Preparation of core-shell structured Fe₃O₄@Sn-MOF composite and photocatalytic performance

Highlights

• A hydrothermal synthesis was used to fabricate the Fe₃O₄@Sn-MOF composite.

• The as-prepared Fe₃O₄@Sn-MOF had a core-shell structure.

• The Fe₃O₄@Sn-MOF had strong photoresponse and effective charge separation.

• Fe₃O₄ as the magnetic nucleus improved the recovery rate of the composite.

Abstract

The magnetic metal-organic framework composite Fe₃O₄@Sn-MOF with core-shell structure was successfully prepared by hydrothermal synthesis. It was characterized by XRD, TEM, BET, DRS, FT-IR, XPS and VSM. The photocatalytic performance of the composite was evaluated by degradation of Acid Red 3R (AR3R) dye under simulated light source. Under the conditions that the additive amount of the catalyst was 6.7 g/L, the initial concentration of AR3R was 50 mg/L and the initial pH of AR3R was 3.0, Fe₃O₄@Sn-MOF revealed the optimized photocatalytic activity with 100% degradation efficiency of AR3R at 30 min. The excellent photocatalytic ability was mainly ascribed to the effective charge separation, strong photo-response and rich active sites. Besides, using Fe₃O₄ as the magnetic nucleus was conducive to improve the recovery rate of the catalyst. The EPR and free radical analysis indicate that •O₂⁻ and •OH played important roles in the photocatalytic reaction. Meanwhile, a possible degradation process and mechanism were proposed by the full band scanning of AR3R, 3D-EEM fluorescence analysis and electrochemical measurement.

Introduction

Prominent environmental issues and energy crises have attracted widespread attention [1], [2], [3]. The efficient and environmentally friendly treatment technology has become a research hotspot for researchers [4]. Photocatalysis is considered to be a promising technology for the degradation of organic contaminants due to its low cost, high efficiency and almost no secondary pollution [5], [6], [7], [8]. It can capture sunlight to produce •OH and •O₂⁻ with the existence of photocatalyst [9].

In recent years, a series of semiconductor materials with photocatalytic properties have been used to remove organic pollutants in water [10]. Although semiconductor photocatalysts have been widely studied, they still have the disadvantages such as easy photo corrosion, small optical response range and difficult post-separation. For instance, ZnO has received widespread attention because of its high chemical stability, environmental protection and low cost [11], [12]. But in fact, the application of ZnO is limited, which attributes to the narrow light response range, the high electron-hole recombination rate and difficulty in recovery after reaction [13]. In terms of these shortcomings, Song et al. [14] introduced metallic Zn in the photocatalyst and synthesized core-shell Zn/ZnO nanomaterials, which obviously hindered the recombination of photo-generated charges and improved the photocatalytic efficiency. Hence, the study of photocatalysts with high efficiency, stability, visible light drive, and low electron-hole recombination rate is an important topic in the field of photocatalysis [15]. Metal-organic frameworks (MOFs) exhibit the enormous advantages in photocatalysis because of their flexible structure characteristics, porous structure and chemical stability compared with traditional photocatalysts [16]. They are new types of functional inorganic-organic hybrid materials. More than 20,000 MOFs has been reported in the past decade [17]. MOFs are a type of porous crystalline material with a periodic network structure composed of metal centers and organic ligands [18]. The unique structural characteristics of MOFs materials make it beneficial to improve the problems of catalyst agglomeration and low specific surface area. However, the stability of MOFs in aqueous solution is poor, because the hydrophilic properties of the metal nodes easily lead to the hydrolysis of metal-linker coordination bonds. Sn-based materials have received extensive attention on account of their richness, environmental friendliness and low cost [19]. Sn, as a kind of hard Lewis acid, could form a strong coordination bond with hard Lewis base (such as carboxylate) to construct a rigid and interconnected frame structure [20]. For example, Wang et al. [21] used Sn-MOF as a template to synthesize SnO₂ with high specific surface area for methanol sensing. Ghosh et al. [20] synthesized Sn(II)-BDC MOF by a facile hydrothermal process for adsorption and removal of toxic anionic dyes in water. Moreover, the core-shell or core-shell-like composites are regarded as one of the most effective and convenient methods to realize the multi-functional synergy [22]. Li et al. [23] synthesized core-shell Fe₃O₄@MIL-100(Fe) for photocatalytic degradation of diclofenac sodium and achieved good removal effect. The core of metal or metal-containing nanoparticles is encapsulated in the MOFs shell, which can greatly improve its anti-agglomeration stability and avert undesirable dissolution or photo-corrosion in the photocatalytic process [16]. In addition, Fe₃O₄ nanoparticle has the advantages of magnetic properties, richness and low toxicity, which is regarded as a promising metal oxide [24], [25], [26], [27]. The incorporation of Fe₃O₄ into MOF-based photocatalysts is beneficial to the separation and recovery of composite.

Therefore, a kind of composite with Fe₃O₄ as magnetic core and Sn-MOF as shell was synthesized in this work. The framework of Sn-MOF was synthesized by Sn as inorganic metal ion and terephthalic acid as organic linker. The magnetism coming from Fe₃O₄ made it separate quickly from the reaction process. The morphology and structure of the prepared Fe₃O₄@Sn-MOF were characterized by different ways. The photocatalytic properties and stability of the composite were evaluated with AR3R as the target pollutant.