Advanced visible light photocatalytic properties of BiOCl micro/nanospheres synthesized via reactable ionic liquids

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Abstract

BiOCl uniform flower-like microspheres and porous nanospheres structures have been successfully synthesized through a one-pot ethylene glycol (EG)-assisted solvothermal process in the presence of reactable ionic liquid 1-hexadecy-3-methylimidazolium chloride ([C16mim]Cl) and [C16mim]Cl–PVP composite system. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and UV–vis diffuse reflectance spectroscopy (DRS). During the reaction process, the ionic liquid acted not only as the solvent and the template, but also as a Cl source for the fabrication of BiOCl microspheres. The effect of the cation alkyl chain length of ionic liquids on morphology and optical absorption properties was investigated and is discussed in detail. In addition, the photocatalytic activity of the BiOCl samples was evaluated by rhodamine B (RhB) dye degradation under visible-light irradiation. It was found that BiOCl porous nanospheres exhibited higher photocatalytic activity than that of the samples prepared with conventional method and TiO2 (Degussa, P25).

Highlights

► Flower-like and porous BiOCl materials have been synthesized with ionic liquid [C16mim]Cl. ► Ionic liquid [C16mim]Cl played the role of solvent, reactant and template simultaneously. ► Porous BiOCl catalyst showed efficient degradation of RhB under the visible light irradiation.

Introduction

Hierarchical nano-structured materials have distinct physical and chemical properties compared with large-size bulk materials [1], [2]. The distinctive properties not only depend on sizes of materials, but also relate to their morphologies. Therefore, studies on the controlled fabrication of hierarchical nanostructures with various sizes and morphologies as well as two-dimensional and three-dimensional superstructures are of great interest and actively being pursued [3], [4], [5]. In particular, many efforts have been devoted to investigate hierarchical semiconductor nanomaterials due to their unique electronic structures [6], [7], [8], [9]. Bismuth oxychloride (BiOCl) is an important V–VI–VII group compound semiconductor with tetragonal matlockite structure, a layer structure characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms. BiOCl can be used as catalysts and photocatalysts [10], [11], [12], ferroelectric materials [13], [14], pigments [15], [16], and so on. Recently, BiOCl has attracted more and more interest and it has been an ideal choice as a new visible light photocatalyst because of its unique and excellent electrical and optical properties [17], [18], [19], [20]. Up to now, several methods have been reported for the preparation of BiOCl micro/nanostructures. For example, Zhang has reported the synthesis of BiOCl nanoplates by a hydrolysis method at room temperature [12]. Zhang and co-workers have synthesized BiOCl microspheres via a general one-pot solvothermal method using ethylene glycol as the solvent [18]. Geng's group has successfully synthesized BiOCl lamellae by a facile sonochemical method in a surfactant/ligand-free system under ambient air [21]. Moreover, BiOCl nanoparticles could be obtained by reverse microemulsions consisting of heptane, non-ionic surfactants, and aqueous salt solutions [22]. Recently, BiOCl nanowire arrays have been prepared employing the anodic aluminum oxide (AAO) template assisted sol–gel method [23]. To our knowledge, the synthesis of highly ordered self-assembled hierarchical BiOCl flower-like microspheres and porous nanospheres structures with high photocatalytic activity has not been reported. Thus, there still remains a great challenge to develop a facile and environmental friendly route to synthesize hierarchical BiOCl flower-like microspheres and porous nanospheres.

