Highly enhanced visible light photocatalytic activity of

Catalysis Communications 90 (2017) 51–55

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Highly enhanced visible light photocatalytic activity of CeO2 through fabricating a novel p–n junction BiOBr/CeO2 Xiao-Ju Wen, Chang Zhang, Cheng-Gang Niu ⁎, Lei Zhang, Guang-Ming Zeng, Xue-Gang Zhang College of Environmental Science Engineering, Key Laboratory of Environmental Biology Pollution Control, Ministry of Education, Hunan University, Changsha 410082, China

a r t i c l e

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Article history: Received 1 April 2016 Received in revised form 29 September 2016 Accepted 17 November 2016 Available online 18 November 2016 Keywords: CeO2 BiOBr p–n junction Visible light Photocatalysis

a b s t r a c t CeO2 microplates were synthesized by a sol-gel auto-combustion method. BiOBr nanosheets were then deposited onto the surface of CeO2 via a facile deposition–precipitation method with the assistance of ethylene glycol. The as-prepared BiOBr/CeO2 samples were characterized by various analytical techniques. The BC-2(Ce/Bi molar ratios is 2) sample exhibited the highest photocatalytic activity. The RhB was degraded (97.3%) under visible light irradiation within 20 min. Furthermore, the BC-2 also exhibited superior degradation activity towards methyl blue (MB) and phenol. The enhanced photocatalytic performance could be ascribed to the enhanced light absorption and the improved separation of photogenerated charge carriers through forming a p-n heterojunction. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Due to its high efficiency, low cost and availability, TiO2 has been widely used as a leading photocatalyst candidate since 1970s [1]. However, TiO2 can only respond in the UV light region, which limit its further application under visible light [2,3]. In addition, the rapid recombination of photogenerated electron–hole pairs also leads to its low quantum efficiency [4]. Therefore, the development of new highly efficient visible light-driven photocatalysts is necessary and urgent. Cerium oxide (CeO2), an ecofriendly photocatalytic material, is an ntype semiconductor [5]. It has high oxygen storage capacity, strong redox capability and been considered as a promising material for photocatalytic applications [6–8]. Compared to TiO2, it possesses a sufficiently long lifetime of photogenerated electron–hole pair [9]. And because of its unique 4f electron configuration, it is widely used as catalyst or a dopant to improve the catalytic ability of other catalysts [10,11]. Recently, it was doped with another metal like Eu and Fe for further improving its photocatalytic activity [12,13]. However, doping can also cause some problem. For example, it may lead to some defects and improve the recombination rate of electron-hole. Thus, a new strategy for improvement its photocatalytic activity is by combining it with other semiconductor [14]. So far, various CeO2-based composites, such as Bi2WO6-CeO2 [15], Ag3PO4-CeO2 [16], C3N4-CeO2 [17], Bi2O3-CeO2 [18], Bi2S3-CeO2 [19], and BiVO4-CeO2 [20], have been reported. These ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (C.-G. Niu).

http://dx.doi.org/10.1016/j.catcom.2016.11.018 1566-7367/© 2016 Elsevier B.V. All rights reserved.

catalysts display much higher catalytic performance for decomposition of organic pollutants or noxious gases than the individual compound. Therefore, seeking an appropriate sensitizer to construct CeO2-based composites is crucial for achieving superior photocatalytic activity. BiOBr, an efficient visible light driven photocatalyst, has recently draw intensive interests of researcher owing to its potential applications in the removal of toxic organic pollutants [21]. However, a high recombination rate of the photogenerated electron–hole pairs of the pure BiOBr restricted its practical applications. To overcome this defect, various efforts have been tried, such as doping it with noble metals or metals [22], coupling it with other semiconductors such as TiO2 [23], C3N4 [24], ZnFe2O4 [25], BiVO4 [26] etc. In particular, fabrication a p–n junction by coupling it with other materials has been considered as a very efficient method to improve the photocatalytic activity of the bare BiOBr. In our previous work, a BiOBr/Fe2O3 p–n junction was successfully fabricated and exhibited high photocatalysis performance [27]. Thus, fabrication BiOBr/CeO2 p–n junction may be an ideal approach to achieve a high photocatalytic activity. Recently, BiOBr/CeO2 heterojunction with enhanced photocatalytic activity via a simple solvothermal method and a microwave-assisted route was reported by researchers, respectively. Similarly, they all synthesized CeO2 nanoparticles, and its load to on the surface of BiOBr nameplates [28,29]. Herein, CeO2 microplates were synthesized by a sol-gel auto-combustion method. A p-n junction was fabricated via a facile deposition–precipitation method with the assistance of ethylene glycol between BiOBr and CeO2. The catalysts were used to degrade the RhB, MB and phenol for assessing the photocatalytic activity of the

