2.1 Optical properties
The UV-Vis diffused reflectance spectroscopy measurement of the as-prepared Bi2O3 nanoparticles was performed to record the absorbance spectra using a JASCO V-670 spectrophotometer. Figure 2a shows the absorbance spectra from DRS measurement, which exhibits maximum absorbance centred at 402 nm associated with a shoulder around 700 nm.
The absorption edge of the synthesized nanoparticles shows that the composites can be excited in the visible region. The catalyst spectra showed a strong absorbance with a direct bandgap of ~2.25 eV (Figure 2b). The bandgap of Bi2O3 can vary between 2.0 and 3.96 eV. According to Hai-Ying et al., α-Bi2O3 is an efficient photocatalyst due to its smaller bandgap, higher valence band hole oxidation power, and non-toxic properties [20]–[22].
The photoluminescence emission spectra of pure α-Bi2O3 were measured to confirm the separation of photogenerated electrons and holes of the catalysts. It is well known that PL emission intensity is related to the recombination rate of excited electron-hole pairs. Lower intensity indicates more excited electrons are transferred or trapped, and higher intensity means a faster recombination rate [23], [24]. From Figure 2c, at 250nm excitation, α-Bi2O3 nanoparticles show a broad emission peak from 340 to 480nm with a maximum of 396nm.
To determine the excited state decay time of the band gap emission of the nanoparticle is studied, and the spectrum is represented in Figure 2d. The excited state decay time of the band gap emission is found to be 1.36 ns.
Fourier-transform infrared spectroscopy (FT-IR) spectroscopy was performed in the range of 600 to 4000 cm-1 to study the detailed chemical bonding as well as the quality of the as-prepared Bi2O3 nanoparticles. Several well-defined absorption bands in the FTIR spectra as shown in Figure 3. The absorption band at ~ 3458 cm-1 is observed due to the O-H stretching vibration of the H2O molecule. The absorption band at 1383 cm-1 can be possibly assigned to the C=O vibration from the organic solvent. The lowest absorption band illustrated in FTIR at ~803 cm-1 can be attributed to the Bi–O stretching vibration bands of BiO6 octahedron. The unwanted absorption bands in the FTIR spectra of Bi2O3 indicate the purity of the synthesized sample.
2.2 Crystal structure
Figure 4a illustrates the XRD spectrum for Bi2O3, which shows only a single, essentially monoclinic phase (α) of Bi2O3, as well as strong and sharp diffraction peaks, implying that the Bi2O3 crystals are highly crystallized. Furthermore, no other secondary crystalline phases were detected by XRD, indicating the high purity of the as-synthesized product. Peak positions and relative intensities match those of bulk monoclinic Bi2O3 crystals well. Oudghiri-Hassani et al. similar research generated the two values with the matching hkl index. So, the peaks observed in the XRD patterns of α-Bi2O3 at 2θ values of 21.54°, 23.70°, 25.50°, 26.71°, 27.23°, 27.82°, 30.07°, 32.60°, 32.98°, 33.76°, 34.88°, 35.22°, 35.75°, 36.82°, 37.46°, 39.91°, 41.29°, 41.76°, 42.19°, 45.03°, 46.20°,46.80°, 48.43°, 49.28°, 49.85° correspond to 020, 102, 002, 111, 102, 012, 107, 211, 122, 022, 212, 031, 102, 130, 112, 222, 131, 213, 122, 023, 041, 140, 104, 133, 113 [25]. The Debye-Scherrer equation was also used to compute the average crystallite size. The Scherrer formula was used to measure the average crystallite size (d) of the nanoparticle, as indicated in equation 1.
Where λ, θB and β are the X-ray wavelength (1.54056), Bragg diffraction angle, and line width at half maximum of the most dominating peak, respectively. The aforementioned equation yielded an α-Bi2O3 crystallite size of 33.43 nm.
Raman spectrum of heavy metal oxide such as Bi2O3 generally consists of four different features as follows (1) low wavenumber Raman modes (30–70 cm-1), (2) heavy metal ion vibrations (70–160 cm-1), (3) bridged anion modes in the intermediate (300–600 cm-1), and (4) non-bridging anion modes (>600 cm-1) [26], [27]. For -Bi2O3 samples (2) and (3) categories are present. In this work, Raman spectra have been taken from 100 cm-1 to 1000 is reflected this is -Bi2O3. This spectrum shows five broad bands at 526, 441.07, 404.70, 308.25 and 274.88 cm-1 for Bi-O stretching and the other four bands at 203.65, 176.22, 143.39 and 111.71 cm-1 for lattice vibrations. These Raman peaks are identical with the Raman data of -Bi2O3 reported by Denisov et al [28]. So, Raman spectral studies confirmed that the as-prepared sample is single phase -Bi2O3(Figure 4b).
The quantitative analysis indicates that the material has a reasonably high concentration of Bismuth and a low concentration of impurities such as copper, iron, and calcium. Bi, Ca, Fe, and Cu have 97.56, 1.81, 0.42, and 0.20%, respectively (Table 1). This result reveals how pure the material is. A similar observation was made for the high-intensity Bi peaks across the full spectrum (Figure 4c). The x-ray tube anode is responsible for the RhLa, RhLb, RhKa, RhKaC, RhKbC, and RhKb peaks. Oxygen does not produce XRF peaks because its fluorescence photons are too low in energy to be carried through the air and are inefficiently detected using some traditional detectors. The same observation was made with the EDS graph, which shows that the nanoparticles are nearly pure (Figure 4d).
Table 1: Quantitative analysis of α-Bi2O3 nanoparticles.
Analyte
|
Result (%)
|
Std.Dev.
|
Line
|
Intensity
|
Bi
|
97.566
|
[ 0.278]
|
BiLa
|
604.8772
|
Ca
|
1.812
|
[ 0.071]
|
CaKa
|
0.5698
|
Fe
|
0.422
|
[ 0.045]
|
FeKa
|
2.9480
|
Cu
|
0.200
|
[ 0.033]
|
CuKa
|
2.6411
|