Ionic liquids (ILs) are room temperature molten salts containing bulky, asymmetric organic cations and inorganic anions. In recent years, ILs have attracted more and more interest for the synthesis of inorganic materials due to their unique combination of attractive properties, such as high ionic conductivity, good thermal stability, good dissolving ability, wide electrochemical window, nonflammability and negligible vapor pressure [24], [25], [26]. Many different materials have been successfully synthesized in ILs, such as Au nanosheets [27], high-quality TiO2 nanocrystals [28], rod-like, star-like and flower-like ZnO nano structures [29], nanoflower Y2O3 [30], PbCrO4, Pb2CrO5 rods [31], ultrathin SmVO4 nanosheets [32], α-Fe2O3 nanostructures [33] and Bi2S3, Sb2S3 nanorods [34]. In these works, ILs mainly act as solvents and templates. Additionally, ILs can also act as reactants for the fabrication of inorganic materials. For example, Taubert [35] synthesized CuCl nanoplatelets from a Cu-containing ionic liquid, which played the roles of the solvent, template and reactant in the reaction. Zhu et al. [36] prepared hierarchical ZnO nanostructures by an ionothermal process. In their work, it is reported that the key to this synthesis methodology was the use of metal-containing ionic liquids, which acted not only as solvents but also as metal precursors. Liu et al. [37] obtained ZnSe hollow nanospheres by using a Se-containing ionic liquid as a new Se precursor and a stabilizer for the ZnSe hollow nanospheres. In the previous work, our group has successfully synthesized a series of Bi-based photocatalysts by ionic liquids [38], [39], [40], which showed high photocatalytic activity. However, the potential of ILs in the controlled synthesis of BiOCl hierarchical nano-structure materials still needs to be fully explored.

Herein, in our work, we demonstrate a novel and environmentally reactable ionic liquid [C16mim]Cl and [C16mim]Cl–PVP system instead of inorganic salts synthesis of BiOCl flower-like microspheres and porous nanospheres by an ethylene glycol (EG)-assisted solvothermal process. During the reaction process, it was found that ionic liquid [C16mim]Cl played the important roles of solvent, reactant and template at the same time, which was vital for the structure of BiOCl hierarchical architectures. The experiment of changing the alkyl chain length in imidazolium cation demonstrated that the long-chain ionic liquid was beneficial to form microstructures with integrity and improve optical absorption properties of the materials. Moreover, the experimental results illustrated that both BiOCl flower-like microspheres and porous nanospheres showed higher photocatalytic activities than that of the samples prepared with conventional method and TiO2 (Degussa, P25). Particularly, BiOCl porous nanospheres exhibited the highest photocatalytic activities.

Section snippets

Material and sample preparation

All chemicals were of analytical grade and used as received without purification. The ionic liquids 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl, n=4, 8, 16) (99%) were purchased from Shanghai Chengjie Chemical Co. Ltd.

Preparation of BiOCl microspheres

In a typical procedure, 0.001 mol of Bi(NO3)3·5H2O was dissolved into an EG solution containing a stoichiometric amount of ionic liquid [C16mim]Cl. The mixture was stirred for 30 min, and then transferred into 25 mL Teflon-lined autoclave up to 80% of the total volume. The

Characterization of BiOCl samples

The purity and crystallinity of the as-prepared samples were examined using powder XRD measurements. Fig. 1a shows the XRD patterns of the as-prepared BiOCl hierarchical architectures synthesized with ionic liquid [C16mim]Cl in ethylene glycol (EG). All the peaks are readily indexed to the tetragonal phase of BiOCl (JCPDS card no. 06-0249), with lattice constants of a=3.890 Å, c=7.890 Å. The narrow sharp peaks suggest that the BiOCl products are well crystallised. No characteristic peaks of other

Conclusions

In summary, BiOCl uniform flower-like microspheres and porous nanospheres structures have been successfully synthesized in the presence of reactable ionic liquid [C16mim]Cl and [C16mim]Cl–PVP composite system. The average diameters of BiOCl porous nanospheres and flower-like microspheres were 0.5–1 μm and 1–2 μm, respectively. During the reaction, ionic liquid [C16mim]Cl acted not only as solvent and template, but also as the Cl source for the fabrication of BiOCl hierarchical architectures.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (Nos. 21007021, 21177050 and 21076099), China Postdoctoral Science Foundation (2012M510125) and Jiangsu University Scientific Research Funding (No. 11JDG0146).

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