Controllable Synthesis and Photocatalytic Activity of Nano-BiOBr Photocatalyst

Nano-BiOBr photocatalysts were successfully prepared by hydrothermal synthesis using the ethylene glycol solution. The nanoBiOBr photocatalysts were characterized and investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), photoluminescence (PL), and UV-vis diffuse reflectance spectroscopy (UV-Vis DRS), and the catalytic ability toward photodegradation of rhodamine B (RhB) was also explored. The results showed that the crystallinity of the nano-BiOBr photocatalyst decreased with the increase of the concentration, while it increased with the amount of the applied deionized water. The morphology of the nano-BiOBr photocatalyst changed from microspheres to cubes and then to a mixture of microspheres and flakes with the increasing of the concentration and from microspheres to flakes with the addition of the deionized water. The results indicated that the concentration and solvents have an essential influence on the bandgap energy values of the nano-BiOBr photocatalyst, and photocatalyst showed an excellent photocatalyst activity toward photodegradation of RhB. The degradation yields of photocatalyst decreased with the increase of the concentration and increased with the addition of the deionized water. PL intensity of photocatalyst increased with the increase of the concentration and weakened with the addition of the deionized water.


Introduction
In recent years, the phenomenon of global water pollution has become a more and more severe issue with the rapid development of the economy, which has attracted widespread attention because of the close relationship between water resources and people's daily work and life [1,2]. Many ways can cause water pollution, one of them being the textile industry and wastewaters with organic dye, which are challenging due to their poor biodegradability [3][4][5]. Semiconductor bismuth halide (BiOX, X = Cl, Br, I)-based photocatalysts have attracted extensive attention from researchers because of their unique structure and excellent photocatalytic properties [6,7]. BiOBr was the target material of the presented investigations because of its moderate bandgap, open layered structure, high oxidation ability, indirect transition mode, high visible light response ability, and excellent stability [8,9]. There are many methods to prepare BiOBr, such as high temperature-based solid-state [10], hydrothermal [11], solvothermal [12], water-(alcohol-) based [13], ultrasound-assisted [14], and electrospinning method [15]. Among them, hydro-and solvothermal methods are the most commonly used synthesis pathways. The structure, morphology, crystallinity, and phase formation of the photocatalysts can be effectively obtained through controllable synthesis because of the slow product formation rate, simple and easy ways to control reaction conditions, and stable reaction environment during the water-(solvent-) based thermal reaction [16]. For example, nano-BiOBr microspheres were synthesized previously by the solvothermal method using ethylene glycol (EG) as a solvent [17]. On the other hand, nano-BiOX microspheres were obtained using other solvothermal methods and the same solvent EG [18]. BiOBr/SrFe 12 O 19 nanosheets were synthesized by the solvothermal method using deionized water (DI) as a solvent [19]. AgBr/BiOBr nano-heterostructure-decorated polyacrylonitrile nanofibers were synthesized by electrospinning technique and solvothermal treatment in the presence of an EG solution as the reductant [20]. Therefore, the present paper is aimed at using Bi(NO 3 )·5H 2 O and CTAB as raw materials, with EG and DI as a solvent, under the condition of different concentrations and different solvents to obtain nano-BiOBr photocatalyst. The influences of different solvents and concentrations of precursors on the structure, morphology, optical properties, and photocatalytic activities were also investigated systematically.

Experiment Section
2.1. Synthesis of Nano-BiOBr Photocatalyst. In the first step, 2 mmol of Bi(NO 3 )·5H 2 O is added to 80 ml of EG, and the solution was ultrasonicated until it was completely dissolved (obtaining solution A). Afterward, 2 mmol of CTAB was introduced into solution A, stirred magnetically until it was completely dissolved (solution B). Next, solution B was introduced into a high-temperature reactor (the filling degree is 80%), and after constant temperature reaction at 180°C for 10 hours in an incubator, the solution was naturally cooled to room temperature, and the precipitate was separated. Finally, the precipitate was washed with DI and alcohol, and then, the nano-BiOBr photocatalyst was finally obtained after the drying procedure at 80°C for 12 hours. Table 1 shows the abbreviation and synthesis parameters of nano-BiOBr photocatalyst obtained with different synthesis conditions, abbreviating them with (a)-(d) in the latter stages of the manuscript.

2.2.
Characterization of Nano-BiOBr Photocatalyst. The crystalline phases were determined by a Bruker D8 Advance X-ray diffractometer (XRD) using a Cu Kα (λ = 0:15418 nm) radiation in the θ~2θ Bragg. The morphologies of the asprepared samples were observed and investigated by a field emission scanning electron microscope (FESEM, Nova NanoSEM 450, FEI). The UV-vis absorption spectra of the samples were measured with a Cary 5000 (Agilent, USA). The photoluminescence (PL) spectra were observed with a He-Cd laser 280 nm.

