Evaluation of Synthetic Methods for Bismuth(III) Oxide Polymorphs

Article pubs.acs.org/crystal

Evaluation of Synthetic Methods for Bismuth(III) Oxide Polymorphs: Formation of Binary versus Ternary Oxides Marcus Weber, Maik Schlesinger, and Michael Mehring* Fakultät für Naturwissenschaften, Institut für Chemie, Professur Koordinationschemie, Technische Universität Chemnitz, 09107 Chemnitz, Germany S Supporting Information *

ABSTRACT: Hydrolysis of the bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] (A), dissolved in EtOH, with a diluted NaOH solution at ambient temperature using Teflon-lined vessels followed by heating of the amorphous product on a silver sheet gave pure γ-Bi2O3. The formation of Bi12SiO20 in addition to γ-Bi2O3 is observed when the hydrolysis of A is carried out in a glass vessel, which provides a low concentration of silicates as a result of glass corrosion. Hydrolysis of the cluster A under microwave-assisted heating in a Teflon-lined vessel gave βBi2O3, whereas Bi(NO3)3·5H2O (B) provided α-Bi2O3 and Bi12O17Cl2 was observed from BiCl3 (C). In glass vessels, alkaline induced hydrolysis of the precursors provided exclusively Bi12SiO20 as final product instead of the isomorphous metastable γ-Bi2O3. This observation is in contrast to recent literature reports. Powder X-ray diffraction studies, attenuated total reflection infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy were used to provide insight into the hydrolysis processes and into the concurring formation of the sillenite-type compound Bi12SiO20 instead of pure bismuth(III) oxide polymorphs.

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spectroscopy.3 In general, bismuth(III) oxides are of interest due to their usage as photocatalysts,14−18 sensors,19,20 and oxidation catalysts.21,22 In addition, the high-temperature modification δ-Bi2O3, which exhibits the highest conductivity of known solid-state oxygen ion conductors due to its crystallization in the deficient fluorite-type, is a well-known material for solid electrolytes (e.g., for solid oxide fuel cells).23−27 However, its limited stability range prevents large scale applications to date. Thus, several studies addressed the stability ranges of this polymorph.28,29 Pathways to synthesize Bi2O3 at moderate conditions such as precipitation reactions are favored due to the mild reaction conditions that allow control over polymorph formation, morphology, and particle size. To date, different precipitation approaches to synthesize polymorphs of bismuth(III) oxide are reported, most of them starting from Bi(NO3)3·5H2O in aqueous HNO3 solution. The type of additive is often discussed as a crucial parameter to control the formation of a specific polymorph (Table 1). However, ambiguous results are reported regarding the role of additives, especially during the formation of metastable γBi2O3 and their impact on the control of polymorph formation (Table 1). This finding prompted us to elucidate the hydrolysis process in more detail by using several bismuth precursors and

INTRODUCTION Bismuth(III) oxide exhibits a distinctive polymorphism with seven established polymorphs reported to date.1,2 An additional modification was recently postulated.3 At room temperature, αBi2O3 (monoclinic) is the stable polymorph, which transforms into δ-Bi2O3 (cubic) upon heating to 730 °C. The δ-Bi2O3 is present up to the melting point (824 °C). By cooling δ-Bi2O3 down a phase transition occurs to give either β-Bi 2O 3 (tetragonal) at approximately 650 °C or γ-Bi2O3 (cubic) at approximately 639 °C. Both β-Bi2O3 and γ-Bi2O3 are (kinetically) stabilized at room temperature.4 γ-Bi2O3 was obtained first by Schumb et al. (1943) after heating β-Bi2O3 in a platinum crucible to 750−800 °C and subsequent cooling.5 It is important to note that within the same decade, Sillén (1937) and Aurivillius and Sillén (1945) have demonstrated the reaction of bismuth(III) oxide with Al2O3 and Fe2O3 to give ternary oxides with the formula Bi12MO20±x (M = Al, Fe), which belong to the family of so-called sillenite-type structures.6 The compounds were reported to be isomorphous to γBi2O3.7,8 Meanwhile, a large number of main group and transition metals in diverse oxidation states were reported to stabilize the sillenite-type structure with the formula which is generally given as Bi12MO20±x.9,10 In addition to the α-, β-, γ-, and δ-modifications of Bi2O3, the metastable polymorphs ε(orthorhombic), ω- (triclinic), and η-Bi2O3 (high-pressure, hexagonal) exist,11−13 and recently the formation of ζ-Bi2O3 was postulated based on PXRD and X-ray photoelectron © XXXX American Chemical Society

Received: April 25, 2016 Revised: August 10, 2016

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DOI: 10.1021/acs.cgd.6b00628 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Selected Synthesis Methods for Bi2O3 Polymorphs with and without the Use of Additives Starting from Bismuth Nitrate and Bismuth Chloride starting material Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O Bi(NO3)3· 5H2O BiCl3 a

solvent HNO3 (aq)

additive NH4VO3

HNO3 (conc)

product

reference

60 °C, NaOH, 24 h

reaction conditions

δ-Bi2O3

18

r.t.a, 12.67 M NaOH, 120 min

α-Bi2O3

30

CH3COOH, EtOH

DMFb

(i) 100 °C, 40 min; (ii) precipitate annealing, 350 °C, 240 min

β-Bi2O3

31

HNO3 (aq)

25 °C, 0.42 M NaOH, 120 min

α-Bi2O3

32

HNO3 (aq)

PEG1000c, PEG4000c or PEG6000c PEG8000c

85 °C, 0.86 M NaOH, 45 min

γ-Bi2O3

33

HNO3 (aq)

PEG400c

70 °C, 0.98 M NaOH, 120 min

γ-Bi2O3

34

HNO3 (aq)

CTABd

(i) r.t.a, 0.19 M NaOH, 30 min; (ii) precipitate annealing, 500 °C, 120 min (i) NH4OH pH = 10; (ii) precipitate annealing, 500 °C, 120 min

γ-Bi2O3

35

γ-Bi2O3

36

150 °C, NaOH pH = 7, 10 min

γ-Bi2O3

37

80 °C, 2 M NaOH, 120 min

γ-Bi2O3

38

HNO3 (aq) HNO3 (aq) EtOH/H2O (1 M HCl)

PEG4000

c

r.t. = room temperature. bDMF = dimethylformamide. cPEG = polyethylene glycol. dCTAB = cetyltrimethylammonium bromide.

