Chinese Chemical Letters  2016, Vol. 27 Issue (06): 867-870   PDF    
Citation
Li Jing, Xu Yan, Yue Wei, Chen Yan, Dub Xi-Hua, Wang Shao-Rong.Synthesis and study of Bi2Ti2O7/TiO2 composites as efficient visible-light-active photocatalysts in the reduction of aqueous Cr(Ⅵ)[J]. Chin. Chem. Lett., 2016,27(06): 867-870  
Synthesis and study of Bi2Ti2O7/TiO2 composites as efficient visible-light-active photocatalysts in the reduction of aqueous Cr(Ⅵ)
Li Jinga,b, Xu Yanb, Yue Weib, Chen Yanb, Dub Xi-Huab, Wang Shao-Ronga     
a College of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, China ;
b School of Chemistry and Chemical Engineering, Xuzhou Institute of Technology, Xuzhou 221111, China
Received Oct 12, 2015, Received in revised form Nov 22, 2015, Accepted Nov 22, 2015, Available online Jun 25, 2016
* Corresponding author. lijingxz111@163.com, ljshan@xzit.edu.cn (J. Li),; srwang@mail.sic.ac.cn (S.-R. Wang).
Abstract: In this work, Bi2Ti2O7/TiO2 composites were synthesized and studied as potential visible-light-activated photocatalysts in the reduction of aqueous Cr(Ⅵ). Bi2Ti2O7/TiO2 composites with tunable compositions were synthesized via a solvothermal-calcination two-step method, simply by changing the molar ratios of Bi(NO3)3·5H2O to tetrabutyl titanate in the reactants. The compositions, structures and optical properties of the as-synthesized Bi2Ti2O7/TiO2 composites were characterized by X-ray diffraction, field emission scanning electron microscopy and UV-vis diffuse reflectance spectra. The photocatalytic activity of the as-synthesized Bi2Ti2O7/TiO2 composites was tested in the reduction of aqueous Cr(Ⅵ) under visible-light (λ > 420 nm) irradiation, and compared with that of TiO2 nanoparticles. It was observed that the as-synthesized Bi2Ti2O7/TiO2 composites exhibited much higher photocatalytic activity than TiO2 nanoparticles, and the most efficient composite (300 mg) can achieve the complete reduction of Cr(Ⅵ) in 300 mL of 50 mg/L K2Cr2O7 aqueous solution under visible-light (λ > 420 nm) irradiation in 90 min.
Key words: Nanocomposites     Functional     Powder technology     Semiconductor    
1. Introduction

Hexavalent chromium (Cr(Ⅵ)) is highly toxic and highly soluble in water,and has been classified as a priority pollutant by many countries in the world [1-3]. Photocatalytic reduction has now been widely considered as a simple,effective,low cost and environmentally-friendly method for the treatment of aqueous Cr(Ⅵ) [4-8]. However,at present,the lack of efficient visiblelight- activated photocatalysts has hindered the industrial application of photocatalysis technology in treating large-scale Cr(Ⅵ) wastewater. For instance,TiO2,the most studied photocatalyst hitherto,can absorb only the UV light (only~5% of solar energy) and exhibits little photocatalytic activity in the reduction of aqueous Cr(Ⅵ) under visible-light irradiation,due to its large bandgap (Eg = 3.2 eV) [8-10].

The modification of TiO2 by smaller bandgap semiconductors has been proved to be a simple and effective way to achieve the visible-light photocatalysis [9-11]. Furthermore,composite photocatalysts can exhibit higher photocatalytic efficiency than their individual components,because of the synergistic effect of their components,such as the enhanced separation of photogenerated electrons and holes [9-12]. Bi2Ti2O7 has a bandgap of 2.58-2.74 eV [13, 14],and has been studied as a visible-light-active photocatalyst in the degradation of organics [14, 15] and production of hydrogen by splitting water [16]. However,the reports on the use of Bi2Ti2O7/TiO2 composites as photocatalysts were scarce so far. Herein,we report the synthesis of Bi2Ti2O7/TiO2 composites by modifying the method proposed to synthesize Bi2Ti2O7 [17],and the initial study of Bi2Ti2O7/TiO2 composites in the photocatalytic reduction of aqueous Cr(Ⅵ).

