The type of functional groups on the surface of synthesized nanostructures were identified by FTIR spectrum, as illustrated in Figure 1. It reveals characteristic vibrating modes of the free carbonate ion with point group symmetry D3h. This is engrossed several internal vibrations including, symmetric stretching mode ν1 at 1066 cm−1, out-of-plane bending mode ν2 at 848 cm−1, antisymmetric vibration ν3 at 1460 and 1384 cm−1, in-plane deformation ν4 at 695 and 673 cm−1 and symmetric stretching plus in-plane deformation modes, ν1 + ν4, at 1756 and 1730 cm−137,38,43. The band groups of CO3−2 at ν1, ν2, ν3, ν4 and ν1 + ν4 reveal the formation of Bi2O2CO3. Moreover, the bands at 844 cm−1 and 300 – 800 cm−1 are corresponded to Bi-O-C and stretching modes of the Bi-O bonds in Bi2O2CO3, and Bi2O3, respectively 53. The bands at 3455 cm−1 and 1400 cm−1 are corresponding to O-H stretching vibration and O-H bending vibration respectively, suggest the presence of (BiO)4CO3(OH)2 in composition54,55.
The XRD pattern of synthesized sample is illustrated in Figure 2. It is observed that there are forty nine different peaks in the pattern which are related to the crystallographic characteristics of α-Bi2O3, (BiO)4CO3(OH)2 and Bi2O2CO3 phases. The crystal system of Bi2O3 is monoclinic with a = 5.8480, b = 8.1660 and c = 7.5100 Å, α = γ = 90° and β = 113° crystallographic parameters and is according to 00-027-0053 reference code 56. The (BiO)4CO3(OH)2 phase is relevant to 00-038-0579 reference code with orthorhombic crystal system with a = 10.7716, b = 5.4898 and c = 14.75740 Å and α = β = γ = 90° crystallographic parameters57. The crystal structure of Bi2O2CO3 is orthorhombic with a = 5.4680, b = 27.3200, c = 5.4680 Å, α = β = γ = 90° crystal parameters and is based on 01-084-1752 standard card58. Based on calculation of semi quantitative analysis of multiphase systems the α-Bi2O3, (BiO)4CO3(OH)2 and Bi2O2CO3 phases have 44%, 36% and 20% share of structure respectively. To describe mechanism of multiphase nanostructures formation, it should be noticed that, establishing electrical arc discharge leads to formation of a high temperature plasma59. The plasma formation process dissociates the solution molecules and produces neutral, ionic and molecular species of bismuth, water and urea constituent atoms. Formation of Bi, O, C and H ions or OH, H2, O2, CO and CO2 molecules in their ground or excited state in the plasma of electric arc discharge has been reported before 51,52. Follow these radicals the formation of oxide, oxide carbonate, hydroxide and carbonate oxide of bismuth could be explained.
Figure 3 shows the FE-SEM images of α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 with nanoflakes structures and the approximate average thickness of nanoflakes were demonstrated by size distribution histogram. The size distribution histogram was obtained by applying a gaussian curve on the size histograms. The average thickness of nanoflakes is almost 22 nm. Furthermore, FE-SEM elemental mapping and energy dispersive spectrometry (EDS) of synthesized nanoflakes indicated the uniform distribution of Bi, O, C and N elements in structures, as shown in Figure 4.
Figure 5 (a) and (b) shows the TEM image of α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes with different magnification, which confirms the SEM observation. There is a strong correlation between morphology and light harvesting performance of a multi-heterojunction nanostructure. Morphology can affect light absorption and electron-hole transport dynamics. Flake structure can enhance carrier extraction via an efficient electron-hole separation which results in an optimal charge transport properties. Reduction of electron-hole recombination rate could be attributed to the enhancement of the electron-hole separation due to the formation of electrical dipole in nanostructures with flake morphology 60–62.
Optical properties specially UV–vis spectrum of the nanoflakes is an effective way to evaluate photocatalytic activity via measuring light harvesting efficiency over different range of visible spectrum 52. The optical absorption spectra of colloidal sample over different times immediately after synthesis, 15 min, 30 min, 1 hour, 4 hours, 4 days and after 1 month are shown In Figure 6a. The spectra are measured in wavelength between 190 and 1100 nm. The optical absorption spectra of the sample show an absorption peak at wavelengths between 190 nm and 400 nm, which is explained by electron transitions from the valance band to conduction band and bandgap absorption of α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2, which is the feature of semiconductor materials 25,39,53. According to Tauc model the optical absorption coefficient of semiconductors with indirect transition band gap near the band edge follows the subsequent equation:
$${\left(\alpha h\nu \right)}^{1/2}= {\rm A}(h\nu -{E}_{g})$$
1
where α is absorption coefficient, հ is Plank constant, ν is light frequency, Eg is band gap energy and A is a constant 53,63. The calculation band gap diagrams of colloidal sample over different times after synthesis are indicated in Figure 6b. The estimated band gap energies are in the range between 2.7 eV and 3.0 eV for as prepared sample and after one month. The optical properties of colloidal sample show that synthesized nanoflakes can be good candidate for visible light harvesting and possible photocatalytic activity.
