γ-Bi2O3 – To Be or Not To Be? Comparison of the Sillenite γ-Bi2O3

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

γ‑Bi2O3 − To Be or Not To Be? Comparison of the Sillenite γ‑Bi2O3 and Isomorphous Sillenite-Type Bi12SiO20 Marcus Weber,† Raul D. Rodriguez,‡,§,∥ Dietrich R. T. Zahn,‡ and Michael Mehring*,† †

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on June 28, 2018 at 13:00:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Fakultät für Naturwissenschaften, Institut für Chemie, Professur Koordinationschemie, Technische Universität Chemnitz, 09107 Chemnitz, Germany ‡ Fakultät für Naturwissenschaften, Institut für Physik, Professur Halbleiterphysik, Technische Universität Chemnitz, 09107 Chemnitz, Germany § Tomsk Polytechnic University, Lenina ave. 30, 634034, Tomsk, Russia S Supporting Information *

ABSTRACT: The “controlled” synthesis of metastable γ-Bi2O3 by solution based approaches was reported several times recently, but the formation of Bi12SiO20 in the presence of trace amounts of silicates renders the results to be questionable. Here, the preparation of the Sillenite γ-Bi2O3 and the Sillenitetype Bi12SiO20 starting from the polynuclear bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] is reported. γ-Bi2O3 crystallizes after calcination at 800 °C of the silicate-free hydrolysis product “[Bi38O45(OH)24]” 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. Bi12SiO20 was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or Na2SiO3 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-Bi2O3 to give αBi2O3 occurred slowly in the temperature range of 348−510 °C (ΔHγ→α = 6.57 kJ·mol−1). The silver-containing γ-Bi2O3 exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV−vis spectrum γ-Bi2O3 exhibits an absorption edge at λ = 485 nm (Eg = 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-Bi2O3 exhibits an absorption edge at λ = 460 nm (Eg = 2.83 eV) and Bi12SiO20 at λ = 422 nm (Eg = 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-Bi2O3 to be slightly more effective compared to Bi12SiO20 and significantly more effective than the silver-free γ-Bi2O3.

■

using PXRD, and a body-centered unit cell with a = (10.245 ± 0.001) Å was determined.2 Studies of Sillén, which were presented in 1937, indicated the existence of a body-centered cubic bismuth(III) oxide, but it came out that the discussed materials were stabilized by impurities such as aluminum or iron to give compounds with the general composition Bi24M2O40 (M = Al3+, Fe3+). A cell parameter of a = 10.16 Å was determined for Bi24Fe2O40.1 Harwig and Gerards demonstrated the crystallization of γ-Bi2O3 by thermoanalysis studies (DTA-TG and DSC experiments) on α-Bi2O3 in a platinum crucible.22 First, heating of α-Bi2O3 to 750 °C provides the formation of δ-Bi2O3 at 729 °C followed by the crystallization of γ-Bi2O3 at 639 °C during the subsequent cooling procedure. In 1945 Aurivillius and Sillén suggested that the cubic unit cell of γ-Bi2O3 with a = 10.24 Å contains

INTRODUCTION Body-centered cubic γ-Bi2O3 (I23) is one out of seven modifications of bismuth(III) oxide reported to date,1−6 and an eighth modification (ζ-Bi2O3) was postulated recently based on PXRD studies.7 Bismuth(III) oxide is a promising material for gas sensors8−10 and solid electrolytes (i.e., δ-Bi2O3 is the most effective oxide ion conductor of all binary metal oxides11,12). Furthermore, bismuth(III) oxide polymorphs are suitable photocatalysts with β-Bi2O3 exhibiting the best photocatalytic activity.13−16 Ternary bismuth(III) oxide based materials such as the Sillenite-type compounds Bi12MO20 (i.e., M = Si4+ or Ge4+) are attractive materials due to their excellent photorefractive sensitivity,17 piezoelectricity,18 and photocatalytic activity in the decomposition of organic pollutants.19−21 In 1943 Schumb and Rittner first reported on the preparation of γ-Bi2O3 that was obtained after heating β-Bi2O3 in a platinum crucible at 750−800 °C followed by cooling in air. Its structural characterization was carried out © XXXX American Chemical Society