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X.-J. Wen et al. / Catalysis Communications 90 (2017) 51–55

samples under visible light irradiation. Furthermore, a mechanism of BiOBr/CeO2 for photocatalytic degradation was also investigated. 2. Experimental The whole experimental part was depicted in Supplementary information. 3. Results and discussion 3.1. Characterization of samples Fig. 1 exhibits the XRD patterns of the BiOBr, CeO2 and BC-2 (the Bi/ Ce molar ratios were 2). It can be clearly observed the XRD peaks of the pure BiOBr were perfectly coincident with the tetragonal phase of BiOBr (JCPDS 73-2061) [24,25]. For pure CeO2, The diffraction peaks are indexed to CeO2 cubic phase, corresponding to JCPDS Card No: 431002 [18]. The peaks of CeO2 at 29.38°, 35.12°, 48.91°, 52.21° and 55.98° were attributed to the (100), (101), (102), (110) and (111) crystal planes of the cubic fluorite structured CeO2 crystal, respectively [16, 19]. The XRD pattern of the BC-2 composite matches well with the diffraction peaks of BiOBr and CeO2. No characteristic peaks for impurities are observed, indicating that only BiOBr and CeO2 phases coexist in the BiOBr/CeO2 composite. The SEM analysis was demonstrated in Supplementary Information. TEM and HRTEM analysis was employed to further confirm the structure of the BiOBr/CeO2 heterojunction. According to Fig. 2Sa–b (Supplementary information), it strongly presents BiOBr nanosheets tightly coupled onto the surface of the CeO2. Furthermore, clear lattice fringes can be observed in the HRTEM image of the BC-2 composite (Fig. 2Sc– d). The characteristic values of lattice constant of 0.2821 and 0.1963 nm match well with the spacing of the (102) and (200) planes of the BiOBr, respectively. In addition, the observed lattice fringes of 0.3119 nm and 0.2701 nm correspond to (111) and (200) planes of the CeO2. Moreover, the corresponding electro-diffraction (SAED) pattern (Fig. S2e) indicates the presence of cubic fluorite-type CeO2 and tetragonal phase BiOBr in BC-2 samples. XPS analysis was applied to investigate the surface compositions and chemical states of BC-2 sample. Fig. 3Sa shows that the BC-2 sample consists of Ce, Bi, O and Br. The Ce 3d core level spectrum was shown in Fig. 3Sb. Six peaks of Ce 3d spectra were observed at 882.48 eV, 889.18 eV, 898.48 eV, 900.78 eV, 907.38 eV and 916.78 eV, respectively. The Ce 3d spectrum is consistent with previous reports, indicating the presence of CeO2 in BC-2 samples [18]. From Bi 4f and Br 3d of XPS spectrum, Bi and Br elements were in +3 oxidation and monovalent oxidation state, respectively [28].The XPS spectrum of O1s is shown in Fig.

Fig. 1. XRD patterns of the pure CeO2, the BC-2 sample, and the pure BiOBr.

