HUANG Ji-To CUI Chun-N, YAN Gui-Yng XING Yong-Lei

a (College of Chemistry and Materials, Ningde Normal University, Ningde, Fujian 352100, China)

b (Fujian Provincial Key Laboratory of Featured Materials in Biochemical Industry, Ningde, Fujian 352100, China)

c (Key Laboratory of green technology in ecological industry of Fujian Province, Wuyishan 354300, China)

d (School of Electronic and Information Engineering,Xi’an Jiaotong University, Xi’an 710049, China)

1 INTRODUCTION

Since the pioneering work of Fujishima’s group in 1972, TiO2-based photocatalysts have aroused extensive concern for the degradation of refractory pollutant[1].However, TiO2can only response to the ultraviolet region accounting for only 5% of solar spectrum, which is due to the large band gap of 3.2 eV[2-4].To date, numerous novel semiconductors have been exploited in water purification because of their better photocatalytic activity under visible light irradiation than that of TiO2[5-7].Recently,Bi24O31Br10, as one of the Bi-based semiconductors with a band gap of 2.5～2.8 eV, has drawn great attention due to its visible-light-driven catalytic activity for the degradation of organic pollutants[8,9].Nevertheless, the defects in the lattice could enhance the recombination of photogenerated hole and electron pairs, decreasing the photocatalytic activity efficiency.As a result, great efforts including doping with elements or coupling with other semiconductors with narrow band gap, have been taken to modify the pure Bi24O31Br10, which can improve the separation of photoinduced carriers[10,11].

2 EXPERIMENTAL

2.1 Method

Bi24O31Br10was prepared according to the reports on BiOBr with a slight modification[16].Shortly, a stoichiometric amount of Bi(NO3)3·5H2O was dissolved in 50 mL of ethylene glycol (EG), and then heated at 80 oC for 1 h.Afterward, 30 mL of EG solution containing certain amount of KBr was added into the above solution.After stirring at 80 oC for 5 h,the mixture was poured into ethanol aqueous solution under vigorously magnetic stirring, followed by centrifugation, washing with distilled water and ethanol, and dried at 75 oC for 5 h.The as-prepared samples were calcined at 600 oC for 2 h.

For In2O3/Bi24O31Br10samples, various mass percentages of In2O3were loaded onto Bi24O31Br10by using a wet impregnation method[12].In brief, 1 g of Bi24O31Br10photocatalyst was added into 3 mL of ethanol containing different amounts of In(NO3)3in a crucible under stirring for 5 h.Then, the solvent was evaporated, and the remaining powders were annealed at 400 oC for 3 h.The obtained products were denoted as 5In2O3/Bi24O31Br10, 10In2O3/Bi24O31Br10,15In2O3/Bi24O31Br10and 20In2O3/Bi24O31Br10.

2.2 Photocatalytic activity measurement

Rhodamine B (RhB) was adopted to evaluate the photocatalytic properties of the samples under visible-light irradiation, using a 500 W xenon lamp (λ≥ 400 nm) as the light source.Typically, 120 mg of the samples was dispersed into 100 mL of RhB (5 mg L-1) solution in a beaker.Before illumination, the solution was stirred for 1 h in the dark to establish adsorption/desorption equilibrium.At given certain time intervals, 5 mL of solution was taken out and centrifuged to remove the photocatalyst for further analysis by testing the absorbance of RhB at 554 nm through a UV-vis spectrophotometer.

2.3 Characterization

X-ray diffractometer (D8Advance, Bruker AXS,Germany) at the scanning speed of 5 o/min was exploited to examine the structure of all the samples.Scanning electron microscopy coupled with an energy-dispersive X-ray (EDAX) spectrometer (SEM,S4800, Hitachi, Japan) and high-resolution transmission electron microscopy (TEM JEM-2100, JEOL Inc., Japan) were employed to observe the morphologies.UV-Vis diffuse reflectance spectra (DRS) were recorded by using a UV/VIS/NIR spectrometer(JASCO Model V-570).BaSO4was used as the reflectance standard.Surface chemical compositions were tested by X-ray photoelectron spectroscopy(XPS, ESCALAB MK II, VG), with C 1s at 284.6 eV as a reference.Photoluminescence spectra (PL)with an excitation wavelength of 330 nm were investigated by using a Jobin-Yvon Fluorolog-3 spectrofluorimeter.