Received: May 7, 2018

A

DOI: 10.1021/acs.inorgchem.8b01249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

prepared “γ-Bi2O3“ was most commonly determined using Powder X-ray Diffraction accompanied by TEM and SEM/ EDX analyses. Comparing the several synthetic approaches, a general scheme becomes obvious that is always based on the same steps: the starting material (most often Bi(NO3)3· 5H2O33−42 or BiCl343) is dissolved in aqueous media (usually dilute nitric or hydrochloric acid, respectively) followed by adding a surfactant (such as CTAB,33,41 polyethylene glycols,36,37,39,41−43 ethylene glycol,38,42 or sodium dodecyl sulfate40) and aqueous NaOH,33,35−43 KOH,42 or NH4OH34 solutions. After heating the mixture (70−90 °C)35−43 some protocols allude annealing of the products at higher temperatures (500−600 °C)33,34,36 which is unexpected due to studies, that report the phase transition of the metastable γBi2O3 into α-Bi2O3 in this temperature range.2,44,45 However, without exception the successful synthesis of the metastable γBi2O3 is postulated.33−43 It might be noted that the formation of ω-Bi2O3 was also postulated, although the PXRD pattern of “ω-Bi2O3” shows an unexplained mismatch with the cited XRD-file of ω-Bi2O3 (ICDD 00-050-1088).39 Noteworthy, ωBi2O3 was prepared by Gualtieri et al. heating α-Bi2O3 to 800 °C on a BeO substrate which most probably is essential to stabilize this metastable polymorph.4 In a preceding paper we already noted that the formation of γ-Bi2O3 using hydrothermal approaches is very unlikely.46 We assume two reasons for the fallacy in the literature reports: (i) formation of watersoluble silicates, as caused by the reaction of SiO2 from glassware with the NaOH, KOH, or NH4OH solutions, that subsequently react with the hydrolyzed bismuth precursors to give Bi12SiO20 instead of pure γ-Bi2O3; (ii) the assignment of products using Powder X-ray Diffraction is not straightforward due to the existence of several different reference files for γBi2O3. We assume that in most cases Bi12SiO20 was formed instead of γ-Bi2O3 as given in several references.33−43 Here we discuss in detail important aspects for the evaluation of analytical results for γ-Bi2O3 in comparison to Bi12SiO20 and present improved synthesis protocols for both compounds. Differences in PXRD patterns, IR- and Raman spectra as well as SEM/EDX analyses, diffuse UV−vis spectra, and thermal analysis (DSC) are focused on. Furthermore, we studied the photocatalytic activity of γ-Bi2O3 and Bi12SiO20 in degradation experiments of aqueous rhodamine B (RhB) solutions under visible light irradiation.

“Bi26O39“ (Z = 13) which nowadays is called Sillenite in order to honor the contributions of Sillén to the understanding of the polymorphism of bismuth(III) oxide.23,24 In 1964 Levin and Roth confirmed the existence of metastable γ-Bi2O3 as reported by Schumb and Rittner,2 but full structure evaluation was still elusive.25 Craig and Stephenson postulated the concept of pentavalent bismuth cations in the unit cell of γ-Bi2O3, where Bi3+ and Bi5+ ions are statistically located at the tetrahedral sites resulting in the formula Bi24[Bi3+, Bi5+]O40.26 The same hypothesis was reported by Watanabe et al. based on the structure of the nonstoichiometric Sillenite-type compound Bi3+23.33[Bi5+0.295P5+1.705]O40.27 All these models are based on the assumption of a fully occupied oxygen lattice.28 More recently, it was noticed that the assumption of pentavalent bismuth cations is inconsistent with regard to the high temperatures needed to prepare the polymorph and the low stability of BiV at higher temperatures.28 The most commonly accepted structure of γ-Bi2O3 to date was reported by Radaev et al. in 1992, presenting the formula Bi 12.8O19.2 (space group I23, Z = 2) that should be given as Bi12[(BixBiO3E)0.8(□’’’BiO4)0.2]O16 according to the KrögerVink notation with a = 10.2501(5) Å.29,30 This model is based on a crystal structure, in which the tetrahedral positions are occupied by Bi3+ to an extent of 80 % accompanied by 20% vacancies (given as □30). Three oxygen atoms coordinate to the Bi3+, and the 6s2 lone pair of electrons completes the coordination sphere of the aforementioned [BixBiO3] group to give a [BixBiO3E] tetrahedron occupying the 2a Wyckoffpositions of γ-Bi2O3. The “tetrahedrally coordinated” vacancy [□’’’BiO4] is statistically distributed over the Bi−O lattice.29 Furthermore, it is now well accepted that the Sillenite γ-Bi2O3 is isomorphous to metal oxides such as Bi12GeO20 and Bi12SiO20 (space group I23), the so-called Sillenite-type compounds, as was demonstrated by X-ray and neutron scattering data (Figure 1).31,32 The replacement of the [(BixBiO3E)0.8(□’’’BiO4)0.2] polyhedra by silicate groups [Si•BiO4] to give the Sillenite-type compound Bi12SiO20 should be given as Bi12[Si•BiO4]O16.30 As mentioned at the beginning, metastable γ-Bi2O3 is accessible by cooling from high temperatures; however, within the last decades numerous studies dealing with the synthesis of γ-Bi 2 O 3 by hydrothermal precipitation reactions were reported.33−43 The structure and composition of the as-