Determination of the Photocatalytic Activity of the
Nano-BiOBr Photocatalyst. The photocatalytic reactions were carried out in a CEL-LAB500E4 multisite photochemical reaction system. The catalytic activity of the target degradation product was evaluated by using nano-BiOBr photocatalyst under the visible light source. In a typical photocatalytic experiment, 0.05 g of the nano-BiOBr photocatalyst was dispersed into 50 ml of RhB (10 mg/l) solution and magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium between RhB and the nano-BiOBr photocatalyst. Then, the light source was turned on, and a sample of 4 ml of the suspension was continually taken from the reaction cell at every 15 minutes and centrifuged. Finally, the absorbance of the supernatant at the maximum absorption wavelength was analysed through an ultraviolet-visible spectrophotometer (UV-1901). The degra-dation efficiencies were calculated according to the expression of degradation rate (1 − A/A 0 ), where A 0 is the absorbance of the target degradation at its maximum absorption wavelength before illumination, and A is the absorbance value after illumination for a certain time.

Results and Discussion
3.1. XRD Analysis. The XRD patterns of nano-BiOBr photocatalyst at different concentrations and different solvents are shown in Figure 1. It is clear that the prepared nano-BiOBr photocatalysts were consistent with the standard diffraction pattern of tetragonal BiOBr (PDF#85-0862) (Figure 1(a)). No other specific diffraction peaks were detected, indicating that the nano-BiOBr photocatalysts prepared in different concentrations and different solvents are pure tetragonal nanoparticles. The intensity of the diffraction peak is weakened with the increase of the concentration, indicating that the concentrations are the key factor affecting the crystallinity of nano-BiOBr photocatalyst, 2-theta was right-shifted, and the crystal space d decreased according to the Prague equation (d sin θ = nλ). The intensity of the diffraction peak increases with the increase of DI, indicating that the increase of DI throughout all the experiments is beneficial to increase the crystallinity of nano-BiOBr photocatalyst; the influence factors were more, and the main reason remains to be further studied.
In order to study the stability of the nano-BiOBr photocatalyst, the XRD of the nano-BiOBr photocatalyst after the photocatalytic reaction was investigated (Figure 1(b)). Compared with the results before the photodegradation, there are no noticeable changes in the crystal phases of the samples. Figure 2 shows the SEM images of nano-BiOBr photocatalysts prepared using different concentrations and different solvents. As shown in Figure 2(a), BiOBr microspheres with a diameter of~2 μm were obtained, and the BiOBr microspheres are self-assembled from irregular nanosheets with a thickness of~10 nm. From Figure 2(b), BiOBr cubes of~2 μm are obtained, BiOBr being also selfassembled from irregular nanosheets in a fixed manner. Compared with (a) photocatalyst, (b) photocatalyst is selfassembled from nanosheets more densely, and the thickness of the nanosheets is higher (~20 nm). As shown in Figure 2(c), irregular sheetlike nano-BiOBr was obtained; compared with (b) photocatalyst, (c) photocatalyst is a nanosheet with a thickness of~15 nm. From Figure 2(d), it can be observed that a mixture of BiOBr microspheres and sheetlike BiOBr is obtained. BiOBr microspheres are tightly assembled from irregular nanosheets with a thickness of~15 nm. Compared with (b) photocatalysts, (d) photocatalysts have more unassembled nanosheets, and the thickness of the nanosheets is smaller and assembled in a way that cubes become microspheres. The results showed that the morphology of the nano-BiOBr photocatalyst changed from microspheres to cubes and then to a mixture of microsphere and flakes with the increasing of the concentration of precursors. Moreover, the morphology of nano-BiOBr photocatalyst changed from microspheres to flakes with the addition of DI.

