Photocatalytic technology has been considered as a promising method for wastewater treatment . Highly efficient photocatalysts with low energy consumption, mild catalytic conditions, and stable chemical properties are most feasible in practice. Recently, bismuth-based photocatalyst has aroused much attention because of the excellent optical properties and visible-light photocatalytic activity  , such as Bi2MoO6  , BiWO4  , Bi4Ti3O12  , BiOX  (X = F, Cl, Br, I), Bi4O5Br2  and so on. Among them, Bi4O5Br2 is a new type of bismuth-based semiconductor photocatalytic material, and possesses good photoelectric properties. The crystal structure of Bi4O5Br2 is similar with BiOX, which belongs to the tetragonal crystal form . Bi4O5Br2 has a layered structure composed of [Bi2O2]2+ and Bi3+. The layers are respectively distributed with X− and , with the lower layer being Bi3+ and the next layer being . X− with the is arranged in a staggered 45 degree. In the layered crystal structure of Bi4O5Br2, the halogen ion layer and the [Bi2O2]2+ ion layer are alternately arranged along the c-axis, with Van der Waals force between the two halogen ion layers allowing the layered structure to dissociate in the easy  direction. There is a strong binding force in the [Bi2O2]2+ layer, and the dissociation of the double halogen ion layer has an electric field, thus the photoelectron-hole separation efficiency of Bi4O5Br2 surface can be greatly improved.
Further, Bi4O5Br2 can be modified to enhance the separation rate of photogenerated electron and holes by using halogen atoms adjustment    and breaking the stoichiometric layered structure to form a nonstoichiometric layered structure     , such as Bi4O5Br2   , Bi3O4Br  and Bi24O31Br10 . Compared with other bismuth-based photocatalyst, Bi4O5Br2 has some advantages such as special facets, excellent catalytic activity, high separation rate of photo generated electron and holes, environmental friendliness and simple synthesis methods  .
However, the shortcomings of small specific surface area, poor thermal stability, difficulty in recovery and agglomeration for Bi4O5Br2 should be overcome, and immobilization of Bi4O5Br2 provided a feasible method. Activated carbon, ZrO2, SiO2 and zeolite molecular sieves had been used as carrier for the immobilization of semiconductors. Among these materials, disordered porous structure and stability limited the application on photocatalyst modification. SBA-15 is a kind of silicon-based molecular sieves with self-ordered mesoporous material, which has uniform pore size, large pore structure, high surface area and good stability in acid and alkaline conditions  . However, no report could be found on SBA-15 supported Bi4O5Br2 semiconductor composite photocatalysts. Moreover, afterBi4O5Br2 loading on SBA-15, the photoelectron migration on the surface of the photocatalyst and the recombination rate of photogenerated electrons and holes have not been studied yet.
In this study, Bi4O5Br2/SBA-15 photocatalytic material was synthesized by sol-gel method, i.e. Bi4O5Br2 was grafted onto the surface of SBA-15. The morphology, specific surface area, loading ratio and optical properties of synthesized Bi4O5Br2/SBA-15 were characterized and the visible light catalytic activity was investigated by using Rhodamine B (RhB) as a target contaminant, the different decolorization capacities of the synthesized photocatalysts (obtained under different loading ratio of Bi4O5Br2/SBA-15) were investigated. At the same time, the photocatalysis stability and reusability of the newly synthesized Bi4O5Br2/SBA-15 was evaluated.
2. Experimental Section
2.1. Synthesis of Bi4O5Br2/SBA-15 Mesoporous Materials
Bi4O5Br2/SBA-15 was prepared through the following two steps. The first step was to synthesize Bi4O5Br2 with solvothermal method. Typically, under stirring at 100 rpm, cetyl trimethyl ammonium bromide (CTAB, 1.5 mmol) and Bi(NO3)3∙5H2O (1.5 mmol) were added and dissolved in the glycerol solution (15 mL glycerol and 6 mL deionized water), and the stirring lasted for 30 min. Then after 0.1 g polyethylene glycol-10,000 (PEG-10,000) being added to the mixture with pH adjusted to 11.0, the obtained mixture was solvothermally treated at 140˚C for 24 h. The solid obtained after centrifugation was washed with water and ethanol for 5 times, and then dried at 50˚C for 12 h.