■

EXPERIMENTAL SECTION

General. The precursor [Bi38O45(O2CC3H5)24(DMSO)9] and its hydrolysis product “[Bi38O45(OH)24]”, as used for the synthesis of silicate-free γ-Bi2O3, were prepared according to the literature.46,47 The aqueous NaOH solution was prepared in a plastic beaker to ensure silicate-free conditions. Silver sheets (99.9%) were purchased from ChemPur. Powder X-ray diffraction was carried out with a STOE STADI P diffractometer (Darmstadt, Germany) using CuKαradiation (40 kV, 40 mA) and a Ge(111)-monochromator. The crystallite size was estimated according to the Scherrer equation: τ = Kλ/βcos θ, where τ is the volume weighted crystallite size, K is the Scherrer constant (here taken as 1.0), λ is the X-ray wavelength, θ is the Bragg angle in ° Theta, and β is the full width of the diffraction line at half of the maximum intensity (fwhm, background subtracted). The fwhm is corrected for instrumental broadening using a LaB6 standard (SRM 660) purchased from NIST. The value of β was corrected from (β2measured and β2instrument are the fwhm’s of measured and standard profiles): β2 = β2measured − β2instrument. Powder X-ray diffraction patterns of γ-Bi2O3 and Bi12SiO20 were calculated from the corresponding cif-files (γ-Bi2O3: 1010313,48 1540059,32 2100844;29 Bi12SiO20: 1010314,48 153322549) using STOE WinXPOW.

Figure 1. “Ball−stick” model of the Sillenite crystal structure. The structure shows γ-Bi2O3 (Bi12[(BixBiO3E)0.8(□’’’BiO4)0.2]O16) if the positions on the corners and the center are occupied by [(BixBiO3E)0.8(□’’’BiO4)0.2]-tetrahedra. In Bi12SiO20 (Bi12[Si•BiO4]O16) the [Si•BiO4]-tetrahedra replace the [(BixBiO3E)0.8(□’’’BiO4)0.2] unit. B

DOI: 10.1021/acs.inorgchem.8b01249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 1. Results of the Calcination Procedures of γ-Bi2O3 in a Tubular Furnace