Table 2. Concentrations of the Starting Materials [Bi38O45(O2CC3H5)24(DMSO)9] (A), Bi(NO3)3·5H2O (B), and BiCl3 (C) in Solutiona

a

solution

starting material

solvent

c [mol·L−1]

A1 A2 B1 B2 B3 C1 C2

[Bi38O45(O2CC3H5)24(DMSO)9] [Bi38O45(O2CC3H5)24(DMSO)9] Bi(NO3)3·5H2O Bi(NO3)3·5H2O Bi(NO3)3·5H2O BiCl3 BiCl3

EtOH EtOH DMSO 1 M HNO3 2 M HNO3 EtOH/H2O (v:v = 1:1, 1 M HCl) EtOH/H2O (v:v = 1:1, 1 M HCl)

1.59 × 10−3 7.95 × 10−3 0.21 0.10 0.15 0.10 0.10

pH

V [mL]

0.6 0.2 1.0 1.0

5.0 2.5 2.5 2.5 20.0 10.0 2.5

See exp. parts A−D.

Here we present studies on the formation of bismuth(III) oxide polymorphs in a one-step procedure under microwave-assisted and conventional hydrolysis/annealing procedures starting from either [Bi38O45(O2CC3H5)24(DMSO)9] (A), Bi(NO3)3· 5H2O (B), or BiCl3 (C). The influence of the NaOH concentration, additive, solvent, reaction time, and the vessel material on the as-prepared hydrolysis products is discussed whereby the question of whether pure specific polymorphs are accessible is of major interest. Additionally, it will be clarified which reaction conditions will result in the formation of ternary bismuth oxides of the sillenite-type family (Bi12SiO20).

additives. We have previously reported on the hydrolysis of preorganized precursors such as the polynuclear bismuth oxido clusters [Bi6O4(OH)4](NO3)6·H2O, [Bi22O26(OSiMe2tBu)14], [Bi38O45(NO3)20(DMSO)28](NO3)4·4DMSO, and [Bi38O45(O2CC3H5)24(DMSO)9].17,39−41 These polynuclear metal oxido clusters are built up by octahedral building units [Bi6O8−x]2(1+x)+, and the extension of this structural motif to a network gives a nearly fcc packing of the bismuth atoms, comparable to the solid state structures of β-Bi2O3 and δ-Bi2O3. Thus, the polynuclear bismuth oxido clusters may be regarded as a “cut out” of the solid state structure of bismuth(III) oxide and are suitable model compounds in the hydrolysis process.1,42 Noteworthy, (thermal) decomposition of silanolate functionalized clusters results in the formation of β-Bi2O3 and amorphous SiO2 at temperatures below 350 °C and provides Bi12SiO20 at 700 °C due to the reaction between β-Bi2O3 and SiO2.41 Our group has also demonstrated the controlled synthesis of pure nanoscaled β-Bi2O3 starting from silanolatefree polynuclear bismuth oxido clusters, e.g., [Bi6O4(OH)4](NO3)6·H2O and [Bi38O45(O2CC3H5)24(DMSO)9] (A), in a simple two-step hydrolysis/annealing procedure.17 Noteworthy, the as-prepared β-Bi2O3 nanoparticles are highly reactive with regard to solid state reaction at temperatures above 480 °C to give Bi12SiO20 and Bi12AlO19.5 by heating on SiO2 and Al2O3 substrates, respectively. This reactivity has to be kept in mind while working with bismuth(III) oxide at higher temperatures.

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EXPERIMENTAL SECTION

General. Powder X-ray diffraction patterns were measured with a STOE STADI P diffractometer (Darmstadt, Germany) using CuKαradiation (40 kV, 40 mA) and a Ge(111)-monochromator. The crystallite size was estimated using the formula determined by the Scherrer equation: τ = Kλ/β cos θ, where τ is the volume weighted crystallite size, K is the Scherrer constant here taken as 1.0, λ is the Xray wavelength, θ is the Bragg angle, and β is the full width of 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 fwhms of measured and standard profiles): β2 = β2measured − β2instrument. TGA/DSC experiments were determined with a Mettler Toledo TGA/DSC1 1600 system with a MX1 balance. The measurement was performed in B



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Table 3. Parameters and Results of the Hydrolysis Experimentsa sample

solution/ solid [mg]

vessel/ substrate

solvent/ precursor

P1a P1b P1a-385 P1b-385 P1b-385-MW60 P1a-800

A1 A1 P1a (52) P1b (56) P1b-385 (160) P1a (14)

glass Teflon silver silver glass

P1b-800 P2-60 P3-60 P3-60-Si

P1b (20) A2 A2 A2

silver glass Teflon Teflon

EtOH EtOH EtOH

P4-60 P5-60 P6-60 P7a-60 P7b-60 P8a-60 P8a-60-Si

A2 A2 A2 B1 B1 B1 B1

glass glass glass glass glass Teflon Teflon

EtOH EtOH EtOH DMSO DMSO DMSO DMSO

P8b-60

α-Bi2O3 (120) B2 B2

glass

DMSO

glass Teflon

P9a-PEG40060 P9b-PEG40060 P9aPEG8000-60 P9bPEG8000-60 P10a-120

B2

glass

HNO3 (aq) HNO3 (aq.) HNO3 (aq)

B2

Teflon

B2

NaOH/ [mL], [M]

additive

EtOH EtOH

5, 0.05 5, 0.05

EtOH

1.4, 0.5

T [°C]

pHc

t [min]

25 25 385 385 100

12.7 12.7

3 3 20 20 60

amorphous (54, 77%)b amorphous (55, 79%)b β-Bi2O3 (49, 97%) β-Bi2O3 (46, 84%) Bi12SiO20 (140, 86%)

180

13.7

product [mg, %]

1.4, 0.5 1.4, 0.5 1.4, 0.5

800 100 100 100

13.8 13.8 13.8

180 60 60 60

γ-Bi2O3 + Bi12SiO20 (12, 86%) γ-Bi2O3 (16, 80%) Bi12SiO20 (178, 99%) α-, β-Bi2O3 (175, 99%) Bi12SiO20 (177, 99%)

1.2, 0.8, 0.4, 1.4, 1.4, 1.4, 1.4,

100 100 100 100 100 100 100

13.5 13.0 11.5 14.3 15.3 14.3 14.3

60 60 60 60 60 60 60

β-Bi2O3 (131, 74%) α-, β-Bi2O3 (155, 88%) amorphous (146, 83%)b Bi12SiO20 (118, 96%) Bi12SiO20 (112, 91%) α-Bi2O3 (113, 94%) Bi12SiO20 (109, 89%)

1.4, 6

100

15.2

60

Bi12SiO20 (104, 85%)

2.5, 12 2.5, 12

100 100

13.5 13.5

60 60

Bi12SiO20 (38, 64%) α-Bi2O3 (35, 59%)

PEG400 (0.125 mL)

2.5, 12

100

13.2

60

Bi12SiO20 (46, 77%)

HNO3 (aq)

PEG400 (0.125 mL)

2.5, 12

100

13.2

60

α-Bi2O3 (45, 76%)

glass

HNO3 (aq)

PEG8000 (0.1 g)

2.5, 12

100

13.3

60

Bi12SiO20 (48, 81%)

B2

Teflon

HNO3 (aq)

PEG8000 (0.1 g)

