One-Step Microwave-Assisted Synthesis and Visible-Light Photocatalytic Activity Enhancement of BiOBr/RGO Nanocomposites for Degradation of Methylene Blue

In this study, bismuth oxybromide/reduced graphene oxide (BiOBr/RGO), i.e. BiOBr-G nanocomposites, were synthesized using a one-step microwave-assisted method. The structure of the synthesized nanocomposites was characterized using Raman spectroscopy, X-ray diffractometry (XRD), photoluminescence (PL) emission spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and ultraviolet-visible diffuse reflection spectroscopy (DRS). In addition, the ability of the nanocomposite to degrade methylene blue (MB) under visible light irradiation was investigated. The synthesized nanocomposite achieved an MB degradation rate of above 96% within 75 min of continuous visible light irradiation. In addition, the synthesized BiOBr-G nanocomposite exhibited significantly enhanced photocatalytic activity for the degradation of MB. Furthermore, the results revealed that the separation of the photogenerated electron–hole pairs in the BiOBr-G nanocomposite enhanced the ability of the nanocomposite to absorb visible light, thus improving the photocatalytic properties of the nanocomposites. Lastly, the MB photo-degradation mechanism of BiOBr-G was investigated, and the results revealed that the BiOBr-G nanocomposites exhibited good photocatalytic activity.


Introduction
Since 1972, the principle and applications of photocatalysis have been extensively investigated. Recently, with the increase in environmental protection awareness, photocatalysis has attracted significant attention for the treatment of environmental pollution and the conversion of solar energy [1][2][3]. As an advanced oxidation process, photocatalysis generates electron-hole pairs via light irradiation of photocatalysts. This photocatalytic degradation reaction can be attributed to the excitation of electron radiation in the valence band (VB) to the conduction band (CB) and the formation of holes with positive charges in the VB. During photocatalysis, the adsorbed material further reacts with the electron and hole to produce numerous oxidized substances that can degrade some organic pollutants [4,5].
Titanium dioxide (TiO 2 ) is a traditional photocatalyst with high photocatalytic activities and low toxicity [6,7]. Currently, TiO 2 is the most commonly used photocatalytic material owing to its low toxicity, which significantly limits its negative impact on the environment [8]. However, the large band gap of TiO 2 (3.2 eV) limits its ability to effectively convert solar energy. This is because it can only be excited by ultraviolet (UV) light (λ < 390 nm), which makes up only approximately 7% of the solar spectrum [9]. To increase the effective utilization of solar energy, researchers have devoted tremendous efforts to developing a new photocatalyst that can perform photocatalysis in the visible light range and clean the environment in an effective and environmentally friendly manner [10]. Bismuth irradiation. The results revealed that the composite exhibited 76.24% MB dye removal after 120 min visible light irradiation.
However, the large energy consumption and long reaction time of these methods have limited their further application. To overcome these challenges, in this study, we employed a simple, efficient, time-saving, and environmentally friendly, one-step synthesis method for the synthesis of photocatalyst nanocomposites. The fabrication of nanomaterials using microwaves is an environmentally friendly and cost-effective method. In addition, the microwave-assisted method can obtain high-purity products, improve product yields, and increase reproducibility [33]. In this study, BiOBr, a photocatalyst with visible light photocatalytic ability, was selected and combined with RGO to synthesize a BiOBr/RGO (BiOBr-G) photocatalyst. Furthermore, the microwave-assisted method was employed to synthesize BiOBr-G within short durations, and the photocatalytic activity of these nanomaterials was investigated. In addition, the crystal structure, morphology, functional groups, absorption spectra, and photocatalytic mechanism were investigated using various methods. The photocatalytic activity of the nanocomposites was investigated by evaluating their MB degradation ability. BiOBr and BiOBr-G photocatalysts were successfully synthesized, and their pollutant degradation abilities were investigated. Lastly, their effects on electron-hole pair separation were also discussed.

The Materials Used
Bismuth nitrate Bi(NO 3 ) 3 ·5H 2 O was obtained from Sigma-Aldrich (St. Louis, MO, USA). Potassium bromide (KBr) and natural graphite powder (99.99%, metals basis) were purchased from Alfa Aesar (Haverhill, MA, USA). All of the chemicals used in the study were of analytical grade. The aqueous solutions were prepared using deionized (DI) water.