In the second step, Bi4O5Br2/SBA-15 was prepared by sol-gel method. In a typical run, 2.0 g polyether P123 (PEO-PPO-PEO) was mixed with 65 mL deionized water at room temperature under stirring for 3 h. 10 ml HCl (2 mol/L) was then dropped into the above mixture at 40˚C under stirring for 1 h. The synthesized Bi4O5Br2 in the first step was added into the above mixture at a different mass ratio of Bi/SiO2 of 20/100, 30/100, 40/100, and 50/100. After stirring for 1 hour, 4.5 g tetraethyl orthosilicate (TEOS) was slowly dropped into the mixture at 4˚C under stirring for 24 h. The obtained mixture was later solvothermally treated at 140˚C for 24 h. The mixture was centrifuged, washed, and dried at 60˚C for 8 h. The obtained solid samples were calcined at 550˚C in air atmosphere for 6 h, which were labeled as Bi20/SBA-15, Bi30/SBA-15, Bi40/SBA-15, and Bi50/SBA-15, respectively.
X-ray diffraction (XRD) measurement was conducted on a D/max-2550 PC X-ray diffractometer. Scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM)) were also used to characterize the obtained photocatalysts. The optical diffuse reflectance spectra (DRS) were acquired with a UV-vis-NIR scanning spectrophotometer. The photoluminescence (PL) spectra were detected by using a spectrophotometer (FS5, England). The Brunauer-Emmett-Teller (BET) surface area was determined by a micromeretics ASAP 2010 apparatus with a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05 - 0.3. The desorption isotherm was used to determine the pore size distribution according to the Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron spectra (XPS) analysis was performed on a PHI-5400 instrument with Mg Kα as the X-ray source under a pressure of 1.33 × 10−7 Pa and a step of 0.05 eV. The C (1s) level (285.0 eV) was taken as the reference binding energy.
2.3. Photocatalytic Activity Measurement
RhB solution (10 mg/L) was used to evaluate the photocatalytic activity of the obtained composites under visible light irradiation. A 300 W xenon lamp with a 420 nm cut-off filter provided visible light irradiation. A same photocatalyst dosage of 0.2 g/L was used for all the tests. Prior to visible light irradiation, the mixtures of RhB and photocatalyst were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption-desorption equilibrium of RhB on the catalyst surface. Then, the solution was irradiated by xenon lamp. At given irradiation time intervals, 4 mL of the mixtures were collected and centrifuged at 10,000 rpm for 10 min to remove the photocatalysts. The residual concentration of RhB was analyzed with a 725N UV-v is spectrophotometer. The RhB solution temperature was controlled at 19˚C ± 2˚C during the whole experiment.
2.4. Active Species Trapping Experiment
To detect the active species produced during the photocatalytic process, such as superoxide radical ( ), hole (h+) and hydroxyl radical (•OH), active species trapping experiments were carried out by adding scavenger into different RhB solutions. The scavengers were p-benzoquinone (p-BQ), sodium oxalate (SO), and iso-propanol (IPA), respectively. The dosage of each scavenger was 1.0 mmol/L.
3. Results and Discussion
3.1. Characterization of Bi4O5Br2/SBA-15 Composites
Figure 1(a) showed the small angle XRD pattern of SBA-15 with different loading ratio of Bi4O5Br2. It can be seen that all the synthesized samples have a strong diffraction peak (2θ = 0.90˚ to 1.0˚) and two weak diffraction peaks (2θ = 1.3˚ to 1.8˚) in the range of small angle (2θ = 0.3˚ to 5˚). These characteristic peaks are
Figure 1. Small angle XRD (a) and wide angle XRD (b) of the synthesized materials.