Microwave assisted heating was carried out using a microwave Discover-S DC 5061 (CEM MATTHEWS, NC) in Teflon lined vessels (35 mL), and a special atmosphere was not required. The heating procedure was carried out with a power of 200 W, and the set point temperature was achieved within 2 min. The cooling procedure was performed within 1 min using compressed air. DSC experiments were carried out with a Mettler Toledo TGA/DSC1 1600 system with an MX1 balance. The measurements were performed in Al2O3 crucibles from 30 to 600 °C with a heating rate of 10 K·min−1 in air flow (60 mL·min−1) and held for 60 min followed by cooling (2.5 K·min−1) to 30 °C. Specific surface analyses were performed using N2 adsorption−desorption isotherms at liquid nitrogen temperature (77 K) using a Micromeritics Gemini 2370. They were evaluated by the Brunauer−Emmett−Teller (BET) method in the p/p0 range of 0.001−0.25. A scanning electron microscope (SEM, NanoNovaSEM, Co. FEI, OR, USA) was used for the energy dispersive X-ray (EDX) spectroscopy experiments. UV−vis spectroscopy was performed using a single-beam simultaneous spectrometer MCS 400 (Co. Carl Zeiss Jena GmbH). The UV and visible radiations were generated using a deuterium lamp CLD 300 and a xenon lamp CLX 11, respectively. Raman spectra were recorded using a Raman spectrometer Horiba LabRam HR800, and the laser excitation of a solid state laser (λ = 514.7 nm) was focused on the sample with a 50× long working distance objective (N.A. 0.5). The scattered Raman signal was collected in the backscattering geometry and analyzed by a diffraction grating with 600 mm−1. ATR-FTIR spectra were recorded on a Spectromat FTS-165 spectrometer. The photodegradation experiments were carried out in a water cooled glass reactor (15 °C). The as-prepared samples (40 mg) were dispersed in an aqueous solution of rhodamine B (RhB, 40 mL, c = 10 μmol·L−1) and stirred in the dark for 30 min to enable the adsorption−desorption equilibrium. The suspension was illuminated with a 300 W xenon lamp (type Cermax VQTM ME300BF, Co. PerkinElmer) equipped with a hot mirror (λ ≤ 700 nm) and a UV cutoff filter (λ ≥ 420 nm, GG420, Co. Schott) to provide only visible light. The UV−vis measurements were carried out by stopping to stir and darkening the light beam by a cover followed by centrifugation of the suspension (5 mL). After the measurement, the powder was suspended and refilled into the main reactor. The measurements were carried out with a 10 min interval up to 60 min, 15 min interval up to 150 min, 30 min interval up to 300 min, and 60 min interval up to 480 min. The degree of conversion is determined by calculating the mathematical area under the UV−vis curve between 450 and 600 nm and plotted as a function of irradiation time. Preparation, Purification, and Thermal Treatment of γBi2O3. I. Preparation. According to the literature procedure, the synthesis of γ-Bi2O3 was carried out under modified conditions.46 The silicate-free hydrolysis product “[Bi38O45(OH)24]” (206.4 mg, 0.023 mmol) of the bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] was placed on a silver sheet and annealed at 800 °C in a preheated tubular furnace in air (50 L·min−1) for 3 min to give a melt. The resulting melted material was quenched in air to room temperature resulting in an orange-brownish solid consisting of γ-Bi2O3 (sample 1, 155.9 mg, 0.33 mmol, 77%). II. Purification. γ-Bi2O3 (sample 1, 326 mg, 0.69 mmol) was suspended for 18 h in a solution of KCN in water (1%, 21 mL). The yellow residue was collected by centrifugation (3000 rpm, 1 min), washed with distilled water (6 × 10 mL), and dried in vacuo (10−3 mbar) for 60 min (60 °C). The silver-free product γ-Bi2O3 was obtained as yellow powder (sample 1a, 290 mg, 88%). III. Thermal Treatment. The as-obtained γ-Bi2O3 (sample 1) was placed on silver sheets and heated to the set point (heating rate: 10 K· min−1, details listed in Table 1) and kept for 60 min in air (50 L· min−1), followed by subsequent cooling (2.5 K·min−1). The resulting solids were collected and were identified using PXRD. Synthesis of Bi12SiO20. According to the literature procedure, the synthesis of Bi12SiO20 was carried out under modified conditions.46 The bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] (250 mg, 0.019 mmol) was dissolved in EtOH (2.5 mL) under stirring, and an aqueous solution of Na2SiO3·9H2O (17.05 mg in 0.05 mL of H2O)

starting material/mg

T/°C

product/mg

γ-Bi2O3 (1), 15.6 γ-Bi2O3 (1), 30.5 γ-Bi2O3 (1), 12.6

300 400 600

γ-Bi2O3 (1b), 15.2 (97%) α-/γ-Bi2O3 (1c), 29.9 (98%) α-Bi2O3 (1d), 12.4 (98%)

was added. After addition of the aqueous NaOH solution (0.5 M, 1.4 mL) under vigorous stirring, a colorless precipitate was formed immediately. The Teflon-lined vessel was closed and transferred into the microwave reactor, and the mixture was heated to 100 °C for 60 min (30 W). After cooling to room temperature using compressed air, the as-prepared product was collected by centrifugation (3000 rpm, 1 min), washed with EtOH/water (1:1) and three times with water, and dried in vacuo (10−3 mbar) for 60 min (60 °C). The as-obtained powder was annealed at 600 °C for 60 min using a heating rate of 10 K·min−1 in air (50 L·min−1), followed by subsequent cooling to room temperature (2.5 K·min−1). The product Bi12SiO20 was obtained as a colorless powder (sample 2, 172 mg, 96%).