2. Experimental

Preparation: 18 mL of absolute ethanol and 7 mL of glycerin were added to a 50 mL Teflon jar,and the mixture was stirred until a homogeneous solution was obtained. A certain amount (0- 1.76 mmol) of Bi(NO3)3-5H2O was added to the absolute ethanol- glycerin solution and the mixture was stirred until the dissolution of Bi(NO3)3-5H2O. 1.5 mL (4.4 mmol) of tetrabutyl titanate was added dropwise to the above solution under magnetic stirring,and after the addition of tetrabutyl titanate,the resulting mixture was further magnetically stirred for 20 min. Diethyl ether (5 mL) was added to the above mixture and the mixture was stirred for 5 min. The Teflon jar containing the reactants was placed into a stainless steel autoclave,sealed and heated at 110℃ for 12 h,then cooled to ambient temperature naturally. The resultant amorphous precipitate was centrifuged,washed with absolute ethanol and deionized water,dried in air at 100℃ for 10 h,and finally calcined at 500℃ for 3 h.

For the convenience of description,the products prepared using the Bi/Ti molar ratios of 0,10%,20%,30% and 40% were abbreviated as BT-0,BT-10,BT-20,BT-30 and BT-40 (Table 1),respectively.

Table 1
Abbreviations and compositions of the products prepared using different Bi/Ti molar ratios in the reactants.

Characterization: The compositions,structures and optical properties of the products were characterized by X-ray diffraction (XRD,German Bruker AXS D8 ADVANCE X-ray diffractometer),field emission scanning electron microscopy (FESEM,Japan Hitachi S-4800 field emission scanning electron microscopy) and UV-vis diffuse reflectance spectra (American Varian Cary 5000 UV-vis- NIR spectrophotometer). Photocatalytic properties of the products (whose dosage was 300 mg) were tested in the reduction of Cr6+ in 300 mL of 50 mg/L K2Cr2O7 aqueous solution under visible-light (λ > 420 nm) irradiation,with the addition of 1.0 mL of 100 mg/mL citric acid aqueous solution as the sacrificing reagent [18]. The photocatalytic reaction process was monitored by measuring the Cr(Ⅵ) concentration using the standard diphenylcarbazide colorimetric method [19].

3. Results and discussion

Fig. 1 shows the XRD patterns of BT-0,BT-10,BT-20,BT-30 and BT-40. All the XRD peaks of BT-0 can be indexed to pure anatase TiO2 (Joint Committee on Powder Diffraction Standards (JCPDS) card number 89-4921). The XRD peaks of BT-10,BT-20 and BT-30 revealed the preparation of the composites of anatase TiO2 and cubic phase Bi2Ti2O7 (JCPDS card number 32-0118),whereas those of BT-40 suggested the formation of a mixture of anatase TiO2,cubic phase Bi2Ti2O7 and orthorhombic phase Bi4Ti3O12 (JCPDS card No. 35-0795). The phase compositions of BT-10,BT-20,BT-30 and BT-40 were estimated by the method of XRD quantitative analysis,and the obtained results are shown in Table 1. The results revealed that the molar ratio of Bi/Ti played a crucial role in the phase compositions of the resultant products. This was also consistent with the previous report [20]. The formation mechanisms of different products may be described as the following equations:

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Figure 1. XRD patterns of BT-0, BT-10, BT-20, BT-30 and BT-40