2.5, 12

100

13.3

60

α-Bi2O3 (37, 62%)

B3

glass

HNO3 (aq)

PEG400 (1 mL)

20, 2

70

1.0

120

P11a-120 P12a-120 P13a-120 P14PEG4000120 P14PEG8000120 P14-180 P15a-120 P15b-Si-120

B3 B3 B3 C1

glass glass glass glass

HNO3 (aq) HNO3 (aq) HNO3 (aq) HCl (aq)

PEG400 (1 mL) PEG400 (1 mL) PEG400 (1 mL) PEG4000 (0.4 g)

20, 20, 20, 10,

70 70 70 80

12.9 13.2 13.7 13.2

120 120 120 120

[Bi6O5(OH)3(NO3)5]·2H2O (590, 68%) α-Bi2O3 (641, 92%) α-Bi2O3 (623, 89%) α-Bi2O3 (630, 90%) Bi12SiO20 (166, 70%)

C1

glass

HCl (aq)

PEG8000 (0.4 g)

10, 4

80

13.2

120

Bi12SiO20 (148, 62%)

C1 C2 C2

glass glass Teflon

HCl (aq) HCl (aq) HCl (aq)

10, 4 2.5, 4 2.5, 4

80 80 80

13.3 13.3 13.2

180 120 120

Bi12SiO20 (104, 44%) Bi12SiO20 (50, 83%) Bi12SiO20 (52, 87%)

P15b-120 P15bPEG4000120 P15bPEG8000120

C2 C2

Teflon Teflon

HCl (aq) HCl (aq)

PEG4000 (0.1 g)

2.5, 4 2.5, 4

80 80

13.3 13.3

120 120

Bi12O17Cl2 (53, 90%) Bi12O17Cl2 (41, 70%)

C2

Teflon

HCl (aq)

PEG8000 (0.1 g)

2.5, 4

80

13.3

120

Bi12O17Cl2 (39, 66%)

P9a-60 P9b-60

a

silver

800

Na2SiO3·9H2O (aq, 1 M, 0.049 mL)

Na2SiO3·9H2O (aq, 1 M, 0.084 mL)

Na2SiO3·9H2O (aq, 1 M, 0.02 mL)

0.5 0.5 0.5 2 6 2 2

4 6 12 4

See exp. parts A−D. bFormation of amorphous Bi2O3 assumed. cFinal reaction solution before heating.

Al2O3 crucibles from 30 to 800 °C with a heating rate of 10 K·min−1 in Ar atmosphere and a volume flow of 60 mL·min−1. A scanning electron microscope (SEM, NanoNovaSEM, Co. FEI, OR, USA) was used for the energy dispersive X-ray (EDX) spectroscopy experiments and morphology studies. ATR-FTIR spectra were recorded on a Spectromat FTS-165 spectrometer. The precursor [Bi38O45(O2CC3H5)24(DMSO)9] (A) and its hydrolysis product as used for the synthesis of γ-Bi2O3 were prepared according to the

literature.17,39 Bi(NO3)3·5H2O, BiCl3, PEG4000, and PEG8000 were purchased from Alfa Aesar. PEG400 was purchased from Lancaster. Microwave-assisted heating was carried out by using a microwave Discover-S DC 5061 (CEM MATTHEWS, NC) in glass vessels (Pyrex, 35 mL, composition see Table S1) or in Teflon-lined vessels (35 mL), a special atmosphere was not required. The heating procedure in the microwave reactor using glass vessels was carried out with a power of 300 W, and the set point temperature was achieved C



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Figure 1. PXRD analyses of P1a-385, P1b-385, P1a-800, and P1b-800 (magnification of the diffractogram; left: 2θ region 20−40°; right: 2θ region 40−70°. reference: ◇ Bi12SiO20, ICDD 00-037-0485; ● β-Bi2O3, ICDD 00-027-0050; ∇ γ-Bi2O3, ICDD 00-045-1344). within 80 s. When Teflon-lined vessels were used, the power was decreased to 30 W, and the heating procedure was increased to 6 min. The cooling procedure of both vessel types was carried out within 1 min using compressed air. A HI 223 Calibration Check pH meter (Hanna Instruments) was used to measure pH values. Glass flasks were purchased from VWR (201-1356 Boro 3.3, composition see Table S1). The aqueous NaOH solutions were prepared in plastic beakers to ensure silicate free conditions. Silver sheets (99.9%) were purchased from ChemPur. The starting materials, solvents, and concentrations of the starting solutions for the hydrolysis studies are listed in Table 2. A. Synthesis of γ-Bi2O3. The hydrolysis of cluster A (solution A1) was carried out in a glass vessel and a Teflon-lined vessel in EtOH using a diluted NaOH solution to give the amorphous precipitates P1a and P1b, respectively (parameters and results listed in Tables 2 and 3).17 The solids P1a and P1b were placed on a silver sheet and heated in a tube furnace (heating rate: 10 K·min−1, argon flow rate: 10 L· min−1). Annealing of P1a as well as P1b at 385 °C for 20 min followed by subsequent cooling (2 K·min−1) results in the formation of β-Bi2O3 as the only crystalline material in both cases (P1a-385, P1b-385). Annealing of P1a to 800 °C in argon atmosphere followed by slow cooling to room temperature (2 K·min−1) provided a mixture of γBi2O3 and Bi12SiO20 (P1a-800), while annealing of P1b gave pure γBi2O3 (P1b-800). B. Hydrolysis Studies under Microwave-Assisted Heating. The solution of the starting material (A2, B1, B2, and C2, respectively) was poured either into a glass or a Teflon-lined vessel (35 mL). In the cases of B2 and C2, polyethylene glycol (PEG400, PEG4000, and PEG8000, respectively) was added to give a clear solution in accordance to the literature.33,34,38 After addition of the aqueous NaOH solution under vigorous stirring, the vessel was closed and transferred into the microwave reactor. Under stirring, the mixture was heated to 80 or 100 °C for a selected time interval (Table 3) and then cooled down to room temperature by compressed air. The as-prepared products were 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 parameters and results are listed in Table 3. Following a similar protocol, hydrolysis of solutions A2, B1, and C2 was also carried out in Teflon-lined vessels in the presence of an aqueous Na2SiO3·9H2O solution (Table 3). C. Microwave-Assisted Heating of Bi2O3 Suspensions. Bismuth(III) oxide was suspended in a solvent (α-Bi2O3 (P8b) in DMSO, 2.5 mL; β-Bi2O3 (P1b-385) in EtOH, 2.5 mL), and aqueous NaOH was added under vigorous stirring. After transferring the suspension into the microwave reactor, the mixture was heated to 100 °C for 60 min under stirring. Then the mixture was cooled down to room temperature, and the obtained products (P8b-60, P1b-385-MW-60) were 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 parameters and results are listed in Table 3. D. Hydrolysis Studies under Conventional Heating. The solution of the starting material (B3 or C1) was poured into a glass flask (100

mL), and polyethylene glycol (PEG400, PEG4000, and PEG8000, respectively) was added to give a clear solution in accordance to the literature.33,34,38 After addition of the aqueous NaOH solution under vigorous stirring, the mixture was heated to 70 or 80 °C at reflux for a selected time interval (Table 3). The as-prepared products were collected by centrifugation (3000 rpm, 1 min), washed three times with water and three times with ethanol, and dried in vacuo (10−3 mbar) for 60 min (60 °C). The parameters were chosen in accordance with literature procedures33,34,38 and listed together with our results in Table 3.