The photocatalytic activity of the novel α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes with multi heterojunctions was evaluated by the degradation of MeO solution under visible light irradiation for 180 minutes. The optical transmission spectra of MeO solution during degradation process by α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes exhibited considerable photocatalytic activity and the MeO absorption peak at 464 nm was considerably reduced.
The concentration of the dye at different times during degradation can be obtain by Langmuir–Hinshelwood model64:
$$r=-\frac{dC\left(t\right)}{dt}={k}_{abc}C\left(t\right)$$
2
where \(r\) is the degradation rate of the dye, \(C\left(t\right)\)reveals concentration at time t and \({k}_{abc}\) is a constant for reaction rate. The degradation kinetics of the dye can be described by following equation at low concentration of the dye:
$$\text{ln}\left(\frac{{C}_{0}}{C}\right)={k}_{abs}t$$
3
where the C0 is initial concentration of MeO. The photocatalytic activity and MeO degradation kinetics for the sample are presented in Figure 7 that obtained at different reaction times during the photo degradation process. Figure 7a illustrates C/C0 plot vs irradiation time and as obviously seen the sample shows highly efficient degradation rate. Figure 7b shows the degradation reaction kinetics, which was measured from the slopes of the −ln(C/C0) plots versus irradiation time 65. The calculated \({k}_{abs}\) value for synthesized nanoflakes is 5.5×10−3 min−1. The promoted photocatalytic performance of α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 can be attributed to its flake morphology and multi-phase structure. Based on previous reports, the large surface area can helps to increase the photocatalytic reaction by means of rising active photocatalytic sites and the promotion of the efficiency of the electron-hole separation60−62. Based on the obtained results of this research, it can be concluded that, the formation of multi-junction flakes can efficiently improve the photocatalytic activity of multi-heterojunction α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst in comparison with single α-Bi2O3, Bi2O2CO3 and (BiO)4CO3(OH)2 photocatalysts 25,45,66.
The recyclability and reusability of photocatalysts are important factors in practical application 6,67. The stability of the photocatalytic activity of a photocatalyst can be evaluated through the monitoring of photocatalytic degradation of MeO under light over several cycles. In fact, the stability of a photocatalyst will be proved by stability in efficiency over every cyclic photocatalytic degradation test68. Hence, the efficiency should not decline noticeably with each cycle. The degradation efficiency of the recovered α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst after four cycles are shown in Figure 8. As it clear, after each cycle the photocatalytic activity of sample was almost unchanged, which demonstrates a high stability of photocatalyst after multiple reuse.
Possible photocatalytic degradation mechanism of MeO by α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 under visible light irradiation were investigated through trapping experiments of reactive species in photocatalytic tests by using different radical scavengers namely EDTA, IPA and BQ for scavenging the active h+, •OH and •O2− during the photocatalytic reaction25,39. Based on Figure 9 degradation values of the MeO decreased in all capturing experiments which reveals all the generated reactive species are effective in photodegradation of MeO. However, BQ as superoxide radicals scavenger, more affected the MeO degradation efficiency. As shown in Figure 9, by capturing •O2− the dye degradation will eliminate more than other h+, •OH species.
According to absolute conduction and valance band energy positions of α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanocomposite and the energy levels of literature, three possible degradation mechanism for α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 multi-heterojunctions before and after junction are proposed in Figure 10 25,69,70. The Fermi level of α-Bi2O3 and (BiO)4CO3(OH)2 which are p-type photocatalyst are close to the valence band and the Fermi level of p-type Bi2O2CO3 is close to the conduction band25,71. By formation of multi-heterojunction the energy band of α-Bi2O3, (BiO)4CO3(OH)2 and Bi2O2CO3 will move upward or downward to lineup the Fermi level of the multi-heterojunction structure25. Under visible light irradiation, the photogenerated electrons on the conduction band of component with less negative edge potential will transfer to upper CB, while the holes in the valence band with more positive edge potential are likely to transfer opposite position70. In this situation, the recombination of photogenerated carries in photocatalyst will reduce and thus formation of p-n-p or p-p-n α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 multi-heterojunctions nanoflakes will improve the separation efficiency of the photogenerated electron and holes which lead better photocatalytic performance60–62.
PL spectrum of the α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 with the excitation wavelength at 355 nm was measured and compared with PL spectrum of Bi2O3 to display the photoactivity enhancement mechanism. Broad emission peaks in the range between 2 and 3 eV are found in the PL spectra at the samples which is agreement with the documents23,39,47,72. In general, a superior PL intensity shows an upper recombination rate of photo generated electron and holes, and thus lower PL intensity express lower recombination rate of photo generated electrons and holes which means improved separation rate of electrons and holes and so better photocatalytic performance39. Based on PL spectra shown in Figure 11, the PL intensities of the multi-heterojunction α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes are all lower than that of the Bi2O3 at the same condition, which means the efficient separation rate of the carriers in α-Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2. The formation of the multi-heterojunction reduce the recombination rate of photo-generated electrons and holes and then was favorable to improve photocatalytic activity.