EXPERIMENTAL

The hydrothermal reaction was performed in a Teflon­lined, stainless steel autoclave with an internal volume of 70 mL. NaBiO3.nH2O and Ba(OH)2.8H2O with 1:1.8 molar ratio and distilled water (50 mL) were placed in this Teflon­lined autoclave, following which the autoclave was maintained at 120 °C for 1h, 2h, 3h ,4h, 6h, 12 h, 24h and 36h. All starting reagents were purchased from Kanto Chemical Co. Ltd. The solid product was separated by filtration, washed with distilled water, and dried at 80 °C. The crystal structure of the product was examined by XRD on a Rigaku X­ray diffractometer (RINT­2000, RIGAKU) with graphite monochromatized CuKα radiation (λ = 1.54056 Å). SXRPD measurements were performed at the BL02B2 powder diffraction beamline at SPring­8, Hyogo, Japan. The powder samples were sealed in a glass capillary with an inner radius of 0.2 mm. The data was collected with a constant wavelength (λ = 0.413829 Å) at room temperature. Crystal structure was refined using the Rietveld program RIETAN­FP¹⁶ and was visualized using VESTA software.¹⁷ The thermal stability was investigated using thermogravimetric analysis (TGA) (Rigaku Thermo Plus) with a heating rate of 10 °C min^­1 from room temperature to 800 °C. The chemical composition was determined by inductively coupled plasma (ICP) analysis (SPS 3520 DD, Hitachi). Diffuse­reflectance spectra (DRS) were collected using a spectrometer (JASCO V­

550 spectrometer) and were converted using the Kubelka­Munk function. Electronic structure calculations based on DFT were performed using the VASP code.^18,19 The photocatalytic activities were examined for the decomposition of phenol (20 ppm) under visible light from a 300­W Xe lamp (UXR­300DU, Ushio Inc.) with 420­nm sharp cut filter.

64 | P a g e 4.3 RESULT and DISCUSSION

Light brown high crystalline BaBi2O6 powder was obtained from a hydrothermal reactions between NaBiO3·nH2O and Ba(OH)2·8H2O in the molar ratio (1:1.8) at 120 °C. The X­ray powder diffraction pattern of the resultant BaBi2O6 as shown in Figure 4.1 (b) was indexed with a hexagonal cell of space group P­31m, which is the same as that of the PbSb2O6­ type structure.²⁰ Previously, Kumada et al. synthesized low crystalline BaBi2O6 phase as shown in Figure 4.1 (a).

Time dependent XRD patterns showed (Figure 4.2) that single phase obtained at above 2 hours. However, the starting material NaBiO3.nH2O did not dissolved until one hour indicated by the same XRD pattern as shown in Figure 4.2 but in between one and two hours the starting material dissolved and reacted with barium hydroxide and the product was recrystallize into BaBi2O6. Previously,

Kumada et al. synthesized low crystalline BaBi2O6 phase. That was because of the ion exchange reactions and probably partially decompose of starting material NaBiO3.nH2O. However, in our case

Figure 4.1 XRD patterns low crystalline (a) and high crystalline BaBi2O6 sample.

65 | P a g e Figure 4.3 Resultant Rietveld refinement pattern from the synchrotron powder diffraction data for BaBi2O6. The markers and solid lines are for the experimental and calculated profiles, respectively. In the middle portion, the short vertical lines denote the positions of possible Bragg reflections.

Figure 4.2 XRD patterns of starting materials with various reaction times for BaBi2O6 at 120 ºC.

CuK

In te n si ty / a .u .

NaBiO₃.nH₂O Ba(OH)₃.8H₂O

1 h 36 h 24 h 12 h 6 h 4 h 3 h 2 h

10 20 30 40 50 60

0

starting material was fully dissolved and recrystallize in to BaBi2O6 phase, which gave the high crystalline phase.

