Enhanced photocatalytic performance and degradation pathway of Rhodamine B over hierarchical double-shelled zinc nickel oxide hollow sphere heterojunction

doi:10.1016/j.apsusc.2017.06.325

Applied Surface Science

Volume 430, 1 February 2018, Pages 549-560

https://doi.org/10.1016/j.apsusc.2017.06.325 Get rights and content

Highlights

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NiO/ZnO hollow spheres were obtained by calcination of MOF via solvothermal method.
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NiO/ZnO heterojunction improved separation of photogenerated electrons and holes.
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The OH radical destroyed dye chromophore structure of RhB during the reaction.
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The degradation pathways for RhB are followed through LC–MS technique.

Abstract

In this study, hierarchical double-shelled NiO/ZnO hollow spheres heterojunction were prepared by calcination of the metallic organic frameworks (MOFs) as a sacrificial template in air via a one-step solvothermal method. Additionally, the photocatalytic activity of the as-prepared samples for the degradation of Rhodamine B (RhB) under UV–vis light irradiation were also investigated. NiO/ZnO microsphere comprised a core and a shell with unique hierarchically porous structure. The photocatalytic results showed that NiO/ZnO hollow spheres exhibited excellent catalytic activity for RhB degradation, causing complete decomposition of RhB (200 mL of 10 g/L) under UV–vis light irradiation within 3 h. Furthermore, the degradation pathway was proposed on the basis of the intermediates during the photodegradation process using liquid chromatography analysis coupled with mass spectroscopy (LC–MS). The improvement in photocatalytic performance could be attributed to the p-n heterojunction in the NiO/ZnO hollow spheres with hierarchically porous structure and the strong double-shell binding interaction, which enhances adsorption of the dye molecules on the catalyst surface and facilitates the electron/hole transfer within the framework. The degradation mechanism of pollutant is ascribed to the hydroxyl radicals ( radical dot OH), which is the main oxidative species for the photocatalytic degradation of RhB. This work provides a facile and effective approach for the fabrication of porous metal oxides heterojunction with high photocatalytic activity and thus can be potentially used in the environmental purification.

Graphical abstract

Introduction

Photocatalytic technology, an advanced oxidation process, has shown a great potential being cost effective, environment friendly for air purification and wastewater treatment using solar energy and complete degradation of organic pollutants [1], [2], [3], [4]. However, the limits of semiconductor photocatalysts are that most of the catalysts have a large band gap and recombination of the photogenerated electrons and holes [5]. In recent years, considerable effort has been made on the design and preparation of heterostructure to improve the photocatalytic activity [6]. The composite heterojunction has been recognized as an effective method to improve photocatalytic efficiencies by enhancing the utilization of sunlight or improving the separation/transportation of the electron-hole pairs [7], [8], [9] (e.g. NiO/hematite [10], ZnO/TiO₂ [11], Cu₂O/ZnO [12], BiPO₄/BiOBr [13]). Recent research found that ZnO-based heterojunctions [14], such as CdS, BiOI, NiO, TiO₂, SnO₂, CdSe, and SnS₂ could significantly promote the transfer of electrons and inhibit the recombination of electrons-holes by heterojunctions. Among these composites, nickel oxide (NiO) is an ideal p-type semiconductor [15]. When NiO and ZnO fabricate a heterojunction, the transfer of photogenerated electron-holes can be enhanced and recombination can be reduced, thereby improving the photocatalytic efficiency [16].

Hollow micro-/nano structures are a class of special micro-/nano materials being named on the basis of their morphologies [17], [18]. They show their advantages on promising photocatalysts in several aspects, such as better charge separation [19]. Many methods have been developed to fabricate this type of micro/nano materials including Kirkendall effect [20], Ostwald ripening [21] and template-directed synthesis [22], [23]. Among these methods, template-directed synthesis, with either hard or soft templates, is one of the most effective methods to fabricate hollow structures.

Metal-organic frameworks (MOFs) are crystalline coordination polymers that form multiple dimensional structures and can be constructed by using a wide range of metal ions and organic ligands. Compared with common inorganic material such as zeolite, MOFs have higher pore volume, specific pore structural, high surface area and structural diversity; therefore have been widely used in various fields. Owing to their permanent porosity, tunable porosity and diversity in metal ions and organic ligands of MOFs makes them feasible as templates for preparation of desired products with unique structures [24], [25].

In this study, a novel facile approach was adopted to fabricate hierarchical double-shelled NiO/ZnO hollow spheres heterojunction derived from zinc nickel bimetal-organic frameworks microspheres. The composites showed significant enhanced photocatalytic activity in RhB and p-nitrophenol degradation, moreover the mechanism of RhB was also investigated systematically.

Section snippets

Materials

Nickel nitrate hexahydrate, terephthalic acid (H₂BDC), nickel nitrate hexahydrate, N, N-dimethylformamide (DMF), ethylene glycol, ethanol, methanol, isopropanol, rhodamine B, p-nitrophenol, sodium hydroxide and sodium sulfate were all obtained from Sinopharm Co. Ltd. (China) and used without any further purification.

Synthesis of Ni/Zn MOF microspheres

Ni/Zn MOF spheres were fabricated by a solvothermal method. The preparation processes were as followed: 0.2 g Ni (NO₃)₂·6H₂O, 0.2 g Zn (NO₃)₂·6H₂O and 0.12 g terephthalic acid (H₂BDC)

XRD patterns analysis

Fig. 1 shows the XRD patterns results of the pure NiO, pure ZnO, and NiO/ZnO composites with different ratios of Ni/Zn and NiO/ZnO without MOFs structure. For pure NiO the same diffraction peaks of NiO have been evidently detected, in which the peaks at 37.2°, 43.3°, 62.9° and 75.4° are corresponding to the diffraction peaks of (111), (200), (220) and (211) crystal planes of NiO, according to the JCPDS standard of No. 47-1049. For the pure ZnO, the peaks at 31.8°, 34.5°,36.7°,47.6°, 56.6°,

Conclusions

In summary, we have demonstrated that hierarchical double-shelled NiO/ZnO hollow spheres heterojunction could be successfully fabricated via a solvothermal method and calcination of Ni/Zn metal-organic frameworks. The results indicate that the NiO/ZnO heterojunction nanocatalyst had best photocatalytic activity when the calcination temperature was at 400 °C for 1.5 h and the initial solution concentration of Ni/Zn ratio was 2:2. We found the hollow structure and p-n heterojunctions play the major

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (21277108, 21476179).

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Full Length ArticleEnhanced photocatalytic performance and degradation pathway of Rhodamine B over hierarchical double-shelled zinc nickel oxide hollow sphere heterojunction

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Synthesis of Ni/Zn MOF microspheres

XRD patterns analysis

Conclusions

Acknowledgements

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Catal. B

J. Environ. Radioactiv.

Appl. Catal. B-Environ.

Appl. Surf. Sci.

Chem. Eng. J.

Appl. Surf. Sci.

Water Res.

Chemosphere

Chem. Eng. J.

Desalination

J. Catal.

Heterojunction Photocatalysts

Adv. Mater.

Hierarchical photocatalysts

Chem. Soc. Rev.

Full Length Article
Enhanced photocatalytic performance and degradation pathway of Rhodamine B over hierarchical double-shelled zinc nickel oxide hollow sphere heterojunction