In situ deposition of Ag/AgCl on the surface of magnetic metal-organic framework nanocomposite and its application for the visible-light photocatalytic degradation of Rhodamine dye

Highlights

• NH₂-MIL-125(Ti) (NMT) metal-organic framework was magnetized by CoFe₂O₄.

• Ag/AgCl was assembled on CoFe₂O₄/NMT (CFNMT) by in situ deposition/photo-reduction.

• Ag/AgCl@CFNMT had fast dye degradation ability under LED visible light.

• Introduced Ag/AgCl on the CFNMT increased absorption of photons in visible region.

• After four times recycling, Ag/AgCl@CFNMT showed high photodegradation ability.

Abstract

Herein, NH₂-MIL-125(Ti) (NMT) as one of the known stable metal-organic frameworks (MOFs) in aqueous solution was successfully magnetized with CoFe₂O₄ nanoparticles through the hydrothermal method. The Ag/AgCl as a plasmonic photocatalyst was assembled on the CoFe₂O₄/NMT (CFNMT) at room temperature by in situ deposition, and photo-reduction methods to improve the photocatalytic activity of CFNMT under LED visible light. The prepared materials were fully characterized by SEM/EDX, TEM, FTIR, XRD, UV-DRS, and VSM analysis. Rhodamin B (RhB) was selected as the pollutant model. The results showed that the Ag/AgCl@CFNMT had super-fast degradation ability of RhB molecule due to the synergetic effect between Ag/AgCl and CFNMT in comparison with NMT and CFNMT. The introduced Ag/AgCl on the surface of CFNMT increased absorption of photons in the visible region and enhanced the transfer and separation of the produced charge on the contact area between Ag/AgCl and CFNMT. Also, after seven times recycling, besides the simple magnetic separation of Ag/AgCl@CFNMT from liquid media, the composite still showed high photodegradation ability (89%).

Introduction

In recent decades, fast development in industrial activities and high increment in world population lead to discharging of highly toxic materials in waterways, as a result, the healthy water sources decreased tremendously. The existence of these toxic materials in the water supply can make serious problems for the ecosystem. Therefore, removing of organic contaminants such as the synthesized dyes, which considered as the major part of organic pollutants in water, due to high stability and non-biodegradability of these materials became a great concern of environmentalist [[1], [2], [3], [4], [5], [6], [7], [8]]. Among all methods of removing contaminants, photodegradation is an eco-friendly, easy to apply, low-cost and high available approach to remediate the organic contaminants [[9], [10], [11], [12]].

Nowadays, metal-organic frameworks (MOFs) were widely used in photocatalyst compositions, nanofibers, and membrane [[13], [14], [15], [16]]. Although, MOFs are great candidates to purify the wastewater, their low water-stability restricted their reusability and utilization in industrial units [17]. Researchers used various composites of MOFs and calcined MOFs in their works to avoid this limitation [[18], [19], [20]]. Some properties of titanium-based MOFs (especially MIL-125(Ti) and NH-MIL-125(Ti)) such as high water-stability, long-term chemical stability, large surface area, and wonderful photocatalytic activity attracted the researchers attention. The NH₂-MIL-125(Ti) and its composites were used for the adsorption and degradation of various pollutants in the aqueous solution (Table 1).

Due to the instability of MOFs in water and falling apart their structure, leaching of metal and ligand to the solution is inevitable. To eliminate this problem, researchers used magnetic MOFs to simplify the separation process by utilizing an external magnetic field and preventing the increment of pollution by MOFs. Many different magnetic particles such as Fe₂O₃, Fe₃O₄, ZnFe₂O₄, MnFe₂O₄, NiFe₂O₄, and CoFe₂O₄ were applied by researchers [[27], [28], [29]]. CoFe₂O₄ as a vigorous magnetic nanoparticle, which has thin optical bandgap and great antibacterial properties, was selected to combine with photocatalysts such as TiO₂ by researchers [30,31].

To date, silver nanoparticle and its compounds such as Ag₃VO₄, Ag₂O, Ag₂S, Ag₃PO₄, Ag₂CO₃, and especially silver halides (AgX, X = Cl, Br, and I) were widely applied for the purification of organically contaminated wastewaters through photodegradation process [[32], [33], [34], [35], [36], [37], [38]]. Assembling of Ag⁰ with silver halides makes them more active in the visible region. Recently, few studies were performed on Ag/AgCl@MOFs as powerful photocatalysts (Table 2).

In this work, the novel composite (CoFe₂O₄/NMT (CFNMT)) was synthesized via the hydrothermal process and boosted by room temperature surface assembling of Ag/AgCl through the reaction between Ag⁺ and Cl⁻ ions for remediation of RhB in aqueous solution under visible light. The morphology, functional group, crystalline structure, photocatalytic activity, and magnetic quantities of the prepared materials were investigated by SEM/EDX, TEM, FTIR, XRD, UV-DRS, and VSM analyses. The comparison test was performed among the synthesized materials to select the superior photocatalyst. Impact of four most effective parameters (amount of H₂O₂, the dosage of catalyst, pH, and RhB concentration) on remediation of pollutant molecules was studied. Several decolorization kinetic tests were performed to investigate the kinetic behavior of Ag/AgCl@CFNMT. Also, the regeneration test was performed to investigate the reusability and performance stability of Ag/AgCl@CFNMT.