coincident with the crystal plane diffraction peaks of (100), (110), and (200) in the two-dimensional hexagonal structure of SBA-15, respectively. This observation reveals the presence of a typical two-dimensional hexagonal ordered mesoporous structure  , further indicating that the loading of Bi4O5Br2 did not change the mesoporous structure of SBA-15. The diffraction peaks shifted to higher angles from Bi20/SBA-15 to Bi50/SBA-15, meaning that their pore diameters gradually decreased along with the increase of loading ratio, and Bi20/SBA-15 has the largest pore diameter . On the other hand, the diffraction peak intensity was first increased and then decreased, with the intensity of Bi30/SBA-15 diffraction peak being the highest. This observation indicated that the mesoporous structure of the synthesized Bi30/SBA-15 is the most regular among the synthesized samples, since the high intensity of peaks is attributable to the incorporation of Bi into the mesoporous silica matrix, creating new crystal facets on mesoporous structure .
Figure 1(b) showed the wide angle XRD patterns of the synthesized Bi4O5Br2/SBA-15 samples. As seen, all the samples had similar characteristic peaks, and the corresponding positions of the characteristic peaks were 11.2˚, 25.0˚, 31.5˚, 39.2˚, 46.2˚, 53.5˚, 57.2˚, 67.1˚, and 76.0˚, respectively. These peaks were corresponding to the tetragonal phase of Bi4O5Br2 (standard card JCPDS No. 09-0699), which were consistent with the following crystal planes of Bi4O5Br2 (001), (011), (012), (110), (112), (020), (211), (220), and (130), respectively. Especially, the strongest diffraction peak intensity was detected at 31.5˚, indicating that the tetragonal Bi4O5Br2 grows along with the (012) crystal plane.
Table 1 summarized the specific surface area, pore volume and average pore size of the composites. The pore diameters of Bi20/SBA-15, Bi30/SBA-15, Bi40/SBA-15, and Bi50/SBA-15 firstly increased and then decreased. This variation trend was consistent with the results from the small angle XRD. With the increase of Bi4O5Br2 loading ratio, both specific surface area and pore volume of the synthesized photocatalysts showed a decreasing trend, possibly due to the relatively bigger size of Bi4O5Br2 particles which are easy to block a part of the pores. This phenomenon implied that the adsorption capacity of Bi4O5Br2/SBA-15 might gradually decrease when compared to SBA-15 . Figure 2 illustrated the nitrogen sorption-desorption isotherm curves of the synthesized composites. All the curves exhibited the Langmuir type IV hysteresis loops, suggesting that the synthesized Bi20/SBA-15, Bi30/SBA-15, Bi40/SBA-15, and Bi50/SBA-15 have the characteristics of mesoporous materials with ordered mesoporous structures.
Table 1. Surface area, pore volume and average pore size of the synthesized materials.
Figure 2. Nitrogen adsorption-desorption isotherms of the samples (Notes: Ds and As denotes adsorbed and desorbed).
The SEM and TEM images of SBA-15, Bi4O5Br2, and Bi4O5Br2/SBA-15 showed in Figure 3. The image of pure SBA-15 clearly has well-ordered mesoporous hexagonal arrays (Figure 3(a)) and one-dimensional channels of the mesoporous structure (Figure 3(d)) with a diameter of 7.3 nm. The results demonstrated the high ordering level obtained from silica synthesis and confirmed the pore size distribution . Figure 3(b) and Figure 3(e) showed the morphology of Bi4O5Br2 with special nanosheets. No diffraction spots or diffraction rings were observed in Figure 3(d), revealing the non-crystallinity silica characteristics of SBA-15. Figure 3(c) and Figure 3(f) show the images of Bi30/SBA-15 sample. Smaller particles were found to spread over larger particles in all the images, and the larger particles were identified as SBA-15 silica because of their characteristic lines of mesoporous material channels. The smaller particles spread on the SBA-15 reflected the same texture as the larger particles, indicating that they belong to the same type of materials. These agglomerations of nanoparticles were mainly in the range of 100 nm. This observation is consistent with the Bi4O5Br2 phase from the XRD patterns. However, as it can be seen from Figure 3(c) and Figure 3(f), Bi4O5Br2 as rods or spheres were distributed on the surface of the SBA-15. Figure 3(c) and Figure 3(f) also showed that Bi4O5Br2 might be chemically bonded to the surface of SBA-15, not simply loaded on the surface, which may produce stable photocatalytic components thus exhibit photocatalytic activity, thereby increase the possibility of recirculation of the photocatalytic material.