3 RESULTS AND DISCUSSION

It is clear to see in Fig.1(a) the XRD patterns of pure In2O3, Bi24O31Br10and In2O3/Bi24O31Br10composites.Significantly, the distinctive peaks located at 28.9o, 29.7o, 30.9o, 31.7o and 39.7o can be indexed to(3ˉ04), (208), (214), (1ˉ17) and (4ˉ06) facets of Bi24O31Br10, respectively[9].And the characteristic peaks at 21.5o, 30.6o, 35.5o, 45.7o, 51.0o and 60.7ocorresponding to (211), (222), (400), (431), (440)and (622) planes of In2O3, respectively can also be observed[15].However, for the In2O3/Bi24O31Br10composites, no typical In2O3peaks can be detected due to the high dispersion of In2O3particles on the surface of Bi24O31Br10[17].Notably, the strongest peak at 39.7o corresponding to the (4ˉ06) facet of Bi24O31Br10decreases with the increasing contents of In2O3,indicating theintensiveinteraction between In2O3and Bi24O31Br10in the composites.Additionally, no other impurity peaks can be observed in thecomposites, elucidating th e high purity of the two components.`

Fig.1.(a) XRD patterns and (b) SEM and EDS element mapping images of 15In2O3/Bi24O31Br10 composites

To further testify the existence of In2O3phase in thecomposites,elementmappingfor 15In2O3/Bi24O31Br10sample was analyzed in Fig.1(b).Apparently, a large number of In2O3particleswith the diameter of about5～10 nm (in Fig.S1 (ESI))wereevenly attachedto the surface of Bi24O31Br10plates.Furthermore, all the elements including Br, O,BiandIn weredispersed uniformly,further illustrating theexistenceof biphaseof In2O3and Bi24O31Br10.

TEMa nd HRTEMwereused toascertainthe construction of15In2O3/Bi24O31Br10composite.As shown in Fig.2, a large number of In2O3nanoparticleswereuniformly dispersed on thesurfaceof Bi24O31Br10plates, which isin accordance with the resultof SEM.Furthermore,twosetsof lattice spacing can berevealed as0.292 and 0.282 nm in Fig.2(b), corresponding to the (222) plane of cubic In2O3[15,17]andthe(117)planeof Bi24O31Br10[11],respectively, which are alsoin good agreement with theabove XRDresults,further confirming the interaction between In2O3and Bi24O31Br10.

Fig.2.(a) TEM image and (b) HRTEM image of the 20In2O3/Bi24O31Br10 composite

Asshown in Fig.3,UV-visdiffusereflectance spectra were adoptedto explore the optical absorption propertiesofallthe obtained materials.Apparently, pure Bi24O31Br10and In2O3show absorption edges at 460 and 485 nm, respectively.According to the Tauc formula, the band gapenergies of Bi24O31Br10andIn2O3were calculated to be 2.68 and 2.57eV, respectively,which isconsistent with the reported result[11,17-18].In comparison, all the In2O3modifiedBi24O31Br10compositesshow enhanced absorption in therange from 460to485nm on account of the interaction between In2O3and Bi24O31Br10.

Fi g.3.(a) UV-Vis DRS and (b) estimated band gap energies of the samp les

Fig.4.(a) a nd (b) Photoc atalytic activit y and (c) PL of the samples

The photocatalytic activity of as-prepared samples wasevaluated by photodegrading RhBaqueous solution under visiblelight (λ ＞ 400 nm) irradiation.As seen in Fig.4(a), only about 35% of RhB aqueous solution was decomposed over In2O3.Most strikingly,15In2O3/Bi24O31Br10displays the best photocatalytic activity for the RhB degradation, where almost 98%of RhBsolution wasdecomposed after30min of visiblelightirradiation,evidencing theappreciable effects of In2O3contents on photocatalytic activity of Bi24O31Br10.As seen in Fig.4(b), the rate constants calculated from the slope of the fitting lines were equal to 0.106, 0.043, 0.041, 0.117 and 0.016 min-1for Bi24O31Br10, 5In2O3/Bi24O31Br10,10In2O3/Bi24O31Br10, 15In2O3/Bi24O31Br10and In2O3.In addition, PL spectra (in Fig.4(b)) were carried out with the excitation wavelength at 330 nm to characterize the recombination rate of the photogenerated carriers.We can also found that the PL intensity of the photocatalyst followed the order:15In2O3/Bi24O31Br10＞ In2O3＞ Bi24O31Br10, indicating that the 15In2O3/Bi24O31Br10has the efficient separation of photogenerated carriers.As we all know, the weaker the peak intensity, the higher the recombination rate is[19].Therefore, it can be concluded that 15In2O3/Bi24O31Br10composite has the lowest recombination rate of photoinduced electron and hole pairs due to the weakest PL intensity, resulting in the highest photocatalytic activity for the degradation of RhB.

4 CONCLUSION

In conclusion, In2O3nanoparticles sensitized Bi24O31Br10plates were synthesized by using hydrolysis, impregnation method and post-thermal process.The In2O3nanoparticles were tightly adhered to the surface of Bi24O31Br10plates forming the In2O3/Bi24O31Br10heterostructures.15In2O3/Bi24O31Br10composite displayed the highest photocatalytic activity, which is contributed by the efficient capability of separation of the photogenerated electrons and holes pairs on the basis of the PL result.Herein, fabrication of In2O3/Bi24O31Br10heterostructured composite could provide a promising strategy in the design of novel Bi-based composites with high photocatalytic activity under the irradiation of visible light.

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