Preparation of Graphene Oxide
In this study, GO was synthesized from graphite powder using the modified Hummers method [34]. Briefly, 70 mL of concentrated sulfuric acid (Japan Chemicsl Industries Co. Ltd, Shimizu-ku, Japan ) was placed in an ice bath and cooled to 5 • C. Subsequently, graphite powder, NaNO 3 , and KMnO 4 were added into the flask and stirred evenly for 2 h. Thereafter, 300 mL of DI water and 10 mL of 30% hydrogen peroxide were added to the solution to stop the reaction. Subsequently, the solution was subjected to suction filtration, after which the product was placed in 500 mL of 5% hydrochloric acid and stirred 30 min to remove the remaining metal ions. Thereafter, the HCl solution was removed and washed with DI water several times until the pH of the GO suspension was neutral. Lastly, the suspension was filtrated and dried in an oven for 12 h at 70 • C to obtain the GO powders.

Synthesis of the BiOBr-G Nanocomposites
BiOBr-G nanocomposites with various GO weight percentages were synthesized using the microwave-assisted method. Briefly, 0.4608 g Bi(NO 3 ) 3 ·5H 2 O was dissolved in 25 mL of ethylene glycol, after which the mixture was dispersed in an ultrasonic bath for 15 min. Simultaneously, GO powders were dissolved in 10 ml of ethylene glycol, after which the mixture was subjected to magnetic stirring for 30 min. Subsequently, 0.2380 g of KBr was added to the Bi(NO 3 ) 3 ·5H 2 O solution, after which the mixture was stirred at room temperature for 0.5 h. Thereafter, the GO solution was carefully added to the afore-mentioned solution, and the solution was stirred continuously for 30 min. Subsequently, the mixture was transferred into a 50 mL Teflon-lined vessel, and the mixture was maintained at 90 • C for 15 min. The precipitates were collected by centrifugation and washed three times with DI water and ethanol. Thereafter, the precipitates were dried in an oven at 80 • C overnight. For comparison, pure BiOBr and RGO were also prepared using a similar process. The BiOBr-G nanocomposites with RGO content of 0.5, 1, and 5 wt% were labeled as BiOBr-G0.5, BiOBr-G1, and BiOBr-G5, respectively. The schematic illustration of the synthesis process is shown in Figure 1. °C overnight. For comparison, pure BiOBr and RGO were also prepared using a similar 148 process. The BiOBr-G nanocomposites with RGO content of 0.5, 1, and 5 wt% were labeled 149 as BiOBr-G0.5, BiOBr-G1, and BiOBr-G5, respectively. The schematic illustration of the 150 synthesis process is shown in Figure 1. The photocatalytic efficiency of the BiOBr-Gs nanocomposites was investigated. The 156 photocatalytic reactions of the nanocomposites were investigated using a PCX50B Dis-157 cover multi-channel photoreactor (Perfect Light Technology Ltd, Beijing, China). A 5 W 158 white light Light-emitting diode lamp was used as the light source to provide visible light 159 with a wavelength of above 420 nm. The MB used in this study was a model pollutant that 160 must be removed. To prepare the sample used for the photocatalytic analysis, 30 mg of 161 as-prepared samples was weighed into a quartz flask, after which 50 ml of 2×10 −5 M MB 162 solution was added. Subsequently, the solution was stirred in the dark for 10 min to 163 achieve adsorption-desorption equilibrium, after which the solution was subjected to ir-164 radiation. The photocatalytic reaction was carried out under visible light irradiation for 165 75 min, and 3 mL of the as-prepared solution was collected every 15 min. The collected 166 MB solutions were centrifuged (14000 RPM, 3 min) to remove the sample powder. The 167 degradation of MB was evaluated by measuring the characteristic absorption of MB solu-168 tion at 664 nm using a CT-2200 UV-vis spectrophotometer (ChromTech Co. Ltd; Apple 169 Valley, MN, USA ). After measurement, the collected samples were re-injected to the 170 quartz flask, and the experimental condition was maintained. 171 The removal efficiency of the target pollutant was determined using the following 172 equation: where C0 and Ct are the initial concentration and concentration of MB at time t, respec-174 tively.

