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

High-Throughput Strategies for the Design, Discovery, and Analysis of Bismuth-Based Photocatalysts

Bismuth-based nanostructures (BBNs) have attracted extensive research attention due to their tremendous development in the fields of photocatalysis and electro-catalysis. BBNs are considered potential photocatalysts because of their easily tuned electronic properties by changing their chemical composition, surface morphology, crystal structure, and band energies. However, their photocatalytic performance is not satisfactory yet, which limits their use in practical applications. To date, the charge carrier behavior of surface-engineered bismuth-based nanostructured photocatalysts has been under study to harness abundant solar energy for pollutant degradation and water splitting. Therefore, in this review, photocatalytic concepts and surface engineering for improving charge transport and the separation of available photocatalysts are first introduced. Afterward, the different strategies mainly implemented for the improvement of the photocatalytic activity are considered, including different synthetic approaches, the engineering of nanostructures, the influence of phase structure, and the active species produced from heterojunctions. Photocatalytic enhancement via the surface plasmon resonance effect is also examined and the photocatalytic performance of the bismuth-based photocatalytic mechanism is elucidated and discussed in detail, considering the different semiconductor junctions. Based on recent reports, current challenges and future directions for designing and developing bismuth-based nanostructured photocatalysts for enhanced photoactivity and stability are summarized.

Polymorphism and Visible-Light-Driven Photocatalysis of Doped Bi2O3:M (M = S, Se, and Re)

δ-Bi₂O₃:M (M = S, Se, and Re) with an oxygen-defective fluorite-type structure is obtained by a coprecipitation method starting from the bismuth oxido cluster [Bi₃₈O₄₅(OMc)₂₄(dmso)₉]·2dmso·7H₂O (A) in the presence of additives such as Na₂SO₄, Na₂SeO₄, NH₄ReO₄, Na₂SeO₃·5H₂O, and Na₂SO₃. The coprecipitation of the starting materials with aqueous NaOH results in the formation of alkaline reaction mixtures, and the cubic bismuth(III)-based oxides Bi₁₄O₂₀(SO₄) (1c), Bi₁₄O₂₀(SeO₄) (2c), Bi₁₄O₂₀(ReO_4.5) (3c), Bi_12.25O_16.625(SeO₃)_1.75 (4c), and Bi_10.51O_14.765(SO₃)_0.49(SO₄)_0.51 (5c) are obtained after microwave-assisted heating; formation of compound 5c is the result of partial oxidation of sulfur. The compounds 1c, 2c, 4c, and 5c absorb UV light only, whereas compound 3c absorbs in the visible-light region of the solar spectrum. Thermal treatment of the as-prepared metastable bismuth(III) oxide chalcogenates 1c and 2c at T = 600 °C provides a monotropic phase transition into their tetragonal polymorphs Bi₁₄O₂₀(SO₄) (1t) and Bi₁₄O₂₀(SeO₄) (2t), while compound 3c is transformed into the tetragonal modification of Bi₁₄O₂₀(ReO_4.5) (3t) after calcination at T = 700 °C. Compounds of the systems Bi₂O₃-SO_x (x = 2 and 3) and Bi₂O₃-Re₂O₇ are thermally stable up to T = 800 °C, whereas compounds of the system Bi₂O₃-SeO₃ completely lose SeO₃. Thermal treatment of 4c and 5c in air results in the oxidation of the tetravalent to hexavalent sulfur and selenium, respectively, upon heating to T = 400-500 °C. The as-prepared cubic bismuth(III)-based oxides 1c-5c were studied with regard to the photocatalytic decomposition of rhodamine B under visible-light irradiation with compound 3c showing the highest turnover and efficiency.