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RESULTS AND DISCUSSION Formation of γ-Bi 2 O 3 versus the Sillenite-type Bi12SiO20. In 1978 Harwig reported that cooling of δ-Bi2O3 from 730 °C to approximately 640 °C results in the formation of γ-Bi2O3.4 Later, Drache et al. proposed that the δ → γ transition from the fluorite cell is possible by building a rhombohedral pseudo cubic cell which exhibits the sillenite cell volume.6,43 The close structural relationship between the rhombohedral cell and the cubic sillenite cell enables the δ → γ transition by rearrangement of the bismuth and oxygen atom sublattices during cooling.43 Thereby, incorporation of low levels of impurities is easily possible, which might result in the formation of sillenite-type compounds Bi12MO20±x (M = main group element or transition metal).8−10 Incorporation of impurities is not observed when crucible materials which resist the attack of bismuth(III) oxide are used. For example, platinum crucibles are appropriate materials for the synthesis of pure γ-Bi2O3 as it was reported by Schumb and Rittner as well as by Gattow and Fricke.5,44 We have chosen silver substrates in the present studies, which are also resistant against the attack of bismuth(III) oxide.7 In a typical procedure, the bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] (A) was hydrolyzed to give the amorphous product P1, which was used as the starting material. The hydrolysis of A was carried out in a glass flask (P1a) and in a Teflon-lined vessel (P1b). The as-obtained powders were placed on the silver sheet and heated in a tubular furnace under argon flow. After heating at 385 °C for 20 min and subsequent cooling to room temperature, both precursors P1a and P1b result in the formation of β-Bi2O3 (P1a-385, dP = (41 ± 4) nm and P1b-385, dP = (31 ± 2) nm; PXRD see Figure 1). A TGA/DSC measurement of P1b in an argon atmosphere on an Al2O3 substrate showed the crystallization of β-Bi2O3 at 295 °C (Figure S1). A monotropic phase transition from β- to α-Bi2O3 occurs at 480 °C, which was also demonstrated by temperature-dependent Raman studies starting from hydrolyzed bismuth oxido clusters on Al2O3.45 Furthermore, at 742 °C a phase transition from α- to δ-Bi2O3 occurs, which is in D



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accordance with literature values.28,29 This behavior was confirmed in an additional experiment under similar conditions, but with a set point temperature of 800 °C which was kept for 60 min followed by subsequent cooling (2.5 K·min−1, Figure S2). The cooling procedure shows first an exothermic peak at approximately 641 °C, which is indicative for the δ-Bi2O3 → γBi2O3 transition in accordance with literature values.28 Further cooling on the Al2O3 substrate results in the phase transition from γ-Bi2O3 to finally give α-Bi2O3 (P1b-TG), which is demonstrated by the exothermic peak in the DSC of P1b at approximately 448 °C and confirmed by PXRD (Figure S3). Noteworthy, decomposition of the precursor P1b gave pure γBi2O36 (dP = (130 ± 57) nm) instead of α-Bi2O3 after heating to 800 °C for 180 min on a silver substrate used instead of Al2O3 (P1b-800) and subsequent cooling to room temperature. Thus, the used substrate seems to affect the transition behavior of the bismuth(III) oxide during the cooling process. Further studies regarding the phase transition of bismuth(III) oxides using different substrates will be part of a ongoing project. In the case that the precursor P1a was heated to 800 °C for 180 min on a silver substrate and cooled slowly down to room temperature, the PXRD shows reflections assigned to both γBi2O3 (dP = (110 ± 42) nm) and the isomorphous sillenitetype Bi12SiO20 (dP = (121 ± 51) nm, P1a-800, Figure 1).6 The IR spectra additionally confirm the presence of Bi12SiO20 in P1a-800 due to the characteristic Bi−O−Si vibration (823− 826 cm−1, Figure 2).46,47 The bands at 632, 573, 524, and 444

the reaction mixture. The cluster A (250 mg, 0.02 mmol) was dissolved in ethanol (2.5 mL, glass vessel with a total volume of 35 mL) and hydrolyzed using a 0.5 M aqueous NaOH solution (final NaOH-concentration of 0.18 M, pH = 13.8, see Table 3). A precipitate was observed and the suspension was then heated at 100 °C for 60 min to give Bi12SiO20 as indicated by PXRD (P2-60, dP = (45 ± 5) nm, Figure 3). β-Bi2O3 containing a tiny

Figure 3. PXRD analyses of P2-60, P3-60, and P3-60-Si (reference: ◇ Bi12SiO20, ICDD 00-037-0485; ● β-Bi2O3, ICDD 00-027-0050; ◆ α-Bi2O3, ICDD 01-070-8243).

amount of α-Bi2O3 (P3-60) was formed when the reaction was carried out in a Teflon-lined vessel under the same reaction conditions. The formation of Bi12SiO20 (P3-60-Si, dP = (128 ± 56) nm) was also observed upon addition of an aqueous Na2SiO3·9H2O solution in a Teflon-lined vessel. Thus, we assume that glass corrosion enables the formation of Bi12SiO20 when using glass vessels. The deviations in the broadness of the reflections in the PXRD and the resulting crystallite sizes for Bi12SiO20 (P2-60, P3-60-Si) are caused by the reaction rate of the hydrolyzed cluster with the silicates. Bi12SiO20 (P3-60-Si) is formed in one step due to the rapid and separate addition of silicate species, and thus sharp reflections of Bi12SiO20 were detected in the PXRD. When the silicates were slowly dissolved from the Pyrex vessel by glass corrosion, Bi12SiO20 (P2-60) is formed slowly and results in a variety of crystallite sizes and causes broadened reflections in the PXRD. Variation of the heating times (0, 1, 10, 20, 30, 45, and 60 min, P2-0 to P2-60, glass vessel, 0.18 M NaOH, 100 °C, EtOH) provided a deeper insight into the formation process of Bi12SiO20 (PXRD and ATR-IR analyses in Figures 4 and 5). The transformation from the amorphous precipitate (P2-0) to crystalline β-Bi2O3 containing a tiny amount of α-Bi2O3 (P21) occurs already after 1 min at 100 °C (Figures 4 and 5). At

Figure 2. ATR-IR spectra of γ-Bi2O3/Bi12SiO20 (P1a-800) and γBi2O3 (P1b-800) as obtained by hydrolysis and annealing of the bismuth oxido cluster A.

