Photocatalysis is one of the most promising methods owing to its great potential to relieve environmental issue. To construct efficient photocatalyst with low energy consumption, mild catalytic conditions, and stable chemical properties are highly desired. In this work, a novel, highly active and environmental friendly mesoporous photocatalyst Bi 4 O 5 Br 2 /SBA-15 was synthesized by hydrothermal method, and its characteristics and visible-light catalytic activity were investigated. The synthesized photocatalyst consisted of Langmuir type IV hysteresis loops, which was confirmed to be a composite material with mesoporous structure. It exhibited a high visible-light absorption intensity and a low recombination rate of photo-generated electrons and holes. When the mass ratio of Bi/SiO 2 was 30/100 during the synthesis, the obtained photocatalyst (Bi30/SBA-15) reflected the fastest Rhodamine B (RhB) removal rate and achiev ed 100% decolorization of RhB by both adsorption and degradation process. This high decolorization efficiency can also be maintained and realized by recycling the used composite in practice. The enhanced visible-light photocatalytic activity of novel Bi4O5Br2/SBA-15 photocatalyst can be ascribed to the existing active sites both inside and outside SBA-15 which enhanced the separation of photo-generated electrons and holes.
Photocatalytic technology has been considered as a promising method for wastewater treatment [
Further, Bi4O5Br2 can be modified to enhance the separation rate of photogenerated electron and holes by using halogen atoms adjustment [
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 [
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.
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.
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.
To detect the active species produced during the photocatalytic process, such as superoxide radical ( • O 2 − ), 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.
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 [
Samples | Specific area (m2/g) | Pore volume (cm3/g) | Average diameter (nm) |
---|---|---|---|
Bi20/SBA-15 | 156 | 0.26 | 6.6 |
Bi30/SBA-15 | 84 | 0.16 | 6.5 |
Bi40/SBA-15 | 62 | 0.11 | 6.3 |
Bi50/SBA-15 | 51 | 0.10 | 6.0 |
The SEM and TEM images of SBA-15, Bi4O5Br2, and Bi4O5Br2/SBA-15 showed in
In order to study the elemental composition and valence state of the photocatalytic materials, XPS analysis of Bi30/SBA-15 was performed (
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 [
α h v = A ( h v − E g ) n / 2
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 (
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.
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 • O 2 − (
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 (
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
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.
The authors declare no conflicts of interest regarding the publication of this paper.
Shu, Y.J., Yan, S.R., Dong, K.J. and Chen, J.W. (2018) Preparation of Novel Mesoporous Photocatalyst Bi4O5Br2/SBA-15 with Enhanced Visible-Light Catalytic Activity. Open Journal of Applied Sciences, 8, 532-544. https://doi.org/10.4236/ojapps.2018.811043