Fig. 2. (a) UV–vis diffuses reflectance spectra of CeO2, BiOBr and BC-2; (b);

3Sd, O1S is primarily assigned to the oxygen from Bi\\O, Ce\\O chemical bonding in the BC-2 heterojunction [18,28]. Fig. 2a displays the DRS of BiOBr, CeO2, and BC-2. It can be clearly observed that the BC-2 and pure CeO2 can absorb UV and visible light with the wavelength less than 500 nm while the absorption band edge of and BiOBr was approximately located 430 nm. The BC-2 displays a broader absorption in visible light region, which might be beneficial to enhance its photodegradation efficiency [13]. The band-gap (Eg) of BiOBr and CeO2 can be obtained from Kubelka–Munk theory shown in Fig. S4 [30]. As presented in Fig. 2b, the Eg values of the as-prepared CeO2, BiOBr and BC-2 were about 2.81, 2.66 and 2.25 eV, respectively. 3.2. Enhancement of photocatalytic activity The photocatalytic activity of all samples was accessed by degrading RhB under visible light irradiation. As presented in Fig. 3a, all the BiOBr/ CeO2 heterostructures showed higher photocatalytic activities than that of pure BiOBr and CeO2. After irradiation for 25 min with visible light, the photodegradation rates of RhB was 84.2%, 81%, 97.3%, 82.4%, 71.2% and 19.8% for BC-4, BC-3, BC-2, BC-1, BiOBr and CeO2, respectively. What's more, the photodegradation rates of the physical mixture of CeO2 and BiOBr is only 64.4%, much lower than that of BC-2, even lower than that of pure BiOBr, suggesting that a heterojunction was formed between CeO2 and BiOBr. The reason that the photocatalytic efficiency of BiOBr/CeO2 samples changes with different BiOBr contents can be concluded to be the following: excess CeO2 in the BiOBr/CeO2 heterostructures could hinder the visible light capture of BiOBr and the high content of BiOBr could not form more heterojunctions compared to BC-2 heterostructure due to the serious agglomeration of excessive BiOBr [31]. Thus, the BC-2 heterostructure shows higher photocatalytic activity than the other BiOBr/CeO2 heterostructures. Furthermore, the pseudo dynamics analysis was discussed in Supplementary Information. Furthermore, methyl blue (MB) and phenol were also chosen as typical organic pollutants and degraded using the BC-2 photocatalyst under the experiment conditions (Fig. 3b). From the Fig. 3b, it can be clearly observed that the degradation efficiency of MB and phenol reached to 95% and 75% within 120 min and 270 min in presence of BC-2 photocatalyst. The results reveal that the BC-2 composite can be used as a catalyst in a relatively wide range for practical applications. The stability of the catalysis was tested and the results are depicted in the Supplementary information. Fig. S7 depicted a UV–Vis spectral change of RhB aqueous solution in the presence of BC-2 heterojunction under visible light irradiation. The characteristic absorption band of RhB at 554 nm decreased significantly with increasing irradiation time, accompanied by a remarkable blue-


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Fig. 3. (a) Photocatalytic activities of CeO2, BiOBr, the BiOBr/CeO2 composites for the degradation of RhB (20 mg/L) under visible light irradiation; (b) Photocatalytic activity of BC-2 for MB and phenol (10 mg/L) under visible light irradiation;

shift of the maximum absorption. The stepwise blue-shift of the main peak from 554 to 496 nm can be attributed to the step-by-step deethylation of RhB [32], while the reduction in absorbance indicates the destruction of the conjugated structure [33]. These absorption peaks disappeared after illumination for 30 min, suggesting that a complete decolorization of the RhB solution was realized. 3.3. Photocatalytic mechanism In general, the generation and separation of photogenerated charges have a significant impact on photocatalyst activity of the catalysts. Triethanolamine (TEOA), Isopropanol (IPA), Benzoquinone (BQ), and were utilized as the quenchers of h+, •OH and •O–2 to probe the active species in the degradation process, respectively [34]. As displayed in Fig. 4, the introduction of IPA had slightly effect on the photodegradation efficiency toward BiOBr and BC-2. Nevertheless, the degradation ratio of RhB for pure BiOBr and BC-2 was significantly inhibited after the addition of BQ and TEOA. It can decipher that both the •O–2 and h+ are dominant reactive species in degradation process. To further explore the mechanism of photocatalytic performance for the BiOBr/CeO2 composites, transient photocurrent responses experiments have been performed. Fig. S9 display the photocurrent transient responses of pure BiOBr, BC-2 and single CeO2 electrodes under 20s