Figure 4.3 shows the observed and calculated patterns obtained from synchrotron powder diffraction. The lattice parameters derived from crystal structure refinement on the synchrotron X­ray powder diffraction (SXRPD) data were a = 5.57534(6) and c = 5.7381(1) Å. The final weighted and unweighted reliability (R) factors in the Rietveld analysis of this structural model led to reasonable values of Rwp = 5.15% and Rp = 3.62% respectively. Crystal data and structural parameters are summarized in Tables 4-(1,2), respectively.

66 | P a g e Figure 4.4 shows the crystal structure of BaBi2O6. This crystal structure consists of two different alternating octahedral layers (BaO6 and BiO6). The BiO6 octahedra are connected to each other by edge sharing, but the BaO6 units are not connected to each other. The BiO6 octahedra show higher distortion than BaO6 due to the smaller size of Bi⁵⁺. Along the a­ or b­

axis it exhibits the layer structure shown in Figure 4.4 (a), but along the c­axis it shows the hexagonal tunnel structure shown in Figure 4.4 (b). This layered configuration and tunnel structure are responsible for enhancing the material’s catalytic properties.²⁰

Atom site x y z Occupancy Biso (Ų)

Ba 1a 0 0 0 1 0.80(2)

Bi 2d 1/3 2/3 1/2 1 0.402(1)

O 6k ­0.3809(7) 0 0.2938(6) 1 0.95(1)

Chemical formula BaBi2O6

Radiation type, λ (Å) Synchrotron ( BL02B2), 0.413829

Temperature (^°C) 25

Crystal System Hexagonal

Space group P­31m (No.162)

Lattice parameters (Å) a = 5.57534(6)

c = 5.7381(1)

Volume (ų) 154.47(4)

Formula weight (g/mol) 651.28

Calculated density(g/cm³) 7.00

Z Value 1

Rwp (%) 5.15

Rp (%) 3.62

RB (%) 1.02

RF (%) 0.38

S 2.42

Table 4-1 Rietveld Refinement crystal data for BaBi2O6 using Synchrotron Radiation.

Table 4-2 Atomic Coordinates, Occupancies and Isotropic displacement parameters as determined by Rietveld Refinement of Synchrotron X­ray Diffraction Data for BaBi2O6.

67 | P a g e In BiO6, the Bi­O bonds are similar in length and the distance is 2.105(3) Å. This value is in agreement with those in other pentavalent bismuthates; i.e. 2.11 Å in LiBiO3,³ 2.10 Å in MgBi2O6,⁴ 2.12 Å in AgBiO3,⁷ and 2.101 Å in SrBi2O6.⁸ The Ba­O distance (2.711(3) Å) is somewhat smaller than that in BaSb2O6 with the PbSb2O6­type structure (2.745 Å).²² The BiO6 bond angles vary significantly, with values from 80.27° to 98.10°. For BaBi2O6, the Bi­O­Bi and Bi­O­Ba angles are as high as 99.73° and 126.62°, respectively.

BaBi2O6 is thermally unstable, as indicated by the thermogravimetric (TG) curve shown in Figure 4.5. Around 350 °C, it begins to decompose and a very sharp weight loss (2.86 %) occurs until 380 °C. This mass loss is due to oxygen evolution and is accompanied by the reduction of Bi⁵⁺ to Bi³⁺. Following this, the material starts to gain weight (1.61 %) until 545 °C due to oxidation. After this, a second mass loss step (2.28 %) occurred, probably again due to oxygen evolution. The overall mass loss was 3.53%. In both mass loss steps, the reactions were endothermic as observed by the differential thermal analysis (DTA) curve at their corresponding temperatures.

Figure 4.4 Crystal structure of BaBi2O6. Alternative layers (a) and hexagonal tunnel (b).

68 | P a g e The optical absorption spectra for BaBi2O6 and three other compounds (NaBiO3, PbSb2O6, and BaSb2O6) are shown in Figure 4.6. PbSb2O6 and BaSb2O6 were prepared by the methods discussed in previous papers.^20,22 These absorption spectra show that the NaBiO3 and BaBi2O6 absorption edges lie within the visible region but those of the PbSb2O6 and BaSb2O6

lie only in the UV region. The band gap energies were estimated from the dependence of (hαν)² on energy and hν (Tauc plot) with the assumption of direct transitions.¹⁴ Tauc plot estimation of the band­gap energy for polycrystalline samples has been reported to give accurate values for monazite­type oxides.²³ The band­gap energies for BaBi2O6, NaBiO3, PbSb2O6, and BaSb2O6 were 2.33, 2.50, 3.40, and 3.98 eV, respectively, as shown in Figure 4.7.