cm−1 are assigned to the asymmetric stretching modes of Bi−O bonds.46 EDX analyses (see Table S2, Supporting Information) show a silicon content of 0.35 wt % in the case of P1a-800, whereas for P1b-800 neither silicon nor silver was detected. The SEM image of P1b-800 shows sintering of the particles due to heating of P1b to 800 °C (see Figure S4). When using glassware, silicates were formed by glass corrosion despite the short hydrolysis time and the use of a low concentrated NaOH solution (0.05 M). The solubility of SiO2-based glass in alkaline solutions is often neglected although being well-known and, e.g., demonstrated in the studies of Laudise et al. by dissolving α-quartz in aqueous NaOH (0.5 M) or in aqueous Na2CO3 solution (0.5 M) to give water-soluble silicates.48 Thus, to ensure silicate-free conditions, alkaline hydrolysis must be carried out in Teflon lined vessels. Hydrolysis Studies on [Bi38O45(O2CC3H5)24(DMSO)9] (A). The NaOH concentration (pH value) and the time of hydrolysis are crucial parameters for the controlled hydrolysis of the bismuth oxido cluster A to finally give β-Bi2O3.17 Microwave-assisted heating was chosen to ensure fast heating of

Figure 4. PXRD analyses of the products P2-0 to P2-60; the heating time in the glass vessels was 0−60 min (bottom to top; reference: ◇ Bi12SiO20, ICDD 00-037-0485; ● β-Bi2O3, ICDD 00-027-0050; ◆ αBi2O3, ICDD 01-070-8243). E



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Figure 5. ATR-IR spectra of the products P2-0 to P2-60; the heating time in the glass vessels was 0−60 min (bottom to top).

Figure 7. PXRD analyses of P2-60, P4-60, P5-60, and P6-60 obtained after hydrolysis at 100 °C in a glass vessel at different NaOH concentrations (reference: ◇ Bi12SiO20, ICDD 00-037-0485; ● βBi2O3, ICDD 00-027-0050; ◆ α-Bi2O3, ICDD 01-070-8243).

heating times of 10−60 min the reaction of β-Bi2O3 to give Bi12SiO20 is gradually observed. The IR spectra of P2-10 to P260, which show an increase of the intensity of the characteristic Bi−O−Si vibration at approximately 820 cm−1, are in line with the PXRD results.46,47 EDX analyses of P2-10 to P2-60 (Figure S5, Table S3; Supporting Information) also indicate the formation of Bi12SiO20. SEM images (Figure 6) illustrate the change from an amorphous precipitate (P2-0) to needle-shaped β-Bi2O3 (P2-1) after treatment at 100 °C for 1 min. Increasing the annealing time to 60 min provides Bi12SiO20 with typical tetrahedra-shaped crystals (P2-60). In addition, ex situ formed β-Bi2O3 nanoparticles (P1b-385, prepared according to the literature in a Teflon-lined vessel17) were used as starting material using microwave-assisted heating in a glass vessel to elucidate its subsequent reaction with silicates to give Bi12SiO20. The PXRD analysis of the final product of the reaction of the β-Bi2O3 nanoparticles (P1b-385) with silicates leached from the glass vessel exclusively shows reflections from Bi12SiO20 (P1b-385-MW-60, Figure S6). Investigations on the influence of the NaOH concentration (pH value) were also carried out for cluster A. Noteworthy, the decrease of the final NaOH concentration from 0.18 to 0.16 M (pH = 13.5) in the hydrolysis procedure of cluster A in EtOH at 100 °C for 60 min results in the formation of β-Bi2O3 (P460). Bi12SiO20 was not observed, even if the reaction was carried out in a glass vessel (PXRD see Figure 7). However, heating the as-prepared β-Bi2O3 (P4-60) to 800 °C for 60 min on a silver sheet followed by subsequent cooling gave a mixture of γ-Bi2O3 and Bi12SiO20 (P4-60-800, Figure S7). This behavior is in line with the results from the heating procedure of P1a (see Figure 1), and due to the contamination of the sample with amorphous silicates the formation of Bi12SiO20 at high temperatures is observed. A mixture of crystalline α-Bi2O3 and β-Bi2O3 (P5-60) is formed when the NaOH concentration was adjusted to 0.12 M (pH = 13.0). At a NaOH concentration of

0.07 M (pH = 11.5), the hydrolysis of A results in an amorphous precipitate (P6-60) after 60 min at 100 °C. We conclude that at low NaOH concentrations of ≤0.16 M in EtOH/H2O the immediate formation of Bi12SiO20 starting from cluster A does not occur, which is most probably a result of a very low silicate concentration. The formation of the bismuth(III) oxide polymorphs can be controlled by adjusting the NaOH concentration (pH value) properly using the here reported hydrolysis protocol for cluster A. However, the oxides will be contaminated with silicates, most probably adsorbed at the surface, which are difficult to remove by the washing procedures and post treatment of the particles at high annealing temperatures will give Bi12SiO20. Hydrolysis Studies on Bi(NO3)3·5H2O (B). Our findings on the reactivity of bismuth(III) oxides and precursors under basic conditions, especially while using glass ware, cast a critical light on reports dealing with the synthesis of metastable γ-Bi2O3 by precipitation reactions (see Table 1). The ambiguous reports prompted us to elucidate the reactions in more detail. The formation of Bi12SiO20 rather than the formation of isomorphous γ-Bi2O3 seems to be most likely using the so far reported reaction conditions, when assuming the common use of glass flasks (see Table 1). In our approach, Bi(NO3)3·5H2O (B, 250 mg, 0.52 mmol) was dissolved in DMSO (2.5 mL, resulting pH = 5.5) using a 35 mL glass vessel. The dissolved Bi(NO3)3·5H2O was precipitated using an aqueous NaOH solution (2 M) to finally give a NaOH concentration of 0.72 M (pH = 14.3, see Table 3). After microwave-assisted heating to 100 °C for 60 min the formation of Bi12SiO20 (P7a-60) is observed exclusively (PXRD, Figure 8). Increasing the final NaOH concentration to 2.2 M (pH = 15.3, see Table 3) by using a 6 M NaOH solution similarly resulted in the formation of Bi12SiO20 (P7b-60), but with a higher degree of crystallinity. The formation of α-Bi2O3 (P8a-60) was observed when the

Figure 6. SEM images of P2-0, P2-1, and P2-60 obtained with a heating time of 0, 1, and 60 min (left to right) in glass vessels. F