$\text{Ti}{{\left( \text{O}{{\text{C}}_{\text{4}}}{{\text{H}}_{\text{9}}} \right)}_{\text{4}}}\text{+6ROH}\to \text{Ti}\left( \text{OH} \right)_{\text{6}}^{\text{2-}}\text{+2}{{\text{R}}^{\text{+}}}\text{+4RO}{{\text{C}}_{\text{4}}}{{\text{H}}_{\text{9}}}$ (1)
$\text{B}{{\text{i}}^{\text{3+}}}\text{+6ROH}\to \text{Bi}\left( \text{OH} \right)_{\text{6}}^{\text{3-}}\text{+6}{{\text{R}}^{\text{+}}}$ (2)
$\text{Ti}\left( \text{OH} \right)_{\text{6}}^{\text{2-}}\to \text{Ti}{{\text{O}}_{\text{2}}}\cdot n{{\text{H}}_{\text{2}}}\text{O+}\left( \text{3}-\text{n} \right){{\text{H}}_{\text{2}}}\text{O}$ (3)
$\text{2Bi}\left( \text{OH} \right)_{\text{6}}^{\text{3-}}\to \text{B}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{3}}}\cdot m{{\text{H}}_{\text{2}}}\text{O}+\left( 3-\text{m} \right){{\text{H}}_{\text{2}}}\text{O+6O}{{\text{H}}^{\text{-}}}$ (4)
$\text{B}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{3}}}\cdot m{{\text{H}}_{\text{2}}}\text{O+2Ti}{{\text{O}}_{\text{2}}}\cdot n{{\text{H}}_{\text{2}}}\text{O}\to \text{B}{{\text{i}}_{\text{2}}}\text{T}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{7}}}\text{+}\left( \text{m+2n} \right){{\text{H}}_{\text{2}}}\text{O}$ (5)
$\text{2B}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{ }\!\!\times\!\!\text{ m}{{\text{H}}_{\text{2}}}\text{O+3Ti}{{\text{O}}_{\text{2}}}\cdot n{{\text{H}}_{\text{2}}}\text{O}\to \text{B}{{\text{i}}_{\text{4}}}\text{T}{{\text{i}}_{\text{3}}}{{\text{O}}_{\text{12}}}\text{+}\left( \text{2m+3n} \right){{\text{H}}_{\text{2}}}\text{O}$ (6)

The microstructures of BT-0,BT-10,BT-20,BT-30 and BT-40 were characterized by FESEM (Fig. 2). The FESEM image of BT-0 showed that the as-prepared TiO2 was comprised of aggregates of nanoparticles. The FESEM images of BT-10,BT-20 and BT-30 showed that the prepared composites of TiO2 and Bi2Ti2O7 were comprised of nanoparticles and micrometer-sized spheres. Furthermore,the number of micrometer-sized spheres increased with the increase of Bi2Ti2O7 content from BT-10 to BT-30. Besides,the higher magnification FESEM image of BT-30 (Fig. 2,BT-30-B) revealed that the micrometer-sized spheres were constructed with nanoparticles. The FESEM image of BT-40 showed that the prepared composite of TiO2,Bi2 Ti2O7 and Bi4Ti3O12 were mainly comprised of micrometer-sized spheres decorated with nanoparticles.

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Figure 2. FESEM images of BT-0, BT-10, BT-20, BT-30 and BT-40

Fig. 3 shows the UV-vis diffuse reflectance spectra of BT-0,BT- 10,BT-20,BT-30 and BT-40 in the absorbance mode. It can be seen that BT-0 (TiO2) displays almost no absorption of visible-light,whereas BT-10,BT-20,BT-30 and BT-40 display distinct absorption of visible-light in the range of about 400-550 nm.