Fig. 4. The effects of different scavengers on the degradation of RhB over BiOBr and BC-2 samples under visible light irradiation.

under intermittent visible light irradiation. In comparison with pure BiOBr and single CeO2, BC-2 exhibits a significantly enhanced transient photocurrent density. The results reveal that the BC-2 possesses the higher separation efficiency of photogenerated electrons and holes. These results indicate that the high photocatalytic activity of BiOBr/ CeO2 composites may be ascribed to the efficient separation of electron-hole pairs. The band position of BiOBr and CeO2 was calculated by empirical equations ECB = X − EC − 0.5Eg and EVB = ECB + Eg [16]. Based on the band gap positions of the BiOBr and CeO2, the EVB of BiOBr and CeO2 were calculated to be 3.03 and 2.775 eV, and their homologous ECB were estimated to be 0.37 and −0.035 eV, respectively. Based on above analyses, a possible photocatalytic mechanism of BiOBr/CeO2 heterojunction is drawn (Fig. 5). Mott-Schottky test was also used to confirm the types of BiOBr and CeO2. As shown in Fig. S10a, the Mott-Schottky plots slope values of BiOBr is negative, demonstrating that BiOBr is p-type semiconductor. The Mott-Schottky plots slope of CeO2 is greater than 0 (Fig. S10b), indicating that CeO2 is an n-type semiconductor [35]. When p-type BiOBr coupled with n-type CeO2, the conduction and valence bands of p-type BiOBr was ascended while that of n-type CeO2 was descended until an equilibrium state was achieved (as presented in Fig. 5b). When the BiOBr/CeO2 heterojunction was irradiated with visible light, BiOBr can generate the electron–hole pairs. Subsequently, the photo-generated electrons are rapidly transferred to the CB of CeO2. The generated electrons (e−) capture the O2 in the system to form ·O− 2 radicals. Meanwhile, the photo-induced holes (h+) still remain on the surface of BiOBr. Due to its strong oxidation ability, it can directly oxidize RhB adsorbed on surface of catalyst [25]. Furthermore, dye sensitization may also exist in process of the RhB degradation. The RhB absorbed on the surface of the catalyst can be activated to produce electrons under visible light irradiation from the highest occupied molecular orbital up to the lowest unoccupied molecular orbital. Subsequently, the electrons in RhB transferred to the conduction bands of BiOBr, which are further transferred to the conduction band of CeO2. The heterojunction between BiOBr and CeO2 improves the separation efficiency of the photo-induced electron-hole pairs. The electrons clustered in the conduction band of CeO2 transfer to the surface and are trapped by captured O2 to produce •O2− radicals. Furthermore, •O2− radicals possess excellent oxidation capabilities and participated in the photodegradation of RhB. Therefore, the photocatalytic process over p–n junction BiOBr/CeO2 composite was + mainly contributed to the •O− 2 radicals and photo-induced holes (h ), which is in accordance with the results of active species trapping

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Fig. 5. Photocatalytic degradation mechanism of RhB for the BiOBr/CeO2 composites under visible light irradiation.

experiments. Furthermore, the photoluminescence (PL) analysis was applied to further prove the enhancement of the separation efficiency of the photo-induced electron-hole pairs. The PL results are depicted in Supplementary information. 4. Conclusion CeO2 microplates were synthesized by a sol-gel auto-combustion method. The BiOBr/CeO2 p-n junction was fabricated by means of a facile deposition method with the assistance of ethylene glycol. The BiOBr/ CeO2 photocatalysts exhibited highly enhanced photocatalytic performance toward degradation of organic pollutants under visible light irradiation. The enhanced photocatalytic activities can be attributed to form a p-n heterojunction between BiOBr and CeO2 interface, leading to easier charge transfer and more efficient separation of electron–hole pairs. This work provides an effective method to enhance the photocatalytic activity of the catalysts, which can also be widely used in other photocatalytic systems. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51541801, 51521006), Natural Science Foundation of Hunan Province (14JJ2045), and a Project Supported by Scientific Research Fund of Hunan Provincial Education Department (13k017). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.11.018.

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