■

DISCUSSION In advance of developing synthetic strategies for γ-Bi2O3 and Bi12SiO20 the literature reports were thoroughly evaluated. Due to the high reactivity of bismuth(III) oxide with various crucible materials at temperatures T > 800 °C, only a few materials such as gold, platinum, silver, or graphite are suitable to prevent the formation of Sillenite-type compounds Bi12MO20±x at higher temperatures (M = main group element or transition metal).1,2,23,25,32,44 Thus, in this work, γ-Bi2O3 (sample 1) was prepared starting from the bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] as a molecular precursor. This was hydrolyzed to give a bismuth oxide hydroxide compound “[Bi38O45(OH)24]”, which does not contain any residuals such as carbon, sulfur, and other impurities. Please note, that the purity of the hydrolysis product is a major feature, but it should be possible to use other precursors to prepare pure bismuth oxide hydroxide. However, we rely here on the easy synthesis of the bismuth oxido cluster and its hydrolysis product “[Bi38O45(OH)24]”; the latter was heated to 800 °C for few minutes on a silver substrate followed by subsequent quenching in air to give γBi2O3 (sample 1, Figure 2). Crystalline Bi12SiO20 (sample 2)

Figure 2. PXRD patterns of the as-prepared silver-containing γ-Bi2O3 (sample 1), pure γ-Bi2O3 (sample 1a), and Bi12SiO20 (sample 2), references: γ-Bi2O3, ICDD 00-045-1344; Bi12SiO20, ICDD 00-0370485; and “γ-Bi2O3“, ICDD 01-074-1375. See full PXRD patterns of samples 1, 1a, and 2 in Figure S1. C

DOI: 10.1021/acs.inorgchem.8b01249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry was obtained from hydrothermal treatment of the bismuth oxido cluster in the presence of Na2SiO3·9H2O and NaOH in a microwave reactor followed by calcination of the collected powder in a tubular furnace at 600 °C in air (Figure 2). Silicate-free conditions have to be ensured, thus avoiding glassware is essential. Otherwise, Bi12SiO20 might be formed as reported by us previously.46 We assume that the only prerequisite for the formation of γ-Bi2O3 is the formation of a “pure” bismuth oxide hydroxide. Due to the preparation conditions at 800 °C, corrosion of the silver-sheet by the bismuth(III)-oxide melt provides the contamination of the resulting γ-Bi2O3 (sample 1) with tiny amounts of silver (EDX: 2.49 wt %, Figure S2, Table S1) which are not incorporated into the Bi−O lattice. Takamori and Boland (1991) reported on a similar corrosion behavior in their crystallization studies of Bi12GeO 20 in platinum crucibles.50 The crucible was corroded by the Bi12GeO20 melt, but platinum was not incorporated in the crystal lattice of Bi12GeO20. However, the as-prepared γ-Bi2O3 (sample 1) fits well with the ICDD file 00-045-1344 (γ-Bi2O3), and the determined cell parameters (a = 10.2605 (4) Å and V = 1080.20 (4) Å3) are close to this ICDD file (a = 10.26700 (20) Å and V = 1082.3 Å3).32 The silver contaminant in γ-Bi2O3 was successfully removed by suspending the powder in an aqueous KCN-solution (1%) for 18 h to give a yellow colored solid still consisting of γ-Bi2O3 (sample 1a, PXRD see Figure 2). The reflection positions in the PXRD pattern of the silver-free γBi2O3 (sample 1a) remain unchanged, and the resulting cell parameters (a = 10.2610 (2) Å, V = 1080.36 (4) Å3) are almost the same compared to the cell parameters of sample 1. Thus, silver was not incorporated into the Bi−O lattice of γBi2O3. The EDX spectrum of γ-Bi2O3 (1a, Figure S2) reveals that the peaks of silver have disappeared. Powder X-ray Diffraction is the main analytical technique used for characterizing γ-Bi2O3.33−43 Interestingly, the ICDD file 01-074-1375 (labeled as γ-Bi2O3 in the database) does not fit with the as-prepared γ-Bi2O3, but the data are very close to those of the Sillenite-type compound Bi12SiO20. Thus we simulated the PXRD patterns as published for γ-Bi2O3 (I − Sillén 1938,48 IV − Radaev 1992,29 V − Harwig 197832) and the isomorphous Sillenite-type compound Bi12SiO20 for better comparison (II − Sillén 1938,48 III − Neov 200249) (Figure 3). It is important to note that several authors refer to the ICDD 01-074-1375 number (I, cif-file 101031348) in order to identify γ-Bi2O3.35,38,41,43 However, simulated PXRD patterns of γ-Bi2O3 (I) and Bi12SiO20 (II) show identical positions of the reflections (Figure 3) and exhibit the same cell parameter a = 10.080 Å.32 The unit cells of Bi12SiO20 (II) and the so-called “γ-Bi2O3“ (I) are close to the unit cell of Bi12SiO20 (III, a = 10.104(1) Å)49 but do not match with γ-Bi2O3 (IV, a = 10.2501(5) Å; V, a = 10.268(1) Å).29,32 It is concluded that the reference file ICDD 01-074-1375 was wrongly assigned to γ-Bi2O3 and represents Bi12SiO20 instead. Unfortunately, it is used as reference file for γ-Bi2O3 in several articles.35,38,41,43 EDX analyses of the as-prepared metal oxides (samples 1, 1a, and 2) confirm the chemical composition for Bi2O3 and Bi12SiO20, respectively (Figures S2 and S3, Table S1). Noteworthy, EDX analysis of Bi12SiO20 should be carried out thoroughly due to the low content of silicon which causes a very tiny signal in the EDX spectrum (Kα at approximately 1.74 keV) which can be easily overlooked. In order to investigate the metastability of the as-prepared γBi2O3 (sample 1), the powder was heated in a tubular furnace

