Microwave-Solvothermal Synthesis of Nanostructured BiOBr with Excellent Visible-Light Photocatalytic Properties

Novel BiOBr hierarchicalmicrospheres have been successfully prepared via a facilemicrowave-assisted solvothermal route and used for visible-light photocatalytic degradation of RhB. The phase and morphology of the products were characterized by powder Xray diffraction (XRD), thermogravimetric analysis (TG), scanning electronmicroscopy (SEM), BET, and UV-vis diffuse reflectance spectra. The SEM observations displayed that BiOBr flower-like nanostructure assembled from nanosheets. The BiOBr flower-like nanostructure, with a narrow band gap (2.63 eV), shows excellent photocatalytic activity in the degradation of RhB dye under visible-light, much higher than those of BiOBr nanosheet and P25 photocatalysts.

In recent years, microwave-hydrothermal reaction has been used as an effective method for the synthesis of nanoparticles of various oxides [20,21].Compared with conventional methods, microwave-hydrothermal synthesis has many distinct advantages: (1) shorter reaction time, (2) more uniform product dimensions and composition, and (3) easiness to tune compositions of the products [22,23].Herein, we report the successful synthesis of BiOBr hierarchical microspheres constructed by nanosheets through a facile and rapid microwave-assisted solvothermal route.Furthermore, the visible-light photocatalytic properties of BiOBr samples were studied.

Experiment Section
2.1.Synthesis.All the chemicals were of analytic purity and used without further purification.In a typical process, 2 mmol Bi(NO 3 ) 3 ⋅5H 2 O and 2 mmol cetyltrimethylammonium bromide (CTAB) were dissolved into 40.0mL ethylene glycol solution while stirring for 10 min at room temperature, respectively.Then, CTAB solution was added dropwise to Bi(NO 3 ) 3 ⋅5H 2 O solution in turn.After being vigorously stirred for 30 min at room temperature, the final clear buffy solution was transferred to a Teflon vessel of the MDS-6 (Microwave Digestion/Extraction System, Shanghai Sineo Microwave Chemical Technology Co., Ltd.) with 80% volume filled.The microwave treatment was maintained at 160 ∘ C for various reaction time (10, 30, 60, and 120 min, which were denoted as BiOBr-1, BiOBr-2, BiOBr-3, and BiOBr-4, resp.)under autogenous pressure and then naturally cooled to room temperature.The as-prepared yellow powders were repeatedly washed with the distilled water and ethanol three times, filtered, and dried in an oven at 60 ∘ C.

Characterization.
The phase of as-synthesized samples was studied by X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer (using Cu K  radiation ( = 1.54056Å) and operating at 40 kV and 40 mA).The morphology of as-prepared samples was examined by scanning electron microscopy (SEM, HITACHI S-4800) and high resolution transmission electron microscopy (HRTEM, JEM-2100).N 2 adsorption-desorption isotherms were measured on a Quantachrome NOVA 4000e at 77 K.The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area.The thermal stability was investigated by a thermogravimetric analyzer (TG 209 F1, NETZSCH).The optical property was analyzed by UV-vis diffuse reflectance spectra (DRS, UV-2400PC, SHIMADZU) equipped with ISR-240A type integrating sphere.

Photocatalytic Test.
The photocatalytic activities of the as-synthesized BiOBr flowers were evaluated in terms of the degradation of RhB in an aqueous solution.A 250 W highpressure mercury lamp (dominant wavelength: 365 nm) used as light source was positioned inside a cylindrical vessel and surrounded by circulating water jacket for cooling.To make sure that the photocatalytic reaction was really driven by visible-light, all the UV lights with the wavelength less than 420 nm were removed by a glass filter (UVCUT420).50 mg of sample was suspended in 50 mL of 2 * 10 −5 mol/L RhB.The solution was continuously stirred for about 30 min at room temperature to ensure the establishment of an adsorptiondesorption equilibrium among the photocatalyst, RhB, and water before irradiation with UV light from the high-pressure mercury lamp.At given time intervals, the concentration of RhB left in the solution was determined by the UV-vis spectroscopy (Shanghai Meipuda Instrument Co., Ltd., UV-1600) at its characteristic wavelength of 553 nm.

