Regular ArticleEco-friendly synthesis of recyclable mesoporous zinc ferrite@reduced graphene oxide nanocomposite for efficient photocatalytic dye degradation under solar radiation
Graphical abstract
Introduction
Ferrites are compounds with the general formula AB2O4, where A and B correspond to various metal cations, typically including the element iron (Fe) [1]. The traditional spinel zinc ferrite (ZnFe2O4, where A = Zn and B = Fe) and its composites have been attracting considerable research attention because of their interesting properties and various applications. For ZnFe2O4, the Zn(II) ions occupy the tetrahedral (A) sites while the Fe(III) ions take up the octahedral (B) sites in a unit cell of 32 oxygen (O) atoms in a cubic close-packed (ccp) arrangement [1]. As a semiconductor for photocatalytic applications, ZnFe2O4 is characterized by its low optical band gap (1.9 eV) [2], [3], high photochemical stability [2], [3], low surface recombination rate [4], low toxicity, ferromagnetic properties at room temperature [5], natural abundance, low cost [2], and environment-friendliness.
Zinc ferrite can be synthesized using a variety of methods, such as a solid-state reaction [6], [7], [8], hydrolysis [9], [10], co-precipitation [5], combustion [3], [11], [12], polyol process [13], thermal decomposition [14], hydrothermal method [8], and solvothermal method [15], [16], [17]. However, due to their high surface energy, zinc ferrite nanoparticles (NPs) tend to agglomerate into large particles, which results in lower surface area and catalytic performance [18]. To overcome this issue, previous researchers have explored the modification of metal oxide NPs, including the use of carbon-based materials as a support material. Graphene, a carbon allotrope, improves the catalytic and photocatalytic efficiencies of semiconductor materials with its high chemical resistance, good electronic conductivity, high specific surface area, and excellent charge transfer properties [18]. The structural and chemical properties of spinel ferrites and their composites are affected by their chemical composition and method of preparation. Furthermore, their magnetic and electric properties vary with cation substitution.
Zinc ferrite-graphene composites have been synthesized by solvothermal method for various applications. Fu and Wang [2] and Xia et al. [19] synthesized ZnFe2O4-graphene composites for photocatalyst and lithium-ion battery applications, respectively, using ethanol as solvent. On the other hand, Li et al. [20] used ethylene glycol as solvent to prepare ZnFe2O4 NPs deposited on reduced graphene oxide (rGO) functionalized with 1,6-hexanediamine for the removal of Cr(VI) by adsorption. ZnFe2O4-rGO nanocomposites were also synthesized by Fei et al. [21] and Hong et al. [22] using ethylene glycol with the addition of ammonium acetate for methylene blue removal and sodium acetate for oxygen-reduction applications, respectively. The acetate ions in the solution acted as particle size controller. In all cases, the solvent ethylene glycol is miscible in water and thus, a potential contaminant for water resources.
To avoid the use of toxic solvents, the hydrothermal method has been employed in the preparation of ZnFe2O4-rGO composites. Hou et al. [23] prepared ZnFe2O4 multi-porous microbricks-graphene composites with high surface area for photocatalysis by a deposition-precipitation reaction with annealing at 500 °C of the ZnFe2O4 precursor, followed by the addition of a graphene oxide (GO) solution using a hydrothermal method. Yang et al. [24] grew spatially confined ZnFe2O4 within ultrasmall graphene sheets using a microwave treatment with water as the solvent in the presence of polyethylene glycol and sodium acetate for the removal of methylene blue under visible-light irradiation. Jumeri et al. [25] synthesized magnetically separable ZnFe2O4-rGO by a microwave-assisted hydrothermal method using FeSO4 and ZnCl as the precursor materials and used the synthesized material for the H2O2-assisted photocatalytic decomposition of methylene blue in water. Most recently, Wang et al. [18] used the hydrothermal method with 6 M NaOH as a pH adjuster to synthesize a GO-ZnFe2O4 nanohybrid for the catalytic thermal decomposition of ammonium perchlorate. The bare ZnFe2O4 and rGO-ZnFe2O4 nanohybrid synthesized in their work displayed low surface areas below 85 m2 g−1.
In this study, mesoporous ZnFe2O4 NPs and ZnFe2O4@rGO nanocomposite were synthesized as they can be separated by a magnetic field from the aqueous solution. To address the issues of using toxic solvents and high temperatures in the synthesis process, a facile and eco-friendly hydrothermal method, which incorporated sodium acetate as a particle-size controller and stabilizer and sodium hydroxide as a pH adjuster or mineralizer, was employed. In addition, the synthesis of the highly crystalline zinc ferrite nanoparticle and zinc ferrite−reduced graphene oxide composite catalysts was carried out in a simple reaction at low temperatures with non-toxic reagents. The hydrothermally synthesized catalysts were tested for the photodegradation of methylene blue under solar irradiation, and a probable mechanism for the dye degradation was proposed.
Section snippets
Materials
Graphite (99.9995%, Junsei Chemical Co., Ltd.), zinc (II) nitrate hexahydrate (Zn(NO3)2·6H2O; >98.0%, Daejung Chemicals & Metals Co., Ltd.), iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O; >98.0%, Daejung Chemicals & Metals Co., Ltd.), anhydrous sodium acetate (CH3COONa; >98.0%, Daejung Chemicals & Metals Co., Ltd.), diethylene glycol (C4H10O3, DEG; >99.0%, Daejung Chemicals & Metals Co., Ltd.), sodium hydroxide (NaOH; >93.0%, Duksan Pure Chemicals), ethanol (>94.5%, Daejung Chemicals & Metals
Phase, morphology, and chemical composition of the samples
Fig. 1a presents the XRD spectra of commercial graphite, the as-synthesized graphene oxide, and the hydrothermally synthesized bare ZnFe2O4 NPs and ZnFe2O4@rGO nanocomposites. For graphite, a sharp peak at 26.0° 2θ (0 0 2) and a low-intensity peak at 54.5° 2θ (0 0 4) revealed the hexagonal graphitic structure of the sample. These peaks are absent in the XRD spectrum of GO, which confirmed the conversion of graphite to GO via the oxidation and exfoliation processes used during the Marcano-Tour’s
Conclusions
A magnetically separable ZnFe2O4@reduced graphene oxide (rGO) nanocomposite was prepared by a facile and eco-friendly hydrothermal method, which was carried out at pH 11 using sodium hydroxide and in the presence of sodium acetate to yield the spinel ZnFe2O4 and its composites instead of Fe2O3. During the hydrothermal process, the simultaneous deoxygenation and reduction of GO and on-site uniform distribution of ZnFe2O4 nanoparticles (NPs) on the rGO nanosheets were achieved, resulting in a
Funding
This study was supported by the National Research Foundation (NRF) of the Republic of Korea under the frameworks of Priority Research Centers Program (NRF-2014R1A6A1031189) funded by the Ministry of Education of the Republic of Korea.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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