Figure 3. PXRD patterns as reported for γ-Bi2O3 (I − Sillén 1938,48 IV − Radaev 1992,29 V − Harwig, 197832) and Bi12SiO20 (II − Sillén 1938,48 III − Neov 200249) calculated from their cif-files (1010313,48 2100844,29 1540059,32 1010314,48 and 1533225,49 respectively).

(10 K·min−1) to different temperatures for 60 min followed by subsequent cooling (2.5 K·min−1). After heating to 300 °C, the residue consists of γ-Bi2O3 (1b), whereas a mixture of α-/γBi2O3 (1c) was formed after heating to 400 °C and α-Bi2O3 (1d) as the sole crystalline phase was obtained after heating to 600 °C (Figure 4). The crystallization of α-Bi2O3 (1d) after

Figure 4. PXRD patterns of γ-Bi2O3 (1) after ex situ calcination (10 K·min−1) to 300 °C (1b), 400 °C (1c), and 600 °C (1d) for 60 min (50 L·min−1) on Ag sheets followed by cooling to room temperature (2.5 K·min−1). PXRD pattern of α-Bi2O3 (1e) obtained after the DSC experiment at 600 °C for 60 min (10 K·min−1) starting from 1 in an Al2O3 crucible in air (20 mL·min−1) followed by cooling (2.5 K· min−1) (references: γ-Bi2O3, ICDD 00-045-1344; α-Bi2O3, ICDD 01070-8243).

heating of 1 to 600 °C is in accordance to previous reports of Schumb and Rittner as well as of Gattow and Schütze, but the presence of a mixture of α-/γ-Bi2O3 (1c) after calcination of 1 to 400 °C indicates an incomplete phase transition.2,44,45 The ex situ calcination study on the metastability of γ-Bi2O3 (1) was accompanied by DSC analysis. A broad signal appeared in the DSC study in the temperature range from 348 to 510 °C with a transition enthalpy of 6.57 kJ·mol−1 indicating an exothermic process (Figure S4).22 The residue after the DSC analysis contains α-Bi2O3 (1e) as the sole crystalline phase (Figure 4) indicating the complete γ → α transition in this DSC experiment. Furthermore, the broad signal of this exothermic process from 348 to 510 °C matches D