Facile and reproducible method of stabilizing [Formula: see text] phases confined in nanocrystallites embedded in amorphous matrix

Bismuth sesquioxide ([Formula: see text]) draws much attention due to wide variety of phases in which it exists depending on the temperature. Among them, [Formula: see text] phase is specially interesting because of its high oxide ion conductivity and prospects of applications as an electrolyte in fuel cells. Unfortunately, it is stable only in a narrow temperature range ca. 730-830 [Formula: see text]C. Our group has developed a facile and reproducible two-stage method of stabilizing [Formula: see text] crystalline phases confined in nanocrystallites embedded in amorphous matrix. In the first stage, glassy materials were obtained by a routine melt-quenching method: pure [Formula: see text] powders were melted in porcelain crucibles and fast-cooled down to room temperature. In the second step, the materials were appropriately heat-treated to induce formation of crystallites of [Formula: see text], [Formula: see text] or [Formula: see text] [Formula: see text] phases confined in a glassy matrix, depending on the process conditions. It was found out that the vitrification of the initial [Formula: see text] and the subsequent nanocrystallization were unexpectedly possible due to the presence of some Al, and Si impurities from the crucibles. Systematic DTA, XRD, optical, Raman and SEM/EDS studies were carried out to investigate the influence of the syntheses processes and allowed us to determine conditions under which the particular phases appear and remain stable down to room temperature.

Phase transformation and room temperature stabilization of various Bi2O3 nano-polymorphs: effect of oxygen-vacancy defects and reduced surface energy due to adsorbed carbon species

We report the grain growth from the nanoscale to microscale and a transformation sequence from Bi →β-Bi₂O₃→γ-Bi₂O₃→α-Bi₂O₃ with the increase of annealing temperature. The room temperature (RT) stabilization of β-Bi₂O₃ nanoparticles (NPs) was attributed to the effect of reduced surface energy due to adsorbed carbon species, and oxygen vacancy defects may have played a significant role in the RT stabilization of γ-Bi₂O₃ NPs. An enhanced red emission band was evident from all the samples attributed to oxygen-vacancy defects formed during the growth process in contrast with the observed white emission band from the air annealed Bi ingots. Based on our experimental findings, the air annealing induced oxidation of Bi NPs and transformation mechanism within various Bi₂O₃ nano-polymorphs are presented. The outcome of this study suggests that oxygen vacancy defects at the nanoscale play a significant role in both structural stabilization and phase transformation within various Bi₂O₃ nano-polymorphs, which is significant from theoretical consideration.

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

We report the synthesis of room temperature (RT) stabilized γ–Bi₂O₃ nanoparticles (NPs) at the expense of metallic Bi NPs through annealing in an ambient atmosphere. RT stability of the metastable γ–Bi₂O₃ NPs is confirmed using synchrotron radiation powder X-ray diffraction and Raman spectroscopy. γ–Bi₂O₃ NPs exhibited a strong red-band emission peaking at ~701 nm, covering 81% integrated intensity of photoluminescence spectra. Our findings suggest that the RT stabilization and enhanced red-band emission of γ‒Bi₂O₃ is mediated by excess oxygen ion vacancies generated at the octahedral O(2) sites during the annealing process.

From a Cerium-Doped Polynuclear Bismuth Oxido Cluster to β-Bi2O3:Ce

The simultaneous hydrolysis of Bi(NO₃)₃·5H₂O and Ce(NO₃)₃·6H₂O results in the formation of novel heterometallic bismuth oxido clusters with the general formula [Bi₃₈O₄₅(NO₃)₂₄(DMSO)_28+δ]:Ce (DMSO = dimethyl sulfoxide; cerium content <1.50%), which is demonstrated by single-crystal X-ray diffraction analysis. The incorporation of cerium into the cluster core is a result of the interplay of hydrolysis and condensation of the metal nitrates in the presence of oxygen. Diffuse-reflectance UV-vis and X-ray photoelectron spectroscopy reveal the presence of Ce^IV in the final bismuth oxido clusters as a result of oxidation of the cerium source. The cerium atoms are statistically distributed mainly on the bismuth atom positions of the central [Bi₆O₉] motif of the [Bi₃₈O₄₅] cluster core. Hydrolysis and subsequent annealing of the bismuth oxido clusters in the temperature range of 300-400 °C provides β-Bi₂O₃:Ce samples with slightly lowered band gaps of approximately 2.3 eV compared to the undoped β-Bi₂O₃ (approximately 2.4 eV). The sintering behavior of β-Bi₂O₃ is significantly affected by the cerium dopant. Finally, differences in the efficiency of the as-prepared β-Bi₂O₃:Ce and undoped β-Bi₂O₃ samples in the photocatalytic decomposition of the biocide triclosan in an aqueous solution under visible-light irradiation are demonstrated.