Controllable Fabrication of Ordered Mesoporous Bi2WO6 and Its High Photocatalytic Activity under Visible Light

Ordered mesoporous Bi 2 WO 6 was fabricated by nanocasting technique using SBA-15 as the template. The effect of the dosage of SBA-15 on the formation of the ordered structure and the photocatalytic ability of mesoporous Bi 2 WO 6 was discussed. It was confirmed that the ordered mesoporous structure was obtained as the dosage of SBA-15 was 0.3 g. It was found that, compared to Bi 2 WO 6 , the RhB degradation rate with ordered mesoporous Bi 2 WO 6 was enhanced under visible light (λ > 400 nm) by the photocatalytic measurements. The enhanced photocatalytic performance of ordered mesoporous Bi 2 WO 6 was attributed to its particular ordered mesoporous structure which could increase the light-harvesting efficiency, reduce the recombination of the photogenerated charge carriers, and promote the surface reaction.


Introduction
Photocatalytic degradation of organic compounds for the purpose of purifying wastewater from industries and households has attracted much attention in recent years [1].Heterogeneous photocatalysis is a common advanced oxidation approach for the removal of hazardous organic compounds from wastewater [2][3][4].Photocatalysts can harness solar energy to drive useful redox chemistry and therefore are of considerable interest for environmental pollutant treatment [5][6][7].Recently, the research of visible-light-driven photocatalysts has drawn the interest of a number of researchers [8][9][10].Bi 2 WO 6 , with a narrow band gap of 2.7 eV is one of the most attractive materials because of its high stability, nontoxicity, and wide solar response [11][12][13].However, just as other photocatalysts, it suffers from the low quantum efficiency.Therefore, some attempts have been devoted to improve the photocatalytic performance of Bi 2 WO 6 [14,15].
Porous materials are of great interest in the application in photocatalysis, owing to their high surface area, which is a basic requirement for an efficient photocatalyst both to enhance the adsorption of photos and reactants and to offer a large number of reactive sites [16][17][18].An ordered porous structure is highly desirable for effective photocatalysis due to their larger surface area and multiple aligned pores, which can further assist the electron/energy transfer within the porous framework [19][20][21][22].
The nanocasting pathways developed over the last ten years, which use hard templates to create ordered replicas and provide promising routes for the preparation of mesostructured materials with novel framework compositions.Yang et al. [23][24][25] have demonstrated that ordered mesoporous silicas, such as two-dimensional (2D) hexagonal (p6 mm) SBA-15 and three-dimensional (3D) bicontinuous cubic (Ia3d) KIT-6, can be used as a hard template to fabricate various ordered crystalline metal oxides.Nanocasting is processed by filling the void of the template with a precursor of the material, subsequent transform of the precursor to the material, and final removal of the template; a replica structure can be obtained [26].The quantity rate of template to the precursor can affect the structure and the morphology of the materials.
Herein, taking SBA-15 as the template, ordered mesoporous Bi  to get enhanced photocatalytic ability under visible light.The effect of the dosage of SBA-15 on the formation of the ordered structure and the photocatalytic ability of mesoporous Bi 2 WO 6 was discussed.RhB, a common pollutant in the industry wastewater, was chosen as a test substance to evaluate the photocatalytic performance of the as-prepared samples under visible light.Relative pressure (P t /P 0 )

Materials and Methods
Relative pressure (P t /P 0 ) Relative pressure (P t /P 0 ) Relative pressure (P t /P 0 ) Relative pressure (P t /P 0 ) the photocatalyst (0.10 g) was added to 100 mL RhB aqueous solution (5 mg/L).During each photocatalytic experiment, 5 mL of the suspension was collected at predetermined time intervals.The suspension was centrifuged at 9500 rpm for 10 min, and the concentration of RhB in the supernatant was analyzed by measuring the absorbance at  = 553 nm with a Shimadzu UV2000 spectrophotometer.