DOI: 10.1021/acs.inorgchem.8b01249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

influence of the silver contaminant on the IR-spectrum of γBi2O3 was not obtained. The remaining vibration bands in Bi12SiO20 are assigned to asymmetric stretching modes of Bi−O bonds (603 cm−1, 573 cm−1, 521 cm−1, and 445 cm−1) which are also present in the IR spectrum of the as-prepared γ-Bi2O3 (629 cm−1, 568 cm−1, 521 cm−1, 492 cm−1, and 441 cm−1).52 The Raman spectra of γ-Bi2O3 (1, 1a) and Bi12SiO20 (2) (recorded using a cobalt-laser at room temperature, λ = 514.7 nm) are shown in Figure 6. The inset picture in the range of

with the result from the ex situ calcination study of 1 at 400 °C which provided a mixture of α- and γ-Bi2O3 (1c) as a result of the incomplete phase transition below 500 °C. To date, the most detailed data on the γ → α transition are based on the thermoanalysis studies of the cooling procedure of δ-Bi2O3 from 750 °C to room temperature reported by Harwig and Gerards in 1979.22 The γ → α transition took place in the temperature range of 639−543 °C and 604−562 °C by DTA and DSC studies (ΔHγ→α = 7.78 kJ·mol−1), respectively.22 The transition enthalpy of the γ → α transition in our DSC analysis is close to the reported value of Harwig and Gerards.22 However, in this work the γ → α transition occurs at T < 510 °C upon heating of 1 to 600 °C which is lower than the temperature range reported by Harwig and Gerards (604−562 °C). Noteworthy, in our previous work the DSC analysis on the hydrolysis product “[Bi38O45(OH)24]” of the cluster [Bi38O45(O2CC3H5)24(DMSO)9] showed the exothermic γ → α transition at approximately 448 °C upon cooling of the in situ formed bismuth(III) oxide from 800 °C.46 In addition to Powder X-ray Diffraction, IR and Raman spectroscopy were used in this work to demonstrate the structural differences between the Sillenite γ-Bi2O3 and the Sillenite-type compound Bi12SiO20. In general, structural elements of the Sillenite-type compound such as Bi12SiO20 are distinguished from those in γ-Bi2O3 by two kinds of structural units: the “quasi-molecular” [SiO4] units and the Bi−O framework.51 The symmetry of the [SiO4] group (Td) is reduced to T when it is placed in the Sillenite Bi−O lattice and all modes of the [SiO4] group are Raman active, whereas only the F modes are infrared active.52 The basic building unit of the bismuth−oxygen framework in Sillenite-type compounds is the [BiO5E] polyhedron which forms edge-shared dimers with neighboring [BiO5E] polyhedra (E is the unshared 6s2 electron pair of bismuth).53,54 Furthermore, Sillenite-type compounds Bi12MyO20±x contain [BiO4E] and [BiO3E] polyhedra when “y” is less than +4 (i.e., in γ-Bi2O3 where the [MO4] tetrahedra are replaced by umbrella shaped [BiO3E] polyhedra).53,54 In the IR spectra of γ-Bi2O3 (1, 1a) and Bi12SiO20 (2) the most obvious difference is shown by an intensive vibration band at approximately 824 cm−1 indicating the vibration of the [SiO4] tetrahedra (F symmetry) in Bi12SiO20 (2) (Figure 5).51,52,55,56 Metastable γ-Bi2O3 (1, 1a) does not show a comparable vibration band in this region of the IR spectrum. A significant

Figure 6. Raman spectra of the as-prepared silver-containing γ-Bi2O3 (sample 1), pure γ-Bi2O3 (sample 1a), and Bi12SiO20 (sample 2) including the inset region from 750 to 875 cm−1.

750−875 cm−1 shows the symmetric stretching mode at 787 cm−1 and the asymmetric stretching mode at 828 cm−1 of the [SiO4] tetrahedra in Bi12SiO20 (2), which are lacking in γBi2O3 (1, 1a).53,57,58 The positions of the Raman modes of 1, 1a, and 2 in this work are in accordance to literature reports (Table S2).53,58 Noteworthy, the Raman modes of 1 and 1a show a lower intensity and are broadened compared to the corresponding Raman modes of 2. The Raman bands in 1 and 1a differ slightly in their intensity, whereas 1 (silvercontaining) exhibits more intense bands compared to 1a (silver-free, see Figure S5). Due to the presence of silver adsorbed to γ-Bi2O3, the surface-enhanced Raman scattering (SERS) effect is observed resulting in more intense Raman bands compared to the silver-free sample of γ-Bi2O3 (1a).59−61 The Raman modes below 600 cm−1 are assigned to the internal bismuth−oxygen framework and are indicative for several stretching, bending, and rocking modes of Bi−O polyhedra in γ-Bi2O3 and Bi12SiO20.58 Whereas [BiO5E] groups constitute the backbone of the Bi−O lattice in Bi12SiO20, γBi2O3 consists alternatively of [BiO4E] and [BiO3E] polyhedra, whereby the latter replace the [SiO4] groups.54 Egorysheva et al. discussed the distinction of vibrations of [BiO5E] and [BiO4E] in the Raman spectra of several Sillenite-type compounds, but these polyhedra involve the same atoms and exhibit almost identical bond lengths as well as bond angles.53 They postulated significant broadening of the Raman bands with increasing concentration of [BiO4E] polyhedra.53 This assumption is in line with the broadening of the Raman modes of 1 and 1a compared to the Raman modes of 2 in this work. The main motivation of several authors to prepare γ-Bi2O3 “by design” was its potential use as photocatalyst in water purification and water splitting.33−35,42,62−64 In contrast to Bi12SiO20, which is known as a wide band gap material with a