SEM Analysis.
3.3. UV-Vis DRS Analysis. Figure 3 shows the UV-vis diffuse reflectance spectra and corresponding bandgap energies of the nano-BiOBr photocatalyst. It can be observed that the absorption edges of (a)-(d) photocatalysts can be found at 439, 450, 446, and 453 nm, respectively, indicating that the absorption wavelength range of (d) photocatalyst is the largest, and the absorption wavelength range of (a) photocatalyst is the smallest, meaning that, more visible light can be absorbed with the increasing of the concentration; the absorbed visible light is reduced with the addition of DI.  Journal of Nanomaterials The position of the absorption edge is closely related to the forbidden bands of the semiconductor photocatalyst. The forbidden bandwidth of the BiOBr photocatalyst is calculated by Equation (1) [21].
where α, h, ν, A, and Eg represent the intrinsic absorption coefficient, the Planck constant, the frequency of light, the proportion constant of photocatalyst, and the bandgap width of semiconductor, respectively. n = 2 for direct bandgap semiconductor, and n = 4 for indirect bandgap semiconductor. n = 4 because BiOBr photocatalyst is an indirect bandgap semiconductor. Using the formula on the hν~ðαhνÞ 1/2 curve, as shown in Figure 3 Figure 4 shows the UV-vis absorption spectral changes of RhB aqueous solution vs. irradiation time in the presence of the (a)-(d) photocatalysts. With the increase of the irradiation time, the absorption maximum of the spectra declined, and the band shifted to a smaller wavelength, which indicates N-demethylation and the destruction of the conjugated structure in the RhB photodegradation process [22]. The blue-shifted band implies that the main photocatalytic degradation path of the RhB is N-demethylation. The major peaks are reduced gradually during visible light irradiation, indicating a step-by-step degradation of RhB. The RhB UVvis absorption spectra of the (a) photocatalyst were approximately a straight line after 60 min of illumination, indicating that the photocatalytic reaction of the (a) photocatalyst to RhB was mainly completed. The absorption peak of RhB in the visible light region of (a), (c), and (d) photocatalysts has completely disappeared after 80 min, and the decolourisation efficiency reaches 100%. The RhB solution photodegraded using (b) photocatalysts had a weak absorption peak at the end of the process. The findings are in accordance with the results obtained from previous sections, as the degradation efficiency of photocatalyst decreases with the increase of the concentration. On the other hand, the degradation efficiency of photocatalyst increased with the addition of DI. Figure 5 shows the degradation performance for RhB by photocatalysts prepared in different concentrations and different solvents. The results showed that the degradation per-formance of photocatalyst declines in varying degrees with the increase of the concentration, which indicates that the concentration can change the degradation performance of the photocatalyst. The degradation performance of photocatalyst improves with the addition of DI. The degradation performance of (a)-(d) photocatalysts to RhB was 96.3%, 68.8%, 88.1%, and 81.3%, respectively, after 60 min of light irradiation. It can be summarised that (a) photocatalyst showed the highest photocatalytic activity for RhB.

Degradation Efficiency for RhB of BiOBr Photocatalysts.
3.5. PL Analysis. Nano-BiOBr photocatalyst using different concentrations and different solvents were characterized by photoluminescence (PL) spectroscopy to verify further the conclusions mentioned above. The excitation wavelength was 280 nm, and the emission range was 400 nm to 600 nm, as shown in Figure 6. The movement, transfer, and recombination rate of photogenerated electron-hole pairs were revealed by PL spectra. The lower the intensity of emission peaks in PL spectra, the higher the separation efficiency of electrons and holes in semiconductors and the higher the photocatalytic activity of photocatalysts that were observable [23]. It can also be seen that the PL intensity increases with the increase of the concentration, while the photocatalytic activity decreases. As can also be observed, the PL intensity is weakened with the addition of DI and the photocatalytic activity increased. It can be seen from the figure that all of the photocatalysts have emission peaks at 437 nm, 449 nm, and 466 nm and the order of luminous intensity is , which means that the recombination rate of electrons and holes of the four materials decreases gradually and the photodegradation performance is gradually enhanced, which is in accordance with the previous photocatalytic experiment results.

Conclusion
Nano-BiOBr photocatalysts were successfully prepared by the hydrothermal method. The influences of different concentrations and different solvents on structure, morphology, optical properties, and photocatalytic properties are investigated systematically. The results show that the crystallinity of the nano-BiOBr photocatalyst decreases with the increase of the concentration and increases with the increase of the DI. The morphology of the nano-BiOBr photocatalyst changes from microspheres to cubes and then to a mixture of microsphere and flakes with the increasing of concentration, from microspheres to flakes with the addition of DI. It was also proved that the concentration and solvents have an essential influence on the bandgap energy values of the nano-BiOBr photocatalyst. The degradation performance of photocatalyst with the decline in the increase of concentrations improves with the addition of DI. The semiconductor (a) showed the highest photocatalytic activity toward RhB. PL intensity of photocatalyst increased with the increase of the concentration and weakened by the addition of DI. Therefore, the high photocatalytic activity of photocatalysts for contaminant aqueous solution makes this research a new platform to develop flexible photocatalyst for practical applications in water purification.   Journal of Nanomaterials

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.