2][3][4] strontium [5][6][7][8][9][10][11][12][13] and barium [14][15][16] have recently been investigated actively as photoactive materials. They are promising photocatalysts because they display high photocatalytic activity toward the decomposition of water, [9,15] the mineralization of various organic compounds, [5,7,8,[10][11][12][13][14][15] and toward the synthesis of organic molecules from carbon dioxide and water precursors.…”