Results and Discussion
The phase and crystallographic structure of the as-obtained samples were examined by XRD. Figure 1 presents the XRD Figure 2 shows the TG and DSC curves of the BiOBr-4 photocatalyst.The TG curve shows that no significant mass loss was recorded from room temperature to 600 ∘ C. It demonstrates that the BiOBr sample is steady below 600 ∘ C, so the prepared samples are of good stability and purity under the reaction temperature (room temperature) [15].The litter mass loss may be caused by the evaporated water molecules.The mass of BiOBr sharply decreased from 600 ∘ C with an exothermic peak on the DSC curve, indicating the decomposition of BiOBr.And remarkable exothermic phenomenon is observed at 600-800 ∘ C, which may be caused by substance change.
The microstructure and morphology of the product were investigated by SEM.The SEM images of the resulting products are presented in Figure 3.At the initial time of 10 min, small nanosheets in irregular shape were formed with the in-plane size of about 150-300 nm and thickness of about 20-30 nm.Then, the nanosheets exhibited the gradual agglomeration and oriented attachment at 30 min.As the solvothermal time prolonged to 60 min, the integrate flowerlike microspheres were formed with larger and thinner nanosheets (about 15 nm thickness).Further increasing the solvothermal time to 120 min could achieve the larger microspheres, in which the diameter was about 1.3-1.5 m and the nanosheet thickness was about 20 nm.Based on the above SEM morphology, the plausible mechanism of BiOBr crystal growth could be proposed.Before the solvothermal reaction, the coordination occurs between ethylene glycol and Bi 3+ and generates stable five-membered-ring coordination cations (Bi(OCH 2 CH 2 OH) 2+ ), which was stable as a dense and linearly aligned structure.CTAB surfactant which acted as both template and Br source was self-assembled to form the lamellar structure [25].The combination of Br − in CTAB lamellas with Bi(OCH 2 CH 2 OH) 2+ induced the formation of BiOBr nanosheets during the solvothermal process.With the reaction time prolonged, the BiOBr nanosheets were agglomerating via the oriented attachment mechanism.In the following growth stage, nanosheets grow into threedimensional (3D) flower-like hierarchical architectures built from oriented nanorods via self-assembly, driven by the minimization of the total energy of the system [26,27].However, the exact formation mechanism for the eventual microstructure and morphology by interaction between the nanoparticles is still unclear at the present time.
The optical properties of the as-prepared BiOBr-4 and BiOBr-1 were characterized by UV-vis diffuse reflectance spectroscopy and the result was shown in Figure 4.For a crystalline semiconductor, the optical absorption near band edge follows the formula (ℎ])  = (ℎ] −   ) where , ℎ, ],   , and  are the absorption coefficient, Plank constant, light frequency, band gap, and a constant, respectively [2,28].BiOBr is an indirect-gap semiconductor;  equals 1/2.The inset of Figure 4 shows the (ℎ]) 1/2 versus (ℎ]) curve of the sample.By extrapolating the straight portion of (ℎ]) 1/2 -to-(ℎ]) plot to the  = 0 point, the band gap of BiOBr-4 and BiOBr-1 was 2.63 and 2.75 eV, which was a key factor in determining its visible-light photocatalytic activity and is close to the value reported in other literatures [15,24].Furthermore, BiOBr-4 showed higher visible-light absorption than BiOBr-1, implying that more light reflection could be achieved within flower-like microspheres assembled with nanosheets.
N 2 adsorption-desorption technique was carried out to investigate the surface area of the BiOBr-10 and BiOBr-120, and the results are given in Figure 5. Type III isotherms were obtained for BiOBr-1 and BiOBr-4.The Brunauer-Emmett-Teller (BET) surface areas of the BiOBr-4 flower nanostructure (28.3 cm 2 /g) are larger than that of BiOBr-4 nanosheets (10.4 cm 2 /g).The large surface area can be due to its flower-like structure, which is beneficial for the diffusion of reactants and intermediates during reaction.Rhodamine B (RhB), a widely used dye, was selected as the model pollutant to evaluate the photocatalytic activity.Its characteristic absorption at about 553 nm has been used to monitor the photocatalytic degradation process [29].The photodegradation of RhB under visible-light irradiation ( > 420 nm) was used as a probe to evaluate the performances of different photocatalysts.Figure 6(a) shows the photocatalytic degradation of RhB on BiOBr sample, presenting the concentration changes of RhB during the degradation process.The degradation of RhB in the absence of photocatalyst is less than 5% at 90 min, which could be neglected.About 93%, close to 100% of RhB, was degraded within 40 min by the sample BiOBr-4, suggesting that the as-synthesized BiOBr hierarchical nanostructure possesses intrinsic photocatalytic activity.As for samples BiOBr-1 and P25, the degradation rate was 75% and 33% of RhB within 90 min.It is evident that the sample BiOBr-4 exhibited higher photocatalytic activity than that of samples BiOBr-1 and P25.It could be attributed to the fact that the hierarchical flower-like morphology assembled with nanosheets in BiOBr-4 with larger BET surface than nanosheet structure of BiOBr-4.Additionally, the hierarchical nanostructures could provide more efficient transportation of reactants in order to improve the photocatalytic activity.
The photocatalysis kinetics of the RhB by BiOBr was obtained by fitting the experimental data to pseudo-firstorder model, and the pseudo-first-order model equation can be expressed by [30], The  0 and  are the concentrations of organic dye in solution at times 0 and , respectively, and  is the pseudofirst-order rate constant.Figure 6 reaction kinetics of RhB degradation in solution on the basis of the data plotted in Figure 6(a).As can be seen, a rather good correlation to the pseudo-first-order reaction kinetics is found.When BiOBr-4, BiOBr-1, and P25 were used as photocatalyst, the pseudo-first-order rate constants are 0.0623 ( = 0.99), 0.0152 ( = 0.99), and 0.0040 ( = 0.99) min −1 , respectively, where the degradation rate constant of BiOBr-4 is four times higher than BiOBr-1 and fifteen times higher than P25.Several reasons may account for the high photocatalytic activity of the sample.First, the BET surface areas of the BiOBr-4 flower nanostructure (28.3 cm 2 /g) are larger than that of BiOBr-4 nanosheets (10.4 cm 2 /g), which lead to more unsaturated surface coordination sites exposed to the pollution molecules in solution.This allows more efficient transport for the reactant molecules to the active sites, hence enhancing the efficiency of photocatalysis.Second, the high photocatalytic activity is not only related to the specific surface area but also associated with hierarchical structure.The rate constant of BiOBr-4 BiOBr-1 is 0.0022 min −1 ⋅m −2 after being normalized by the surface area, which is 1.5 times higher than that of BiOBr-1 (0.00146 min −1 ⋅m −2 ); the reason may be attributed to the fact that the hierarchical microspheres composed of high surface-to-volume ratios of nanosheets are in favor of the transfer of electrons and holes and facilitate the degradation of pollution molecules.
The lifetime of photocatalyst is important for practical applications.The photodegradation of RhB over BiOBr-4 has been repeated 5 times under the same reaction conditions, by repeating the centrifugation of the BiOBr sample after completing the photocatalysis reaction and redispersing into 50 mL RhB solution in each run.In the lifetime test, photodegradation of RhB was 92.3%, 92.0%, 91.6%, 91.8%, and 91.5%, which indicates that BiOBr-4 exhibits nearly similar photocatalysis performance.It proves that BiOBr is stable and suitable for photocatalyst.

Conclusions
In summary, a novel BiOBr hierarchical microsphere has been successfully prepared via a facile microwave-assisted solvothermal route.The obtained BiOBr nanostructure is indirect transition semiconductors with intrinsic optical band gaps of approximately 2.63-2.75eV depending on the synthesis conditions.These BiOBr flower-like nanostructures are more active for photodegradation of RhB dye under visible-light irradiation.Our work provides a facile, rapid, low-cost pathway to novel BiOBr hierarchical nanostructures.

Figure 6 :
Figure 6: (a) Photodegradation efficiencies of RhB as a function of irradiation time; (b) photocatalysis kinetics of the RhB degradation. 0 and  referred to the initial RhB concentration and the RhB concentration determined at different reaction time, respectively.