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sillenite-type compound Bi12SiO20 increases by extending the heating time to 20 min (P7a-20). IR spectroscopic measurements (Figure S9) are in line with the PXRD analyses both confirming the reaction of intermediately formed α-Bi2O3 with silicate-species to finally give Bi12SiO20. EDX analyses show the presence of silicon for all finally obtained materials P7a-1 to P7a-60 (Table S4). Unexpectedly, an increase of the final NaOH concentration (2.2 M in DMSO/H2O, glass vessel) did not result in the immediate formation of Bi12SiO20, but resulted in the formation of crystalline α-Bi2O3 (P7b-0) immediately after precipitation at room temperature (PXRD analyses and IR spectra see Figures S10 and S11). Furthermore, the crystallization of αBi2O3 (P7b-1) is obtained after 1 min at 100 °C without crystallization of any Bi12SiO20. We assume that the high concentration of hydroxide results in a layer of negatively charged OH− at the surface of the α-Bi2O3 particles that impedes the reaction with the negatively charged silicates to give Bi12SiO20. This behavior is in accordance with the studies of Iyyapushpam et al. that demonstrated the formation of αBi2O3 instead of “γ-Bi2O3” upon increasing the NaOH concentration in the hydrolysis of Bi(NO3)3·5H2O (B) in aqueous HNO3.36 However, it should be kept in mind that Bi12SiO20 rather than γ-Bi2O3 would be expected to be formed. In our experiments, increasing the reaction time to 10 min partially provides the sillenite-type compound and after 30 min α-Bi2O3 is completely converted to Bi12SiO20 (P7b-30 to P7b60). EDX analyses show signals of silicon for P7b-10 to P7b-60 in accordance with the formation of Bi12SiO20 (Table S5). SEM images illustrate the evolution of crystalline α-Bi2O3 (needles in P7a-1 and polyhedral-shaped crystals in P7b-1) into typical Bi12SiO20-tetrahedra (P7a-60 and P7b-60, Figure 10). The different morphologies of α-Bi2O3 result from the use of different NaOH concentrations. The change from needles to polyhedra of α-Bi2O3 by increasing the NaOH concentration is consistent with the studies of Wu et al. They demonstrated the synthesis of needle-shaped α-Bi2O3 by precipitation of Bi(NO3)3·5H2O dissolved in nitric acid using aqueous NaOH solution. In addition, the formation of plate-like morphologies

Figure 8. PXRD analyses of the products P7a-60, P7b-60, P8a-60-Si, and P8a-60 (reference: ◇ Bi12SiO20, ICDD 00-037-0485; ◆ α-Bi2O3, ICDD 01-070-8243).

hydrolysis (2 M NaOH) was carried out in a Teflon-lined vessel, while the addition of aqueous Na2SiO3·9H2O results in the formation of Bi12SiO20 (P8a-60-Si). However, reduction of the heating time (1 min, 100 °C, glass vessel, NaOH concentration: 0.72 M) gave α-Bi2O3 (P7a-1), accompanied by a tiny amount of Bi12SiO20 only (PXRD see Figure 9, complete screening in S8). The amount of the

Figure 9. PXRD analyses of the products P7a-0 to P7a-60; the heating time in the glass vessels was 0−60 min (bottom to top; reference: ◇ Bi12SiO20, ICDD 00-037-0485; ◆ α-Bi2O3, ICDD 01-070-8243).

Figure 10. SEM images of α-Bi2O3 and Bi12SiO20 obtained from 0.72 M NaOH solution (top left to right, P7a-0 to P7a-60) and from 2.2 M NaOHsolution (bottom left to right, P7b-0 to P7b-60) obtained at an annealing time of 0, 1, and 60 min (left to right) in glass vessels. G



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or polyhedra by increasing the NaOH concentration was shown. The discussed shielding effect by adsorbed OH−-ions at the (001) face of the α-Bi2O3 crystals suppresses the crystal growth to needles, and thus the growth of polyhedra is observed.30 We performed further studies using commercially available αBi2O3 (P8b, Alfa Aesar, 99%, 0.25 mmol, 120 mg) as the starting material in DMSO (2.5 mL), which was heated in a microwave-reactor to 100 °C for 60 min (glass vessel, NaOH concentration: 2.2 M). PXRD analysis of P8b-60 shows exclusively reflections of Bi12SiO20 (Figure S12). We conclude from our experiments that α-Bi2O3 similar to β-Bi2O3 reacts readily with silicates to finally provide Bi12SiO20 at NaOH concentrations ≥0.72 M in DMSO/H2O. By contrast, several literature reports based on the hydrolysis of Bi(NO3)3·5H2O in aqueous HNO3 postulate the formation of “γ-Bi2O3”.33−38 Thus, we additionally carried out the hydrolysis of Bi(NO3)3· 5H2O (B) in aqueous HNO3 (1 M) using microwave-assisted heating to elucidate the influence of the solvent. The dissolved Bi(NO3)3·5H2O (B, 0.1 M in 1 M HNO3, 2.5 mL, pH = 0.6) was hydrolyzed using an aqueous NaOH solution (12 M, 2.5 mL, pH = 13.5, see Table 3). After microwave-assisted heating to 100 °C in a 35 mL glass vessel, the resulting powder was analyzed by PXRD that exclusively shows the formation of Bi12SiO20 (P9a-60, Figure 11). Polyethylene glycols (PEG400

was the only crystalline product, even in the presence of PEG400 or PEG8000 (P9b-60, P9b-PEG400-60, and P9bPEG8000-60; Figure 11). The pH value (see Table 3) was just marginally affected by addition of the polyethylene glycols. The formation of Bi12SiO20 (P9a-60, P9a-PEG400-60, and P9a-PEG8000-60) was confirmed by IR spectroscopy showing the characteristic Bi−O−Si vibration (823−826 cm−1, Figure S13).46,47 EDX analyses showed signals of silicon for P9a-60 to P9a-PEG8000-60 that prove the formation of Bi12SiO20 (Table S6). Thus, we conclude that the use of polyethylene glycols does not exhibit any influence to enable the formation of metastable γ-Bi2O3 in aqueous HNO3. The formation of the isomorphous sillenite-type compound Bi12SiO20 is more likely due to the glass corrosion. SEM images illustrate that the addition of polyethylene glycols such as PEG400 and PEG8000 does slightly influence the morphology of the resulting sillenitetype material Bi12SiO20 (P9a-60, P9a-PEG400-60, and P9aPEG8000-60; Figure 12). The tetrahedral-shaped particles of P9a-PEG8000-60 are smaller as compared to P9a-60 and P9aPEG400-60. However, the influence of the polyethylene glycols on the resulting morphology can be neglected at high NaOH concentrations. The results show that glass corrosion also occurs in aqueous nitric acid and induces the formation of Bi12SiO20 instead of γBi2O3. The required NaOH concentration for the formation of Bi12SiO20 seemed to be slightly higher as compared to reported reaction protocols in the literature, and thus the hydrolysis of Bi(NO3)3·5H2O (B) was repeated under the reported conditions (details listed in Table 3).34 Again, an unexpected result was observed. The hydrolysis of B in an aqueous HNO3 solution (2 M) using a 2 M aqueous NaOH solution in a 50:50 volume ratio gave pH = 1.0, and the hexanuclear bismuth oxido cluster [Bi6O5(OH)3(NO3)5]·2H2O (P10a-120) was formed as a product of partial hydrolysis, instead of γ-Bi2O3 or Bi12SiO20 (PXRD, Figure 13).34 An increase of the NaOH concentration results in the formation of α-Bi2O3 (in glass vessels; 4 M, pH = 12.9: P11a-120, 6 M, pH = 13.2: P12a-120 and 12 M, pH = 13.7: P13a-120) even if PEG400 was added. The formation of metastable γ-Bi2O3, as reported in the literature, was not observed.34 Additionally, we conclude that the influence of the added PEG400 does not induce the formation of metastable γBi2O3. These results support the assumption that Bi12SiO20 is readily formed under so far reported reaction conditions, which do not take care for the inhibition of glass corrosion. Hydrolysis Studies for BiCl3. The influence of polyethylene glycols on the hydrolysis behavior of bismuth(III) containing compounds for the selective synthesis of “γ-Bi2O3” was reported recently.33,34 Since our studies on the bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] (A) and Bi-