Photocatalytic Activity
The photocatalytic efficiency of the BiOBr-Gs nanocomposites was investigated. The photocatalytic reactions of the nanocomposites were investigated using a PCX50B Discover multi-channel photoreactor (Perfect Light Technology Ltd, Beijing, China). A 5 W white light Light-emitting diode lamp was used as the light source to provide visible light with a wavelength of above 420 nm. The MB used in this study was a model pollutant that must be removed. To prepare the sample used for the photocatalytic analysis, 30 mg of as-prepared samples was weighed into a quartz flask, after which 50 mL of 2 × 10 −5 M MB solution was added. Subsequently, the solution was stirred in the dark for 10 min to achieve adsorption-desorption equilibrium, after which the solution was subjected to irradiation. The photocatalytic reaction was carried out under visible light irradiation for 75 min, and 3 mL of the as-prepared solution was collected every 15 min. The collected MB solutions were centrifuged (14,000 RPM, 3 min) to remove the sample powder. The degradation of MB was evaluated by measuring the characteristic absorption of MB solution at 664 nm using a CT-2200 UV-vis spectrophotometer (ChromTech Co. Ltd; Apple Valley, MN, USA). After measurement, the collected samples were re-injected to the quartz flask, and the experimental condition was maintained.
The removal efficiency of the target pollutant was determined using the following equation: where C 0 and C t are the initial concentration and concentration of MB at time t, respectively.

Characterization
The as-synthesized BiOBr-G nanocomposites were synthesized using a microwave (Flexiwave T660, Milestone srl, Sorisole, Italy). The crystal structure of the as-synthesized nanocomposites was analyzed using powder X-ray diffraction (XRD, D8A25 eco, BRUKER Co. Ltd, Billerica, MA, USA) with CuKα X-ray radiation (λ = 1.5418 Å) operated at 40 kV and 25 mA. The morphology of the particles was observed using transmission electron microscopy (TEM, Hitachi H-7500, Tokyo, Japan) at an accelerating voltage of 80 kV. The surface morphology of the samples was investigated using scanning electron microscopy (SEM, Jeolism -6930, Tokyo, Japan) equipped with a system of energy-dispersive spec-troscopy (EDS, INCAx act, Munich, Germany). The photoluminescence (PL, Hitachi F-7000, Tokyo, Japan) spectra of the samples were obtained using a Hitachi F-7000 (Tokyo, Japan) spectrometer at an emission wavelength of 300 nm. The Raman spectra were determined using a Princeton Instruments Acton SP2500 (Acton, MA, USA) monochromatic/photographic spectrometer equipped with a nitrogen-cooled CCD detector and a Spec-10 system. The light absorption properties of the samples were investigated using UV−vis diffuse reflectance spectroscopy (DRS, JASCO IBXL0005-V770-EA, JASCO, Tokyo, Japan). The chemical state of the composites was measured using Fourier-transform infrared spectrometer (FT-IR, JASCO FT/IR-6700, JASCO, Tokyo, Japan). A multi-channel photochemical reaction system (Perfect Light Technology Ltd, PCX-50B, Beijing, China) was used during the photocatalytic reaction experiment. The MB concentration was measured using a CT-2200 ultraviolet-visible (UV-Vis) spectrophotometer (ChromTech Co. Ltd.; Apple Valley, MN, USA).