Figure 5. IR spectra of the as-prepared silver-containing γ-Bi2O3 (sample 1), pure γ-Bi2O3 (sample 1a), and Bi12SiO20 (sample 2). E

DOI: 10.1021/acs.inorgchem.8b01249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry band gap from 3.15 to 3.25 eV,65 γ-Bi2O3 is supposed to exhibit good optical properties such as a wider absorption in the visible light region (550 nm) compared to the α- (420 nm) and β-polymorph (450 nm) based on first-principle calculations by Wang et al.66 Calculations of the band structure and density of states (DOS) by Wang et al. showed that γ-Bi2O3 exhibits a direct band gap with a simulated value of 2.07 eV.66 In this work, the as-obtained silver-containing γ-Bi2O3 (1) is a yellow-brownish solid and exhibits an absorption edge at 485 nm in the diffuse UV−vis absorption spectrum, which corresponds to a band gap of Eg = 2.76 eV (Figure 7).

Bi12SiO20 (2) is a colorless solid and exhibits a narrow absorption band in the UV region (Figure 7) with an absorption edge at 422 nm and a resulting band gap of 3.16 eV, which is in the range of reported values (2.80−3.25 eV).19,21,65 The samples 1, 1a, and 2 exhibit low specific surface areas of ABET = 0.21 m2·g−1, 1.79 m2·g−1, and 0.84 m2·g−1, respectively, as a result of the calcination procedures. Due to the washing procedure of 1 for 18 h with KCN-solution, the γ-Bi2O3 particles are slightly ground, and thus the surface area of 1a is slightly increased. SEM images of samples 1, 1a, and 2 confirm the presence of μm scaled particles (crystallite size approx. 120 nm for 1 and 2) resulting in low surface areas (Figure 8).

Figure 8. SEM images of the as-prepared silver-containing γ-Bi2O3 (sample 1), pure γ-Bi2O3 (sample 1a), and Bi12SiO20 (sample 2). Noteworthy, the particle shapes of Bi12SiO20 (2) are very similar to the “flowerlike” agglomerates of as-prepared “γ-Bi2O3“ by hydrothermal treatment as shown in several studies.36,38 In various literature reports, the synthesis of “γ-Bi2O3“ under hydrothermal conditions was reported for approaches with33,36−40,43 and without surfactants,34,35 and the used additives such as ethylene glycol and polyethylene glycols are discussed to affect the resulting crystal structure of the bismuth(III) oxide. The presence of such surfactants accelerates the formation of isomorphous Bi12SiO20 and affects the resulting morphology of the particles, but their effect on the formation of metastable γ-Bi2O3 could not be verified in a recent study.46

Figure 7. Diffuse reflectance UV−vis spectra (top) and Tauc plots for the direct band gaps (bottom) of the as-prepared silver-containing γBi2O3 (sample 1), pure γ-Bi2O3 (sample 1a), and Bi12SiO20 (sample 2).

The as-prepared samples γ-Bi2O3 (1, 1a) and Bi12SiO20 (2) were investigated with regard to their photocatalytic activity in the decomposition of aqueous rhodamine B (RhB) solution (1 × 10−5 mol·L−1) under visible light irradiation (400 nm < λ < 700 nm, Figure 9). Samples 1, 1a, and 2 (40 mg) were suspended in the aqueous RhB solution (40 mL) and stirred in the dark for 30 min to enable the adsorption/desorption equilibrium followed by illumination with visible light for 480 min or stirring in the dark. When the suspension was stirred in the dark for 30 min, the adsorption of RhB was negligible (