Results and Discussion
3.1.XRD Analysis.The measurements of WXRD and LXRD patterns were performed to identify the crystalline phase and mesoporous ordering of as-prepared samples (see Figure 1).The WXRD patterns of Bi 2 WO 6 , S 1 , S 2, and S 3 were shown in Figure 1(a).The crystal forms of the Bi 2 WO 6 and mesoporous Bi 2 WO 6 could be identified to the orthorhombic type according to JCPDS card number 73-2020.And no other peaks were found in the spectra of S 1 , S 2, and S 3 , indicating that no other crystal type was formed, and the template of SBA-15 was completely removed.One obvious peak (2 = 0.80 ∘ ) and a little peak (2 = 0.98 ∘ ) in the LXRD pattern of S 2 could be found, inferring that the periodically ordered structure of S 2 was obtained.However, no ordered mesoporous Bi 2 WO 6 was found in S 1 and S 3 , based on the result that there were no obvious peaks in the LXRD spectra of them.The results indicated that the dosage of SBA-15 could affect the formation of the ordered mesoporous structure.structure was found in S 1 and S 3 , but ordered replicas of SBA-15 were found in S 2 .The HRTEM image of S 2 was presented in Figure 3(f).It could be measured that the  spacing was 0.320 nm, which matched well to the lattice spacing of (113) of orthorhombic type Bi 2 WO 6 according to JCPDS card number 73-2020.Furthermore, some big particles with nonporous structure were found in S 1 , and there were some larger pores in S 3 .The ordered porosity of the S 2 resulted from interparticle voids.In the filling process, the pores of SBA-15 were completely filled.After the template was removed, the interparticle would construct the ordered mesoporous structure.SBA-15 dosage of S 1 was only 0.15 g, which was lack of enough pores to make the Bi 2 WO 6 particles highly be organized.As a result some particles gathered together.And for the S 3 with 0.6 g SBA-15, there were too much pores for the precursor of Bi 2 WO 6 to permeate.The distance of the particles would be larger than the pore size of SBA-15.The Bi 2 WO 6 particles maybe gathered after the template was removed.So many pores may make the Bi 2 WO 6 arrange irregularly.

Optical Absorption Ability.
The photo absorption abilities of the samples were detected by UV-vis absorption spectra, and the results were shown in Figure 4(a).All the samples showed intense absorption in the region from 200 to 450 nm.An obvious red shift of band gap edge was found in the spectra of mesoporous Bi 2 WO 6 .The band gaps of Bi 2 WO 6 and S 2 were determined to be 2.72 and 2.59 eV, respectively (Figure 4 Because Bi 2 WO 6 is the directly allowed optical transition semiconductor,  in the function equals 1.The narrower band gap of S 2 meant that S 2 could be excited by more photos and had enhanced light absorption ability. 3.4.FL Spectra Analysis.FL analysis was used to reveal the separation efficiency of the photogenerated electrons and holes in semiconductors, and the results were shown in Figure 5.The lower peak indicated lower recombination rate of them.The FL intensity of Bi 2 WO 6 was the biggest one, inferring that Bi 2 WO 6 had the highest recombination rate of photogenerated charge carriers.The results confirmed that the separation of photogenerated charge carriers could be improved through the construction of mesoporous Bi 2 WO 6 , leading to the enhancement of photocatalytic activity.The FL intensity of S 2 got the lowest one among those of mesoporous Bi 2 WO 6 , indicating that S 2 should have higher photocatalytic ability.in Figure 6(a).In 30 min, only 23.8% of RhB was removed with Bi 2 WO 6 .However, RhB removal rate with S 1 , S 2 , and S 3 was 69.2%, 91.5%, and 39.0%, respectively.The results indicated that the photocatalytic ability of Bi 2 WO 6 could be enhanced by constructing mesoporous structure.And the photocatalytic ability of S 2 was higher than those of S 1 and S 3 , inferring that the ordered mesoporous structure could further enhance the photocatalytic performance of Bi 2 WO 6 .Because of the ordered mesoporous structure, the harvesting of incident light of S 2 became more efficient; the separation efficiency of photogenerated charge carriers was enhanced.Thus, an increased number of electrons and holes would arrive at the surface of Bi 2 WO 6 particles, participating in oxidative reactions with RhB.High surface area could also provide more active sites for oxidative reactions and subsequently promote RhB removal.The adsorption performance of RhB of S 2 was measured, and the result has been presented in Figure 6(b).The removal rate of RhB (26.0%) was largely lower than that in photocatalytic process, indicating that ordered mesoporous Bi 2 WO 6 had excellent photocatalytic ability.

Conclusions
Ordered mesoporous Bi 2 WO 6 was successfully synthesized by nanocasting technique using SBA-15 as the template.The dosage of SBA-15 could affect the formation of ordered structure of mesoporous Bi 2 WO 6 .The as-prepared ordered mesoporous Bi 2 WO 6 exhibited an excellent photocatalytic decomposition of RhB under visible light irradiation.The high photocatalytic activity could be ascribed to its particularly ordered mesoporous structure which could increase the light-harvesting efficiency, reduce the recombination of the photogenerated charge carriers, and promote the surface reaction.Based on the results got here, it is confirmed that construction of ordered mesoporous structure can efficiently improve the photocatalytic performance of photocatalysts, and photocatalysts with ordered mesoporous structure will be a promising candidate for the removal of hazardous organic compounds from wastewater.

Figure 4 :
Figure 4: (a) UV-vis absorption spectra of Bi 2 WO 6 , S 1 , S 2, and S 3 and (b) calculation of the band gap by Bi 2 WO 6 and S 2 by Kubelka-Munk function.

3. 5 .Figure 6 :
Figure 6: C  /C 0 of the RhB concentration versus reaction time for (a) photocatalytic degradation with Bi 2 WO 6 , S 1 , S 2 , and S 3 under visible light and (b) adsorption with S 2 .