XRD Analysis
The GO, RGO, BiOBr, and the BiOBr-G nanocomposites synthesized using the microwaveassisted method were analyzed using XRD ( Figure 2). The XRD pattern of the GO exhibited a distinct strong reflex at 2θ = 12.6 • , corresponding to the (001) crystal plane of GO. In addition, notable peaks were observed in the XRD pattern of the RGO at 2θ = 25.0 • and 43.1 • , corresponding to the RGO (002) and (102) crystal plane, respectively [35]. However, the (001) crystal plane of GO was not observed in the XRD pattern of the RGO, confirming the successful reduction of GO by the microwave-assisted method. The XRD patterns of BiOBr and BiOBr-G nanocomposites were consistent with the standard card number of pure tetragonal phase BiOBr (JCPDS 09-0393) [36]. The main peaks of the XRD pattern were observed at 2θ = 10.  [37]. However, the (001) crystal plane of GO was not observed in the XRD patterns of all the BiOBr-G samples, confirming the successful reduction of GO. Furthermore, the typical diffraction peaks of RGO at 2θ = 25.0 • and 43.1 • were not observed in the XRD patterns of the BiOBr-G nanocomposites, which could be attributed to the relatively low diffraction peak intensity of RGO [38].  Figure 3 shows the Raman spectrum of GO, RGO, and BiOBr-G. Two notable bands, 229 the D and G bands, were observed in the Raman spectra of GO and RGO, respectively. 230 The D band corresponded to the k-point phonon mode, which could be attributed to the 231 sp 3 defects in the carbon material, such as vacancies and edge effect. The G band could be 232 attributed to the sp 2 carbon atoms vibration model [39,40]. The D and G bands of GO were 233 observed at 1348 cm −1 and 1587 cm −1 , respectively. In addition, the D and G bands of RGO 234 were observed at 1343 cm −1 and 1572 cm −1 , respectively. However, no notable peak was 235 observed in the Raman spectrum of BiOBr. In addition, two bands were observed in the 236  Figure 3 shows the Raman spectrum of GO, RGO, and BiOBr-G. Two notable bands, the D and G bands, were observed in the Raman spectra of GO and RGO, respectively. The D band corresponded to the k-point phonon mode, which could be attributed to the sp 3 defects in the carbon material, such as vacancies and edge effect. The G band could be attributed to the sp 2 carbon atoms vibration model [39,40]. The D and G bands of GO were observed at 1348 cm −1 and 1587 cm −1 , respectively. In addition, the D and G bands of RGO were observed at 1343 cm −1 and 1572 cm −1 , respectively. However, no notable peak was observed in the Raman spectrum of BiOBr. In addition, two bands were observed in the Raman spectrum of BiOBr-G at 1365 and 1601 cm −1 . The D band to G band intensity ratio of BiOBr-G (I D /I G = 0.99) was slightly lower than that of the pure GO (I D /I G = 1.01). This confirms the decrease in the sp 3 domain of the carbon atoms in BiOBr-G and the increase in the production of the graphene sp 2 structure in BiOBr-G. According to the literature, these results correspond to the high electron transport rate of BiOBr-G [41,42]. In addition, the I D /I G value of BiOBr-G was higher than that of RGO (I D /I G = 0.89), indicating that the RGO loaded with BiOBr has more defects than RGO. The shift of the D and G band in the BiOBr-G Raman spectrum confirmed that the microwave-assisted synthesis achieved both the reduction of GO and the formation of Bi-OBr-G [24,43]. Figure 3 shows the Raman spectrum of GO, RGO, and BiOBr-G. Two notable bands, 229 the D and G bands, were observed in the Raman spectra of GO and RGO, respectively. 230 The D band corresponded to the k-point phonon mode, which could be attributed to the 231 sp 3 defects in the carbon material, such as vacancies and edge effect. The G band could be 232 attributed to the sp 2 carbon atoms vibration model [39,40]. The D and G bands of GO were 233 observed at 1348 cm −1 and 1587 cm −1 , respectively. In addition, the D and G bands of RGO 234 were observed at 1343 cm −1 and 1572 cm −1 , respectively. However, no notable peak was 235 observed in the Raman spectrum of BiOBr. In addition, two bands were observed in the 236 Raman spectrum of BiOBr-G at 1365 and 1601 cm −1 . The D band to G band intensity ratio 237 of BiOBr-G (ID/IG = 0.99) was slightly lower than that of the pure GO (ID/IG = 1.01). This 238 confirms the decrease in the sp 3 domain of the carbon atoms in BiOBr-G and the increase 239 in the production of the graphene sp 2 structure in BiOBr-G. According to the literature, 240 these results correspond to the high electron transport rate of BiOBr-G [41,42]. In addition, 241 the ID/IG value of BiOBr-G was higher than that of RGO (ID/IG = 0.89), indicating that the 242 RGO loaded with BiOBr has more defects than RGO. The shift of the D and G band in the 243 BiOBr-G Raman spectrum confirmed that the microwave-assisted synthesis achieved both 244 the reduction of GO and the formation of Bi-OBr-G [24,43].

FTIR Spectroscopy
The FT-IR spectra of BiOBr and BiOBr-G nanocomposites are shown in Figure 4. A notable band was observed in the FT-IR spectra of the BiOBr nanosheets at 514 cm −1 , which could be attributed to the typical symmetric A 2u type of the Bi-O bond vibrations. In addition, a similar band was observed in the FT-IR spectra of the BiOBr-G nanocomposite [44,45]. Furthermore, a wider absorption band was observed in the FT-IR spectra of these samples at 3404 cm −1 , which could be attributed to the O-H stretching mode of adsorbed water or hydroxyl groups [46,47]. Moreover, the BiOBr-G sample exhibited the typical FT-IR spectrum of BiOBr-G. This indicates that the addition of GO to BiOBr during the microwave-assisted synthesis had no effect on the crystal structure of BiOBr [48]. Figure 5a,b show the SEM image of BiOBr and BiOBr-G nanocomposites. The pure BiOBr exhibited a thin flake structure with a two-dimensional relatively smooth surface, which was assembled to flower-like microstructures [49]. However, the BiOBr-G nanocomposite exhibited a spherical structure formed by the assembly of smaller and denser nanosheets. This indicates that the addition of RGO affected the crystallization process of BiOBr and destroyed the existing micro-flower nanostructure of BiOBr, thus increasing the dispersion and photocatalytic activity of BiOBr-G [50]. Furthermore, TEM was conducted to investigate the structural characteristics of the BiOBr and BiOBr-G nanocomposites, and the results are shown in Figure 5c, d. As shown in Figure 5c, BiOBr exhibits a thin sheet stacked structure with a flat surface, which is consistent with the SEM results and observation [51]. After the addition of RGO, the particle size of BiOBr reduced significantly, and the dispersion on the wrinkled RGO increased. This indicated the successful prepara-tion of BiOBr-G. This is because RGO effectively controlled the crystal size of BiOBr and prevented the agglomeration of nanoparticles [52]. In addition, after the addition of RGO, the surface area of the BiOBr-G nanocomposite in contact with dye increased compared to that of pure BiOBr owing to its smaller particle size. Consequently, the electron transfer and photocatalytic activity of BiOBr were significantly enhanced [53].