Figure 11. PXRD analyses of P9a-60 to P9b-PEG8000-60 obtained from the hydrolysis of Bi(NO3)3·5H2O (B) in 1 M HNO3 using glass vessels (reference: ◇ Bi12SiO20, ICDD 00-037-0485; ◆ α-Bi2O3, ICDD 01-070-8243).

and PEG8000), which are reported to enable the formation of metastable γ-Bi2O3 in aqueous HNO3,33,34 were added to elucidate their influence on the resulting crystal structure. The hydrolysis of Bi(NO3)3·5H2O in glass vessels in the presence of PEG400 or PEG8000 similarly results in the formation of Bi12SiO20 (P9a-PEG400-60 and P9a-PEG8000-60). If the hydrolysis of B was carried out in Teflon-lined vessels, α-Bi2O3

Figure 12. SEM images of Bi12SiO20 (P9a-60, P9a-PEG400-60, and P9a-PEG8000-60) obtained from the hydrolysis of Bi(NO3)3·5H2O in 1 M HNO3 in glass vessels. H



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In the literature, the formation of “γ-Bi2O3” as obtained from precipitation reactions was mainly confirmed by PXRD analysis using several references to identify the obtained reflections in the PXRD patterns (Bi12.8O19.2, ICDD 01-081-0563;33,34 γBi2O3, ICDD 00-045-1344;35 γ-Bi2O3, ICDD 01-071-0467;36 γBi2O3, ICDD 00-006-0312;37 γ-Bi2O3, ICDD 01-074-137538). We compared the database files of γ-Bi2O3 and several files of the sillenite-type compound Bi12SiO20 with those of the asprepared Bi12SiO20 (P15a-120, PXRD, see Figure 15).

Figure 13. PXRD analysis of the products P10a-120 to P13a-120 obtained from the hydrolysis of Bi(NO3)3·5H2O (B) in 2 M HNO3 (reference: △ Bi6O5(OH)3(NO3)5·2H2O, ICDD 00-054-0627; ◆ αBi2O3, ICDD 01-070-8243).

(NO3)3·5H2O (B) revealed some interesting details on the formation of bismuth(III) oxide polymorphs and Bi12SiO20, we decided to additionally investigate the reaction of BiCl3 (C) in accordance with literature procedures38 as well as under modified reaction conditions in the presence/absence of polyethylene glycol (PEG4000 and PEG8000). Furthermore, the influence of heating of the reaction mixture in (i) an oil bath and (ii) using microwave-assisted heating was studied. In a general procedure using an oil bath, the BiCl3 solution (C, 10 mL of 1 M HCl in EtOH/H2O mixture, 1:1, pH = 1.0) was stirred in a glass flask, and PEG4000 (0.4 g) was added to give a clear solution in accordance with the literature (details listed in Table 3).38 However, after the addition of the aqueous NaOH solution (10 mL, 4 M, pH = 13.2) and stirring of the mixture for 120 min at 80 °C, Bi12SiO20 was observed instead of γ-Bi2O3 (P14-PEG4000-120, PXRD Figure 14). The same procedure

Figure 15. Comparison of the (310) reflection (hkl) of Bi12SiO20 (P15a-120) with references of Bi12SiO20 from the literature (A Bi12SiO20, ICDD 00-037-0485; B Bi12SiO20, ICDD 01-072-7675; C Bi24Si2O40, ICDD 01-080-0627; D Bi12Si0.87O20, ICDD 01-084-0090) and γ-Bi2O3 (E γ-Bi2O3, ICDD 00-006-0312; F γ-Bi2O3, ICDD 00045-1344; G γ-Bi2O3, ICDD 01-071-0467; H γ-Bi2O3, ICDD 01-0741375; I Bi12.8O19.2, ICDD 01-081-0563).

The reference files of the metastable γ-Bi2O3 (E−I) clearly do not fully match the PXRD pattern of the as-prepared Bi12SiO20 (P15a-120), but a good correlation with the reference files of Bi12SiO20 (A−D) is observed (PXRD, see Figure S14). We assume that the ICDD 01-074-1375 reference H is wrongly assigned, most probably being a sillenite-type compound. However, largely broadened reflections may result in wrong assignments. Although deviations between the reference files of Bi12SiO20 (A−D) also exist, the differences in the positions of the (310) reflection (hkl) are not as significant as in the references of γ-Bi2O3. In conclusion, the identification of bismuth(III) oxide polymorphs by PXRD using existing reference files has to be carried out very carefully to differentiate pure bismuth(III) oxide from ternary bismuth(III) oxides of the sillenite-type. The characterization by PXRD should be supported by independent methods such as infrared spectroscopy. The IR spectra explicitly confirm the formation of Bi12SiO20 (P14-PEG4000-120 to P15a-120) due to the characteristic Bi−O−Si vibration (821 cm−1, Figure S15).46,47 EDX analyses of these samples confirm the presence of silicon, and thus we conclude that Bi12SiO20 is obtained starting from a BiCl3 solution with Bi12O17Cl2 being an intermediate phase (Table S7). In the presence of PEG4000 and PEG8000, the reaction of Bi12O17Cl2 to give Bi12SiO20 seems to be accelerated but γ-Bi2O3 is not formed. Thermal treatment of the reaction mixture using both microwave-assisted heating or an oil bath finally results in the formation of Bi12SiO20. A microwaveinduced effect can be neglected. The influence of PEG4000 and PEG8000 on the hydrolysis of C was additionally studied by using Teflon-liners in the reaction under microwave-assisted heating (2.5 mL BiCl3 solution, 2.5 mL 4 M NaOH, 120 min, 80 °C, addition of 0.1 g of PEG4000 and PEG8000 to give P15b-PEG4000-120 and P15b-PEG8000-120, respectively; see Table 3). The