Figure 4.5 TG­DTA curve of hydrothermally prepared BaBi2O6 sample at 120 °C.

69 | P a g e Figure 4.6 UV−Vis absorption spectra for NaBiO3, BaBi2O6, PbSb2O6 and BaSb2O6.

Figure 4.7 Tauc plot for the estimation of the band gap for BaBi2O6, NaBiO3, PbSb2O6 and BaSb2O6.

70 | P a g e The photocatalytic activities of these compounds were characterized by the decomposition of phenol at an initial concentration of 20 ppm using a 0.15 g sample in 50 mL ultrapure water. Figures 4.8 and 4.9 show the time profiles of C/C0 under visible­light irradiation (λ > 420 nm) and UV­Vis irradiation, respectively. When the suspensions were magnetically stirred in the dark (30 min) to ensure extent of phenol adsorption on the sample surface, the phenol concentration decreased only a little; however, after 90 min, the phenol was almost completely degraded by both NaBiO3 and BaBi2O6. It is considered that the strong dispersion in the hybridized Bi 6s and O 2p orbitals that were identified at the bottom of the conduction band (CB), shown in Figure 4.10 (a), contributes to the high photocatalytic activity of NaBiO3. In the case of BaBi2O6, the CB bottom is also primarily composed of hybridized Bi 6s and O 2p orbitals, as shown in Figure 4.10 (b). This large dispersion in CB narrowing the band gap like the parent compound is responsible for the high photocatalytic activity in the visible region.

Figure 4.8 Time dependence of photocatalytic degradation of phenol under visible radiation (λ > 420 nm) for BaBi2O6.

C /C

Elapsed time (min.)

BaSb₂O₆ PbSb₂O₆ BaBi₂O₆ NaBiO₃

Dark Visible light

0 30 60 90 120 150 180

0 0.2 0.4 0.6 0.8 1

71 | P a g e On the other hand, no hybridized orbitals were found at the CB for PbSb2O6 and BaSb2O6, as shown in Figure 4.10 (c) and Figure 4.10 (d), respectively. They exhibit larger band gaps of 3.40 eV and 3.98 eV, respectively, which are responsible for the absence of photocatalytic activity under visible light (λ > 420 nm), as shown in Figure 4.8. Additionally, phenol decomposition rates using these catalysts seem to be relatively slow under UV­Vis radiation (Figure 4.9), despite their tunnel structure, which is also likely due to their large band gap.

Figure 4.9 Time dependence of photocatalytic degradation of phenol under UV­Vis irradiation for NaBiO3, BaBi2O6, PbSb2O6 and BaSb2O6.

C /C

Elapsed time (min.)

Dark UV irradiation

BaBi₂O₆ BaSb₂O₆ NaBiO₆ PbSb₂O₆

0 30 60 90 120 150 180

0 0.2 0.4 0.6 0.8 1

72 | P a g e 4.4 CONCLUSION

Highly crystalline sample BaBi2O6 obtained by using NaBiO3.nH2O and Ba(OH)2.8H2O under low temperature hydrothermal conditions. The crystal structure of BaBi2O6 was refined for the first time using synchrotron X­ray powder diffraction. The structure consists of a layered configuration with hexagonal tunnels that are probably favorable for photocatalytic activity.

DFT calculations showed that it has Bi 6s and O 2p hybridized orbitals at the bottom of the conduction band, which makes it a potential candidate for the degradation of organic compounds under visible light, similar to its parent compound.

Figure 4.10 DOS curves simulated by first­principle DFT calculation for NaBiO3 (a), BaBi2O6 (b), PbSb2O6 (c)and BaSb2O6 (d).

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76 | P a g e

Chapter 5