FTIR Spectroscopy 262
The FT-IR spectra of BiOBr and BiOBr-G nanocomposites are shown in Figure 4. A 263 notable band was observed in the FT-IR spectra of the BiOBr nanosheets at 514 cm −1 , which 264 could be attributed to the typical symmetric A2u type of the Bi-O bond vibrations. In ad-265 dition, a similar band was observed in the FT-IR spectra of the BiOBr-G nanocomposite 266 [44,45]. Furthermore, a wider absorption band was observed in the FT-IR spectra of these 267 samples at 3404 cm −1 , which could be attributed to the O-H stretching mode of adsorbed 268 water or hydroxyl groups [46,47]. Moreover, the BiOBr-G sample exhibited the typical FT-269 IR spectrum of BiOBr-G. This indicates that the addition of GO to BiOBr during the mi-270 crowave-assisted synthesis had no effect on the crystal structure of BiOBr [48].  BiOBr exhibited a thin flake structure with a two-dimensional relatively smooth surface, 285 which was assembled to flower-like microstructures [49]. However, the BiOBr-G nano-286 composite exhibited a spherical structure formed by the assembly of smaller and denser 287 nanosheets. This indicates that the addition of RGO affected the crystallization process of 288 BiOBr and destroyed the existing micro-flower nanostructure of BiOBr, thus increasing 289 the dispersion and photocatalytic activity of BiOBr-G [50]. Furthermore, TEM was con-290 ducted to investigate the structural characteristics of the BiOBr and BiOBr-G nanocompo-291 sites, and the results are shown in Figure 5c, d. As shown in Figure 5c, BiOBr exhibits a 292 thin sheet stacked structure with a flat surface, which is consistent with the SEM results 293 and observation [51]. After the addition of RGO, the particle size of BiOBr reduced signif-294 icantly, and the dispersion on the wrinkled RGO increased. This indicated the successful 295 preparation of BiOBr-G. This is because RGO effectively controlled the crystal size of Bi-296 OBr and prevented the agglomeration of nanoparticles [52]. In addition, after the addition 297 of RGO, the surface area of the BiOBr-G nanocomposite in contact with dye increased 298 compared to that of pure BiOBr owing to its smaller particle size. Consequently, the elec-299 tron transfer and photocatalytic activity of BiOBr were significantly enhanced [53].  Figure 6a shows the UV-Vis diffuse reflectance spectra (DRS) results of BiOBr and 304 BiOBr-G nanocomposites. As shown in the image, both BiOBr and BiOBr-G exhibit a wide 305 and high absorption in the visible light range. In addition, the absorption peak of BiOBr 306 was observed at 445 nm. After adding RGO, the BiOBr-G absorption edge slightly red-307 shifted to 462 nm. The optical band gap (Eg) of the BiOBr and BiOBr-G composites was 308 obtained using the Tauc relation, as follows [54]: where A is a constant, which depends on the transition probability; h is the Planck con-310 stant; ν is the frequency of light; α is the absorption coefficient; and n is based on the 311 transition property of the semiconductor [55]. For example, n = 2 corresponds to the indi-312 rect transition of the semiconductor, whereas n = 1/2 corresponds to the direct transition 313 of the semiconductor. Previous studies have reported that the n of BiOBr is 2, indicating 314 that the transition property of BiOBr is indirect [56]. The plot of (αhν) 1/2 vs. the photon 315 energy is shown in Fig. 6b. As shown in the image, the Eg value of the BiOBr and BiOBr-316  Figure 6a shows the UV-Vis diffuse reflectance spectra (DRS) results of BiOBr and BiOBr-G nanocomposites. As shown in the image, both BiOBr and BiOBr-G exhibit a wide and high absorption in the visible light range. In addition, the absorption peak of BiOBr was observed at 445 nm. After adding RGO, the BiOBr-G absorption edge slightly red-shifted to 462 nm. The optical band gap (E g ) of the BiOBr and BiOBr-G composites was obtained using the Tauc relation, as follows [54]:

UV-Vis Diffuse Reflectance Spectra
where A is a constant, which depends on the transition probability; h is the Planck constant; ν is the frequency of light; α is the absorption coefficient; and n is based on the transition property of the semiconductor [55]. For example, n = 2 corresponds to the indirect transition of the semiconductor, whereas n = 1/2 corresponds to the direct transition of the semiconductor. Previous studies have reported that the n of BiOBr is 2, indicating that the transition property of BiOBr is indirect [56]. The plot of (αhν) 1/2 vs. the photon energy is shown in Fig. 6b. As shown in the image, the E g value of the BiOBr and BiOBr-G nanocomposites are 2.83 and 2.69 eV, respectively. The E g value of the BiOBr is similar to the previous study [24].
obtained using the Tauc relation, as follows [54]: where A is a constant, which depends on the transition probability; h is the Planck con-310 stant; ν is the frequency of light; α is the absorption coefficient; and n is based on the 311 transition property of the semiconductor [55]. For example, n = 2 corresponds to the indi-312 rect transition of the semiconductor, whereas n = 1/2 corresponds to the direct transition 313 of the semiconductor. Previous studies have reported that the n of BiOBr is 2, indicating 314 that the transition property of BiOBr is indirect [56]. The plot of (αhν) 1/2 vs. the photon 315 energy is shown in Fig. 6b. As shown in the image, the Eg value of the BiOBr and BiOBr-316 G nanocomposites are 2.83 and 2.69 eV, respectively. The Eg value of the BiOBr is similar 317 to the previous study [24].

PL Analysis
The PL spectra of BiOBr and BiOBr-G were obtained to investigate their electron-hole recombination properties. With an increase in the PL intensity of a sample, its photon separation rate decreases, thus increasing the electron-hole pair recombination. Consequently, this reduces the photocatalytic activity of the sample [57]. Figure 7 shows the PL spectrum of the as-synthesized BiOBr and BiOBr-G nanocomposites at an exciting wavelength (λ ex ) of 300 nm. The highest peaks of BiOBr and BiOBr-G were observed at 468 nm. However, the PL spectral intensity of the pure BiOBr was significantly stronger than that of BiOBr-G. With an increasing in the RGO content, the intensity of the BiOBr-G emission peak decreased, indicating that the addition of RGO enhanced the separation of carriers. These results confirmed that photoelectrons moved from BiOBr to RGO and that RGO facilitated the suppression of the electron-hole pair recombination [58]. This indicates that a higher quantum efficiency can be achieved during photocatalytic reactions by utilizing the hierarchical structure of BiOBr-G. The PL spectra of BiOBr and BiOBr-G were obtained to investigate their electron-338 hole recombination properties. With an increase in the PL intensity of a sample, its photon 339 separation rate decreases, thus increasing the electron-hole pair recombination. Conse-340 quently, this reduces the photocatalytic activity of the sample [57]. Figure 7 shows the PL 341 spectrum of the as-synthesized BiOBr and BiOBr-G nanocomposites at an exciting wave-342 length (λex) of 300 nm. The highest peaks of BiOBr and BiOBr-G were observed at 468 nm. 343 However, the PL spectral intensity of the pure BiOBr was significantly stronger than that 344 of BiOBr-G. With an increasing in the RGO content, the intensity of the BiOBr-G emission 345 peak decreased, indicating that the addition of RGO enhanced the separation of carriers. 346 These results confirmed that photoelectrons moved from BiOBr to RGO and that RGO 347 facilitated the suppression of the electron-hole pair recombination [58]. This indicates that 348 a higher quantum efficiency can be achieved during photocatalytic reactions by utilizing 349 the hierarchical structure of BiOBr-G.