In order to study the elemental composition and valence state of the photocatalytic materials, XPS analysis of Bi30/SBA-15 was performed (Figure 4). The survey XPS spectrum of Bi30/SBA-15 confirmed the existents of Si, C, O, Bi, and Br elements. The appearance of element C may be caused by CO2 from the sample absorption of air during the analysis . In Figure 4(b), the peak at 103.6 eV is corresponding to Si 2p spectrum, which is the characteristic peak of Si-O-Si band in SiO2 of SBA-15 and the characteristic peak of Si4+   . A high resolution energy spectrum of O 1s with two main peaks (Figure 4(c)),
Figure 3. SEM images of SBA-15 (a); Bi4O5Br2 (b) and Bi4O5Br2/SBA-15 (c); and TEM images of SBA-15 (d); Bi4O5Br2 (e) and Bi4O5Br2/SBA-15 (f).
Figure 4. XPS spectra of Bi30/SBA-15: full spectrum (a); Si 2p (b); O 1s (c); Bi 4f (d); and Br 3d (e).
530.6 and 533.0 eV, respectively, was obtained. The main peak 530.6 eV is assigned to the lattice oxygen of Bi-O bond in [Bi2O2]2+ in the layered Bi4O5Br2/SBA-15, while the peak at 533.0 eV is chemisorbed oxygen and OH− adsorbed on the surface of the material . The Bi 4f spectrum displayed in Figure 4(d) contains two peaks at binding energy of 159.4 and 164.8 eV, which is attributed to Bi 4f7/2 and 4f5/2  . Figure 4(e) showed that the high-resolution energy spectrum belongs to Br 3d. As its two partial peaks are 68.4 and 69.5 eV     , respectively, and its corresponding spectra were 3d5/2 and 3d3/2, thus its valence was −1. Results of the XPS spectra indicated that the measured elements corresponded to the elements in the synthesized samples.
Figure 5 showed the UV-vis DRS of Bi4O5Br2/SBA-15 at different loading ratio of the two major materials. As seen, all the samples had a certain absorption under UV light and the absorption efficiency of Bi20/SBA-15 was the highest. As shown in Figure 5(a), all the composite samples exhibited significant absorption sidebands in the visible light range around 410, 415, 430, and 435 nm, respectively. In addition, with the increase of the loading ratio, the absorption wavelength of visible light by the photocatalytic material was red-shifted, and the light response range was expanded. Among them, the bandgap of Bi50/SBA-15 was determined to be the narrowest, resulting in the highest utilization rate of visible light. This could be further analyzed by using the Kubelka-Munk formula  :
where α is the light absorption index, namely Abs, h is the Prandtl constant, v is the frequency, A is the constant, Eg is the semiconductor band gap energy. Since Bi4O5Br2 is an indirect bandgap semiconductor, n = 2. Eg = hv = 1240/λ, the unit is electron volts (eV), Eg is the abscissa, (αhv)1/n was plotted on the ordinate, and band gap energy spectra of different samples were obtained (Figure 5(b)). It can be seen from Figure 5(b) that the bandgap energy of the samples decreased from about 3.30 eV to 3.10 eV with the increase of Bi4O5Br2 loading ratio, and this may be caused by the agglomeration of nanoparticles .
Figure 5. UV-vis absorption spectrogram (a) and (αhv)1/2-(hv) curves (b) for different synthesized materials.
3.2. Evaluation on Photocatalytic Performance
Figure 6(a) demonstrated the photodegradation curves and degradation curves for Bi20/SBA-15, Bi30/SBA-15, Bi40/SBA-15, and Bi50/SBA-15 at 25˚C, respectively. The abscissa was the reaction time for dark adsorption and photodegradation, and the ordinate was the concentration of RhB remained. From Figure 6(a), it can be seen that the Bi4O5Br2 at different loading ratio possessed strong absorption capacity for RhB, which could remove RhB from 30% to 65% within 1 h of adsorption in dark, following a descending order of Bi30/SBA-15 > Bi20/SBA-15 > Bi40/SBA-15 > Bi50/SBA-15. The residual RhB in solution was removed by photodegradation under visible light. During the whole reaction process, the removal of RhB with Bi30/SBA-15 was the fastest. Although the band gaps of Bi50/SBA-15 and Bi40/SBA-15 were narrower than that of Bi30/SBA-15, the RhB removal rates for Bi50/SBA-15 and Bi40/SBA-15 were lower than that for Bi30/SBA-15. It might be the structure of SBA-15 was destroyed with big size nanoparticles which blocked the porous for adsorption of RhB onto the surface of SBA-15.