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Figure 3. UV–vis diffuse reflectance spectra of BT-0, BT-10, BT-20, BT-30 and BT-40

Fig. 4 shows the photocatalytic reduction of aqueous Cr(Ⅵ) over BT-0,BT-10,BT-20,BT-30 or BT-40 in the presence of citric acid as the sacrificing reagent,as well as the results obtained in the absence of a catalyst under visible-light (λ > 420 nm). In the absence of a catalyst,the Cr(Ⅵ) concentration remained unchanged,suggesting that Cr(Ⅵ) cannot be reduced directly by citric acid under visible-light (λ > 420 nm) irradiation. In the presence of BT-0 (TiO2),the Cr(Ⅵ) concentration decreased slowly with the increase of irradiation time. In contrast,the Cr(Ⅵ) concentration decreased much more quickly with the increase of irradiation time in the presence of BT-10,BT-20,BT-30 or BT-40. These results indicated that BT-10,BT-20,BT-30 and BT-40 all had higher visiblelight- driven photocatalytic activity than BT-0 in the reduction of aqueous Cr(Ⅵ). The photocatalytic activity of the samples followed an order of BT-0 < BT-10 < BT-20 < BT-40 < BT-30. In the presence of the most efficient BT-30,a complete reduction of Cr(Ⅵ) can be achieved under visible-light (λ > 420 nm) irradiation in 90 min.

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Figure 4. Photocatalytic reduction of aqueous Cr(Ⅵ) over BT-0, BT-10, BT-20, BT-30 or BT-40 in the presence of citric acid as the sacrificing reagent, as well as the results from mere irradiation (no catalyst) under visible-light (λ > 420 nm). Note: C0 and Ct are the Cr(Ⅵ) concentrations at the irradiation times of 0 and t min, respectively

The possible mechanisms involved in the photocatalytic reduction of aqueous Cr(Ⅵ) over Bi2Ti2O7/TiO2 under visible-light (λ > 420 nm) irradiation in the presence of citric acid as the sacrificing reagent were proposed in Fig. 5. The electrons in the valence band of Bi2Ti2O7 can be excited upon visible light irradiation to its conduction band with the simultaneous generation of an equal number of holes in its valence band,but TiO2 cannot absorb the incidental visible-light (λ > 420 nm) due to its wide band gap (Eg = 3.2 eV). Because the valence band and conduction band potentials of Bi2Ti2O7 are more negative than those of TiO2 [21],the photo-generated electrons (e-1) can migrate from the conduction band of Bi2Ti2O7 to the conduction band of TiO2,whereas the photo-generated holes (h+) still remain on the valence band of Bi2Ti2O7. The e-1 can reduce Cr(Ⅵ) to Cr(Ⅲ),and at the same time,the h+ are consumed by the oxidation of citric acid [18]. The matched band structure and heterojunction interface between Bi2Ti2O7 and TiO2 could benefit the transfer and separation of e-1-Hh+ pairs,contributing to the higher photocatalytic activity of Bi2Ti2O7/TiO2 composite than TiO2 [21]. In addition,the more content of the visible-light-active Bi2Ti2O7 in the Bi2Ti2O7/TiO2 composite resulted in the higher photocatalytic activity. Thus,BT-30 with the largest content of Bi2Ti2O7 exhibited the highest photocatalytic activity. Nevertheless,the presence of Bi4Ti3O12 in the composite would decrease the photocatalytic activity (such as BT-40),because Bi4Ti3O12 cannot absorb the incidental visible-light (λ > 420 nm) either due to its larger band gap of 3.08 eV [22].

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Figure 5. Schematic diagram for photocatalytic reduction of aqueous Cr(Ⅵ) over Bi2Ti2O7/TiO2 under visible-light (λ > 420 nm) irradiation in the presence of citric acid as the sacrificing reagent

4. Conclusion

Bi2Ti2O7/TiO2 composites with tunable compositions were synthesized via a solvothermal-calcination two-step method,simply by changing the molar ratios of Bi(NO3)3·5H2O to tetrabutyl titanate in the reactants. The as-synthesized Bi2Ti2O7/TiO2 composites exhibited much higher photocatalytic activity than TiO2 nanoparticles in the reduction of aqueous Cr(Ⅵ) under visible-light (λ > 420 nm) irradiation.

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