Photocatalytic Activity
To investigate the photocatalytic properties of the fabricated samples, the MB degradation properties of commercial TiO 2 (P25), BiOBr, and each BiOBr-G sample under visible light were investigated (Figure 8a). The results revealed that the pure MB did not exhibit self-degrading properties under visible light; however, the addition of the photocatalyst significantly improved the removal efficiency [59]. The order of MB removal percentage of each material was: TiO 2 P25 (29.74%) < BiOBr (67.25%) < BiOBr-G0.5 (71.37%) < BiOBr-G1 (90.80%) < BiOBr-G5 (96.41%). The photocatalytic activity of P25 under visible light was lower than those of the other samples, which could be attributed to the fact that the band gap of P25 is higher than that of BiOBr and BiOBr-Gs. This indicates that the P25 sample exhibited the lowest MB removal rate compared to the other samples. In addition, the photocatalytic activity of all the BiOBr-G nanocomposites was significantly higher than that of BiOBr. These findings are consistent with the PL spectra in Figure 7. Furthermore, the photoelectron-hole pair separation efficiency of the BiOBr-G sample was higher than that of the BiOBr sample, and it also exhibited an optimum photocatalytic activity performance. The addition of RGO to BiOBr significantly enhanced the photocatalytic activity of BiOBr, with the optimum RGO concentration being 5 wt%. In addition, the photo-induced electron transfer rate from the BiOBr surface to the RGO surface increased with an increase in the RGO loading, thus increasing the photocatalytic activity of the BiOBr-G nanocomposite. The prepared photocatalyst follows the first-order kinetic model (Figure 8b), which can be expressed using the following equation [60]: where k and t are the rate constant and lighting time, respectively. The k values of P25, BiOBr, BiOBr-G0.5, BiOBr-G1, and BiOBr-G5 are 0.00426, 0.0090, 0.0313, 0.0370, and 0.0392 min −1 , respectively. The increase in the k value of BiOBr-G compared to that of BiOBr indicates the short-term degradation of MB dye. In addition, the BiOBr-G5 nanocomposite exhibited the optimum photocatalytic effect, which was 4.36 times higher than that of BiOBr. This result indicates that the addition of graphene to BiOBr significantly improved the photocatalytic efficiency of BiOBr [61]. Table 1 shows the comparison of the MB removal and rate constant of BiOBr-G5 to those of different materials investigated in previous studies. The MB-removal percentage of BiOBr-G5 was similar to that of other materials; however, the rate constant of BiOBr-G5 was higher than those of the other materials. This result illustrates the enhanced MB-removal efficiency of BiOBr-G5. induced electron transfer rate from the BiOBr surface to the RGO surface increased with 386 an increase in the RGO loading, thus increasing the photocatalytic activity of the BiOBr-G 387 nanocomposite. The prepared photocatalyst follows the first-order kinetic model (Figure 388 8b), which can be expressed using the following equation [60]: where k and t are the rate constant and lighting time, respectively. The k values of P25, 390 BiOBr, BiOBr-G0.5, BiOBr-G1, and BiOBr-G5 are 0.00426, 0.0090, 0.0313, 0.0370, and 0.0392 391 min −1 , respectively. The increase in the k value of BiOBr-G compared to that of BiOBr in-392 dicates the short-term degradation of MB dye. In addition, the BiOBr-G5 nanocomposite 393 exhibited the optimum photocatalytic effect, which was 4.36 times higher than that of  OBr. This result indicates that the addition of graphene to BiOBr significantly improved 395 the photocatalytic efficiency of BiOBr [61]. Table 1 shows the comparison of the MB re-396 moval and rate constant of BiOBr-G5 to those of different materials investigated in previ-397 ous studies. The MB-removal percentage of BiOBr-G5 was similar to that of other materi-398 als; however, the rate constant of BiOBr-G5 was higher than those of the other materials. 399 This result illustrates the enhanced MB-removal efficiency of BiOBr-G5.    3.3. Photocatalytic Mechanisms Figure 9 shows the photocatalysis mechanism of BiOBr-G under visible light irradiation. The excitation of the electrons (e − ) of the VB into the CB under visible light irradiation leads to the generation of electron-hole pairs. The E CB and E VB of BiOBr were evaluated using the equation below [67]:

436
In summary, in this study, BiOBr-G nanocomposites were successfully synthesized 437 using a facile one-step microwave-assisted method and characterized by XRD, Raman, 438 FTIR, SEM, TEM, UV-vis DRS, and PL. The XRD analysis confirmed the successful syn-439 thesis of GO, RGO, BiOBr, and all the BiOBr-G nanocomposites. However, the RGO dif-440 fraction peak was not observed in the XRD pattern of BiOBr-G. In addition, D and G bands 441 were observed in the Raman spectrum of the BiOBr-G nanocomposites, indicating the 442 graphene structure of the synthesized nanocomposite. Furthermore, a Bi-O vibrational 443 peak was observed in the FT-IR spectra of BiOBr and Bi-OBr-G. The SEM and TEM results 444 revealed that the presence of RGO in the BiOBr-G nanocomposites prevented the nano-445 particles' agglomeration and reduced the particle size. In addition, the band gap of BiOBr-446 G was lower than that of BiOBr, indicating that the addition of RGO to BiOBr enhanced 447 the photocatalytic activity of BiOBr. The PL results revealed that BiOBr-G exhibited higher 448 quantum efficiency during the photocatalytic process compared to BiOBr. The photocata-449 lytic activity of the as-synthesized nanocomposites under visible light irradiation was in-450 vestigated. BiOBr-G5 exhibited the optimum photocatalytic activity, and the removal per-451 centage of MB achieved more than 96% in 75 min. The prepared photocatalyst follows the 452 first-order kinetic model, and the rate constant of BiOBr-G5 (0.0392 min −1 ) was higher than 453 According to the DRS measurements, E e is 4.5 eV, and it represents the energy of free electrons on the hydrogen scale. In addition, E VB and E CB correspond to the edge potentials of VB and CB, respectively. From the DRS results, χ is semiconductor electronegativity. The χ of BiOBr was 6.17 eV, and the E g of BiOBr-G was 2.69 eV. Using the afore-mentioned equation, the E VB and E CB of BiOBr are approximately 3.02 and 0.33 eV, respectively [68]. The VB position of BiOBr is above OH − /•OH (2.38 eV to Normal Hydrogen Electrode, NHE) and H 2 O/•OH (2.72 eV to NHE) [69]. The photogenerated electron-hole of BiOBr can oxidize OH − and H 2 O to the •OH free radicals, and •OH can degrade the MB molecules.
The RGO exhibits an extremely efficient electron collection and separation structure. After the charge separation of BiOBr, the excited electrons were transferred to the RGO surface, thus decreasing the rate of electron-hole recombination [70]. Photogenerated electrons reacted with O 2 molecules adsorbed on the surface of the RGO to produce O 2 − radicals. Simultaneously, the OH − and H 2 O reacted with the holes in the VB, thus generating •OH radicals. The RGO nanosheets enhanced the electron-hole pair separation and also increased the adsorption of reactants owing to π-π interactions and electrostatic force effects. In addition, the high surface area of RGO facilitated the adsorption of MB and the active oxides (O 2 − and •OH radicals), thus enhancing the contact probability with radical and dye molecules and enhancing the photocatalytic activity of the catalyst. Finally, the MB molecules were degraded by O 2 − and •OH radicals, thus producing non-toxic small molecules (H 2 O, CO 2 , SO 4 2− , NO 3 − , and NH 4 + ). This photocatalytic process can be represented using the following chemical equation [20,71].

Conclusions
In summary, in this study, BiOBr-G nanocomposites were successfully synthesized using a facile one-step microwave-assisted method and characterized by XRD, Raman, FTIR, SEM, TEM, UV-vis DRS, and PL. The XRD analysis confirmed the successful synthesis of GO, RGO, BiOBr, and all the BiOBr-G nanocomposites. However, the RGO diffraction peak was not observed in the XRD pattern of BiOBr-G. In addition, D and G bands were observed in the Raman spectrum of the BiOBr-G nanocomposites, indicating the graphene structure of the synthesized nanocomposite. Furthermore, a Bi-O vibrational peak was observed in the FT-IR spectra of BiOBr and Bi-OBr-G. The SEM and TEM results revealed that the presence of RGO in the BiOBr-G nanocomposites prevented the nanoparticles' agglomeration and reduced the particle size. In addition, the band gap of BiOBr-G was lower than that of BiOBr, indicating that the addition of RGO to BiOBr enhanced the photocatalytic activity of BiOBr. The PL results revealed that BiOBr-G exhibited higher quantum efficiency during the photocatalytic process compared to BiOBr. The photocatalytic activity of the as-synthesized nanocomposites under visible light irradiation was investigated. BiOBr-G5 exhibited the optimum photocatalytic activity, and the removal percentage of MB achieved more than 96% in 75 min. The prepared photocatalyst follows the first-order kinetic model, and the rate constant of BiOBr-G5 (0.0392 min −1 ) was higher than those of the other materials. These results were also superior to those of various catalysts reported in previous studies. The BiOBr-G nanocomposite synthesized in this study is an efficient, cost-effective, and environmentally friendly photocatalyst, with promising potential for wastewater treatment.

Conflicts of Interest:
The authors declare no conflict of interest.