Figure 6(b) showed the fluorescence spectra of synthesized materials at different loading ratio of Bi4O5Br2. The test was conducted at room temperature with the excitation wavelength of 250 nm. It can be seen that the peak shapes and positions of the four samples are similar, with the largest peak around 432 nm and another two weak peaks, indicating that the Bi4O5Br2/SBA-15 was excited at 432 nm by the sunlight with the occurrence of electronic transitions and generation of photogenerated electron-hole pairs. Photoluminescence spectroscopy is an important approach to characterize the photoelectron-hole recombination rate of photocatalysts. The recombination rate of photoelectron-holes is proportional to the light intensity. Therefore, the photogenerated electron-holes of photocatalysts can be compared by using the light intensities. The light intensity of Bi50/SBA-15 was the largest, followed by Bi40/SBA-15, Bi30/SBA-15, and Bi20/SBA-15. The separation effect of Bi30/SBA-15 was relatively high, which might be the major reason for its better photocatalytic effect than others. The
Figure 6. Photocatalytic performance (a) and photoluminescence spectra (b) of the tested samples.
reason was the active site was present inside and outside the SBA-15 first increasing with Bi4O5Br2 loading increase, which indicated the separation rate of electron and hole was enhanced, and then decreased which explored that the active site decrease with excess loading of nanoparticles of Bi4O5Br2 blocked the porous.
3.3. Active Species and Stability Analysis
Bi4O5Br2 is a lamellar structure composed of a [Bi2O2]2+ layer and a double Br-layer interlaced, with an internal electrostatic field established between the layers. When a semiconductor was excited under visible light irradiation, photocurrent could be formed if the photoelectrons move in a fixed direction. A stronger photocurrent indicated more effective separation of electron-holes and higher photocatalytic performance of the semiconductor as well. In the photocatalytic decolorization of RhB, the capture agents SO, IPA and p-BQ were individually added to capture the active species h+, •OH and (Figure 7(a)). As shown in Figure 7(a), the photocatalytic activity of Bi30/SBA-15 slightly decreased when the capture agent IPA was added. In the case of capture agent p-BQ, the photocatalytic activity of the synthesized photocatalyst was significantly reduced after its addition in the photocatalytic process. When adding SO, the photocatalyst showed almost no effect on the decolorization of RhB, implying the catalytic activity was significantly inhibited. The result indicated that h+ played the main role in RhB degradation under visible light.
For evaluation on the stability of the photocatalytic activity of the prepared Bi4O5Br2/SBA-15, sample Bi30/SBA-15 was selected to remove RhB for 5 times (Figure 7(b)). Results showed that after 5 cycles of reutilization of Bi30/SBA-15, its photocatalytic activity remained almost unchanged, and about 99.42% RhB was removed.
A sol-gel thermal method was used to synthesize the Bi4O5Br2/SBA-15 photocatalytic materials. Bi4O5Br2 loading on the SBA-15 had some positive effects on
Figure 7. Effects of various scavengers (SO, IPA and p-BQ) on RhB removal with Bi30/SBA-15 (a) and cycle runs of Bi30/SBA-15 for RhB decolorization (b).
RhB degradation. The photocatalytic activity of Bi4O5Br2/SBA-15 firstly increased and then decreased with the increase of Bi4O5Br2 loading. The highest RhB removal rate was obtained by using Bi30/SBA-15. The reusability of the catalyst Bi30/SBA-15 was investigated. This work provided a new thought for designing the photocatalyst with high efficiency and stability for environmental remediation.