Facile Green Synthesis of BiOBr Nanostructures with Superior Visible-Light-Driven Photocatalytic Activity

Novel green bismuth oxybromide (BiOBr-G) nanoflowers were successfully synthesized via facile hydrolysis route using an Azadirachta indica (Neem plant) leaf extract and concurrently, without the leaf extract (BiOBr-C). The Azadirachta indica leaf extract was employed as a sensitizer and stabilizer for BiOBr-G, which significantly expanded the optical window and boosted the formation of photogenerated charge carriers and transfer over the BiOBr-G surface. The photocatalytic performance of both samples was investigated for the degradation of methyl orange (MO) and phenol (Ph) under the irradiation of visible light. The leaf extract mediated BiOBr-G photocatalyst displayed significantly higher photocatalytic activity when compared to BiOBr-C for the degradation of both pollutants. The degradation rate of MO and Ph by BiOBr-G was found to be nearly 23% and 16% more when compared to BiOBr-C under visible light irradiation, respectively. The substantial increase in the photocatalytic performance of BiOBr-G was ascribed to the multiple synergistic effects between the efficient solar energy harvesting, narrower band gap, high specific surface area, porosity, and effective charge separation. Furthermore, BiOBr-G displayed high stability for five cycles of photocatalytic activity, which endows its practical application as a green photocatalyst in the long run.

Supplementary Materials S1: In order to optimize the concentration of plant extract (P.E), BiOBr-G was synthesized with different amounts of plant extract i.e., 4 mL, 6 mL, 8 mL, 10mL, and 12 mL. The as-prepared samples were tested for the photodegradation of methyl orange (MO). From Figure S1, it can be clearly seen that the highest photocatalytic activity was observed with an 8 mL plant extract (P.E). Hence, 8 mL of plant extract was utilized for the experimentation.

S3:
The dark experiment was conducted at different time intervals i.e., 30 min, 60 min, and 90 min. As shown in Figure S3, the highest adsorption occurred at 60 minutes and became constant at 90 minutes. Hence, adsorption-desorption equilibrium was attained in 60 minutes. Figure S3. Showing the dark study at 30 min, 60 min, and 90 min for BiOBr-C and BiOBr-G.

S4:
The average crystallite size of BiOBr-G and BiOBr-C was calculated using Debye Scherer's Equation.
where D is the size of the particles; K is the shape factor (0.9); λ is the wavelength of emitted x-rays (0.15418 nm); β is the full width at half maximum of the corresponding XRD peak; and θ is the angle of the incidence of the x-ray beam.

S5:
This represents the corresponding TEM images of BiOBr-G and BiOBr-C ( Figure S5). It can be clearly seen that the use of plant extract has introduced internal cavities in the mesoporous BiOBr-G nanostructures exhibiting a synergistic effect. Hence, the combined effect of internal cavities and lower

S6: Photocatalytic Performance of Plant Extract
In order to investigate the role of leaf extract alone, a control experiment with raw leaf extract (10 mL) was conducted following the same method reported in the manuscript for the degradation of MO. As shown in Figure S6, the leaf extract did not influence any degradation of MO under 90 minutes of visible light irradiation. This suggests that the leaf extract alone does not possess any photocatalytic activity. For the prospect of dye sensitization, a typical colorless pollutant i.e., phenol was also chosen to further evaluate the photocatalytic activity of the as-prepared BiOBr-C and BiOBr-G. In the dark experiment, about 9.3 % and 13.6 % of the Ph was adsorbed on the BiOBr-C and BiOBr-G surface, respectively. Figure S6a shows that in the presence of BiOBr-C and BiOBr-G, the photodegradation rate of Ph was found to be 52.01 % and 68.67 % within 600 minutes, respectively. In addition, no change in the degradation efficiency of Ph was observed without the photocatalyst, indicating the stability of Ph under visible light irradiation. From Figures S6b,c it can be clearly seen that upon increasing BiOBr-C and BiOBr-G concentration from 50-150 mg, the photodegradation efficiency of Ph was further increased and was found to be 65.70% and 81.73%, respectively. In this study, 125 mg was found to be the optimized photocatalytic concentration in this case. Once again, the photocatalytic efficiency of BiOBr-G was found to be nearly 16 % more than that of BiOBr-C.
In order to investigate the active species responsible for Ph degradation, a similar study as that of MO was carried out in this case. As shown in Figures S6d,e, almost no inhibition of the photocatalytic performance was observed when isopropanol and benzoquinone were used to quench . OH and . O2 -. This indicates that . OH and . O2 -showed a comparatively weak effect on the Ph degradation. However, a discerning inhibition of photocatalytic activity was seen when sodium oxalate was used to quench h + and confirms the importance of h + in the photooxidation process. Both BiOBr-G and BiOBr-C consumed more time in the degradation of Ph when compared to that of MO. This can be explained as in the case of MO, where all three active species actively participated in the photooxidation process while in the case of Ph; it wasonly the holes that were solely responsible for the degradation of Ph. Hence, using Equations (3),-(5), the photo-oxidation process that occurred in this case would be: Ph + h + → degraded product (8) Figure S6f shows the pseudo-first-order kinetic model for Ph degradation. The rate constants for BiOBr-C and BiOBr-G were calculated using Equation (7) as used earlier, and were found to be 0.0012 min -1 and 0.0019 min -1 , respectively. Interestingly, in this case, the calculated value of the rate constant for BiOBr-G was also greater than that of BiOBr-C, which explained the superior photocatalytic activity of BiOBr-G towards phenol.

S8: Band Position Calculations
For a compound, at the point of zero charge, the valence band (VB) position can be calculated by the following empirical formula 8 : where X is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms; E e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); EVB is the VB edge potential ;and Eg is the band gap of the semiconductor. The conduction band (CB) position can be deduced by ECB = EVB -Eg. Given the equations above, the top of the VB and the bottom of the CB of BiOBr-G were calculated as 3.091 and 0.261 eV with respect to the normal hydrogen electrode (NHE), respectively. Accordingly, in the case of BiOBr-C, the VB and CB were estimated to be 3.196 and 0.156 eV, respectively.