γ-Bi₂O₃ – To Be or Not To Be? Comparison of the Sillenite γ-Bi₂O₃ and Isomorphous Sillenite-Type Bi₁₂SiO₂₀

Abstract

The "controlled" synthesis of metastable γ-BiO by solution based approaches was reported several times recently, but the formation of BiSiO in the presence of trace amounts of silicates renders the results to be questionable. Here, the preparation of the Sillenite γ-BiO and the Sillenite-type BiSiO starting from the polynuclear bismuth oxido cluster [BiO(OCCH)(DMSO)] is reported. γ-BiO crystallizes after calcination at 800 °C of the silicate-free hydrolysis product "[BiO(OH)]" on a silver sheet. Corrosion of the substrate causes contamination with silver, which is not incorporated into the Bi-O lattice, and was removed by treatment with an aqueous KCN-solution. BiSiO was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or NaSiO in a Teflon-lined reactor vessel followed by calcination at 600 °C. PXRD studies, scanning electron microscopy, nitrogen adsorption measurements, IR- and Raman spectroscopy, diffuse UV-vis spectroscopy, and DSC were used for characterization. The phase transition of γ-BiO to give α-BiO occurred slowly in the temperature range of 348-510 °C ( Δ H = 6.57 kJ·mol). The silver-containing γ-BiO exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV-vis spectrum γ-BiO exhibits an absorption edge at λ = 485 nm ( E = 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-BiO exhibits an absorption edge at λ = 460 nm ( E = 2.83 eV) and BiSiO at λ = 422 nm ( E = 3.16 eV). The photocatalytic activity of the compounds was investigated in the decomposition of aqueous rhodamine B under visible light irradiation, showing silver-containing γ-BiO to be slightly more effective compared to BiSiO and significantly more effective than the silver-free γ-BiO.

Structural and Enhanced Optical Properties of Stabilized γ‒Bi2O3 Nanoparticles: Effect of Oxygen Ion Vacancies

“…The Raman spectra of γ-Bi2O3 NPs displayed 20 vibration modes, consistent with the literature, where so far, only 8 to 15 modes have been resolved in the same range [4,30,34,42]. The Raman bands in Figure 3 were assigned by comparing with the reported values for γ-Bi2O3 and the theoretical calculations in the literature [30,[41][42][43][44][45]. A very weak mode at 481 cm −1 and a broad mode at 567 cm −1 , which have been reported for γ-Bi2O3, are so far not assigned to any vibrational mode.…”

“…The excitation usually occurs from the 1 S 0 ground state to the 3 P 1 and 1 P 1 excited states [49]. The estimated value of a direct bandgap for γ-Bi 2 O 3 NPs using a UV-visible diffuse reflectance spectrum is~2.9 eV (434 nm) (data are not shown), which is slightly lower than the reported value of 2.95 eV [3] and higher than 2.76-2.83 eV [30]. Therefore, the emission peak at 394 nm (2% of PL spectra) can be indexed as the band-to-band recombination in a direct transition manner ( 3 P 1 → 1 S 0 ).…”

“…A very weak mode at 481 cm −1 and a broad mode at 567 cm −1 , which have been reported for γ–Bi 2 O 3 , are so far not assigned to any vibrational mode. The Raman modes below ~650 cm −1 are usually assigned to the internal bismuth–oxygen framework and are indicative of several breathing, stretching, rocking, and bending modes of Bi–O polyhedra in γ–Bi 2 O 3 [ 30 , 41 , 42 , 43 , 44 , 45 ]. Raman modes in 70–190 cm −1 are quite sharp, whereas most of the modes from 190 to 660 cm −1 are quite broadened with FWHM varying between 14 and 54 cm −1 .…”

“…Raman modes in 70-190 cm −1 are quite sharp, whereas most of the modes from 190 to 660 cm −1 are quite broadened with FWHM varying between 14 and 54 cm −1 . Salazar-Pérez et al [30] reported the Raman spectra from oxygen-deficient (confirmed using EDS) γ-Bi 2 O 3 NPs prepared by annealing 10 nm Bi NPs in the air at 700-750 • C for a duration of 30 min. Comparatively, (i) in this work γ-Bi 2 O 3 is formed at a relatively low temperature, which could be possibly related to the use of different size Bi NPs; (ii) all the modes from 190 to 660 cm −1 show red-shift with a magnitude varying between 6 and 25 cm −1 ; (iii) the intensity of modes at 89, 206, and 281 cm −1 is relatively low, and to the naked eye, it appears that these peaks are broadened.…”

“…Hereof, in recent decades, various physical and chemical methods have been introduced for the synthesis of RT stabilized γ–Bi 2 O 3 nanostructures with various morphologies [ 1 , 9 ]. Versatile forms of nanostructures could be obtained either by heating the mixture at low temperature (40–90 °C) [ 2 , 3 , 24 , 25 , 26 , 27 , 28 , 29 ] or in some methods at high temperature (300–800 °C) [ 4 , 6 , 7 , 30 , 31 , 32 ].…”