Figure 14. PXRD analyses of P14-PEG4000-120 to P15a-120 obtained by the hydrolysis of BiCl3 in glass vessels (reference: ◇ Bi12SiO20, ICDD 00-037-0485; solid rectangle over downward triangle Bi12O17Cl2, ICDD 00-037-0702).

was carried out using PEG8000 (0.4 g, pH = 13.2), and Bi12SiO20 was formed as the only crystalline product (P14PEG8000-120). A mixture of Bi12SiO20 and Bi12O17Cl2 (P14120) was obtained after 120 min at 80 °C if PEG derivates were not added to the BiCl3 solution (pH = 13.3). The reaction mixture which partially provided Bi12O17Cl2 (P14-120) was stirred for additional 60 min and resulted in the formation of pure Bi12SiO20 (P14-180). After microwave-assisted heating of a mixture of the BiCl3 solution (C, 2.5 mL, 0.1 M) and aqueous NaOH-solution (2.5 mL, 4 M, pH = 13.3) in a 35 mL glassvessel for 120 min at 80 °C, the sillenite-type Bi12SiO20 (P15a120) was obtained exclusively. γ-Bi2O3 was not observed in any case. I



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latter can easily be provided by (unintended) glass corrosion. Hydrolysis of A in a Teflon-lined vessel results in an amorphous precipitate first, which is transformed into γ-Bi2O3 by subsequent heating to 800 °C on a silver sheet. The use of a glass vessel during hydrolysis of cluster A results in silicate impurities that cannot be removed by several washing steps. Thus, finally the formation of a mixture of Bi12SiO20 and γBi2O3 is observed after heating the contaminated precipitate to 800 °C. The hydrolysis of different Bi(III)-containing materials in glass vessels results in the formation of Bi12SiO20 instead of metastable γ-Bi2O3 even at moderate conditions (T ≤ 100 °C). Thus, hydrolysis of cluster A provides needle-shaped β-Bi2O3, whereas α-Bi2O3 with variable morphologies was formed by hydrolysis of B in DMSO or aqueous HNO3. Bi12O17Cl2 is formed upon the reaction of C with NaOH in aqueous HCl. Depending on the starting material and solvent, the formation of Bi12SiO20 usually appears at pH > 13, and the addition of additives affects the pH value just marginally. In the hydrolysis of cluster A at 100 °C, the formation of α-, β-Bi2O3 or Bi12SiO20 depends strongly on the pH value and heating time. Hereby, it has to be kept in mind that the washing procedure does not remove the amorphous silicates completely from the bismuth(III) oxide polymorphs, which were always present while using glassware. The use of PEG400 during the hydrolysis of B in aqueous HNO3 does not show any influence on the formation of γ-Bi2O3. Bi12SiO20 is formed in glass vessels in the presence and absence of PEG400. The hydrolysis of C in a glass vessel at a final NaOH concentration of 2 M also provides Bi12SiO20 instead of isomorphous γ-Bi2O3, in the presence as well as in the absence of polyethylene glycols. The hydrolysis of C in the presence of PEG4000 or PEG8000 solely results in the formation of the bismuth oxychloride Bi12O17Cl2 if Teflon-lined vessels were used. Thus, the polyethylene glycols do not exhibit any influence on the resulting crystal structure, which is in contrast to previous literature reports. The presence of the polyethylene glycols seems to accelerate the formation of Bi12SiO20 and to affect the morphology of the resulting sillenitetype compounds. In the presence of polyethylene glycols during the hydrolysis of C in aqueous HCl, tetrahedra-shaped crystals are observed, whereas microcubes of Bi12SiO20 are formed in the absence of polyethylene glycols. From our findings, we concludein contrast to the literature reports that it is not possible to synthesize metastable γ-Bi2O3 by precipitation under alkaline conditions in glass vessels due to the strong tendency to incorporate silicon or other impurities to give sillenite-type compounds Bi12MO20±x. The precipitation should be carried out in Teflon-lined vessels to prevent contamination with silicates. PXRD studies on the influence of additives on polymorphism and morphology should be

PXRD analyses show the formation of Bi12O17Cl2 in both cases. We finally conclude that PEG4000 and PEG8000 do not exhibit any effect as “capping agents” with regard to the controlled synthesis of “γ-Bi2O3” (Figure 16) as reported elsewhere. 33−38 In the presence of a silicate source isomorphous Bi12SiO20 is the final product.

Figure 16. PXRD-analyses of P15b-120 to P15b-Si-120 (reference: ◇ Bi12SiO20, ICDD 00-037-0485; solid rectangle over downward triangle Bi12O17Cl2, ICDD 00-037-0702).

In addition, the influence of glass corrosion was confirmed by the formation of Bi12SiO20 (P15b-Si-120) in a Teflon-lined vessel, when an aqueous Na2SiO3·9H2O solution was added (Figure 16). Bi12O17Cl2 (P15b-120) is obtained when the hydrolysis was carried out without the addition of a silicate source. However, SEM images illustrate that the presence of PEG4000 and PEG8000 during hydrolysis of C in glass vessels gave Bi12SiO20 with different morphologies (Figure 17). Crystals of P14-PEG4000-120 exhibit a tetrahedral-shaped morphology, whereas P14-PEG8000-120 consists of tetrahedra and microcubes. Cubic crystals of Bi12SiO20 (P15a-120) were obtained exclusively without additives. Thus, the use of polyethylene glycols such as PEG4000 and PEG8000 influences the morphology of the resulting sillenite-type compounds, but its influence on polymorph formation can be excluded.

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CONCLUSION In this work, we focused on the hydrolysis behavior of [Bi38O45(O2CC3H5)24(DMSO)9] (A), Bi(NO3)3·5H2O (B), and BiCl3 (C) in dependence of several synthesis conditions, like vessel materials (glass and Teflon), additives (polyethylene glycols), solvents, and NaOH concentrations. The results demonstrate that depending on the precursor α- and β-Bi2O3 and bismuth oxychloride are formed first. They easily react with silicates to give the sillenite-type compound Bi12SiO20. The

Figure 17. SEM images of Bi12SiO20 (P14-PEG4000-120 to P15a-120) obtained in the presence and absence of PEG4000 and PEG8000 in the hydrolysis of BiCl3 (C) with aqueous NaOH. J



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accompanied by IR spectroscopy, which easily allows the identification of sillenite-type structures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00628. EDX spectra, TGA/DSC analyses, PXRD analyses, ATRIR spectra, and composition of the used glass vessels and silver sheets (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work has been supported by the Deutsche Forschungsgemeinschaft (SPP1415: Crystalline Nonequilibrium phases preparation, characterization and in situ studies of formation mechanisms). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Prof. S. Spange for access to the ATR-IR instrument and Prof. M. Hietschold for access to the scanning electron microscope. We acknowledge M. Sc. Benjamin Büchter for performing SEM and EDX-analyses. We thank Julian Noll for performing the TGA/DSC analyses.

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REFERENCES

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