Bimetalic ZIFs(Zn-M) (M: Co, Ni) for photocatalytic removal of organic pollutant in aqueous solution

. In this study, bimetallic ZIFs(Zn-M) of 2-methyl imidazole were successfully synthesized with the participation of a second metal, herein cobalt or nickel, into the ZIF network. All ZIFs samples were synthesized at room temperature, then characterized by XRD, FT-IR, SEM, DRS, nitrogen adsorption and desorption isotherms. All samples possessed a high crystallinity with a large surface area. The photocatalytic ability of all samples was examined through the degradation of indigo carmine under a solar light simulator. The presence of a second metal enhanced the photon harvesting ability of ZIF(Zn) when the materials can absorb photons in both UV and visible region. Among synthesized ZIFs, ZIF(Zn-Co) exhibited an extremely high photocatalytic ability when removed 100 % of indigo carmine from an aqueous solution. After 2 h, the ZIF(Zn-Co) adsorbed 45 % and degraded 55 % of indigo carmine under 1.5 h of solar irradiation. However, while the existence of Co 2+ increased the surface area of ZIFs, Ni 2+ reduced it. As a result, the photocatalytic efficiency of ZIF(Zn-Ni) was not higher than ZIF-8(Zn).


Introduction
Metal-organic frameworks (MOFs) are hybrid, crystalline, porous materials constructed from metal-ligand coordination, forming infinite networks.The inorganic nodes or vertices in the framework consist of metal ions or clusters, namely secondary building units (SBUs).These nodes are connected by coordination bonds to organic linkers, which commonly contain carboxylate, phosphonate, pyridyl, imidazolate or other azolate functional groups.The combination of ease of tunability, atomic-level structural uniformity, compositional (elemental) variety and high porosity is, in large measure, what distinguishes MOFs from other porous materials such as activated carbon, porous organic polymers and zeolites [1].MOFs have exceptional porosity and a wide range of potential uses including gas storage, separations, and catalysis [2].
Zeolitic imidazolate frameworks (ZIFs) are a new class of porous crystals with extended three-dimensional structures constructed from tetrahedral metal ions (e.g., Zn, Co) bridged by imidazolate (Im).By combining metal salts with imidazole in solution, a large number of crystalline ZIFs have been made; some of these possess topologies found in zeolites, and others have yet to be made as zeolites [3].
Among ZIFs family, ZIF-8(Zn) has been explored as highly thermal and chemical stability [1], high porosity, and ease of synthesis, so become an attractive candidate for many applications such as separation and storage of gases, removal of organic pollutants in solution [4][5][6][7].However, due to the large band gap (~5 eV), its photocatalytic ability is limited.Jing et al. [8] used ZIF-8 as a catalyst for photodegradation of methylene blue under UV irradiation, even though, the efficiency of dye removal is not very practically valuable.

Methods
Synthesis of ZIF-8(Zn): ZIF-8 was synthesized with slight modification to a previously reported procedure [10].Specifically, Zn(NO3)2.6H2O(3 mmol) was dissolved in 30 mL of methanol (MeOH) to form a clear solution, which was subsequently poured into 10 mL of MeOH containing Hmim (12 mmol).After thorough mixing, the resulting solution was incubated at room temperature for 24 h.The as-obtained precipitates were centrifuged and washed with ethanol and dry in a vacuum overnight.

Results and Discussion
The phase purity of bimetal ZIFs was confirmed by XRD patterns (Figure 1).two Hmim ligands and one N-H stretching vibration at 1846 cm -1 (Fig. 2).These two adsorption bands disappeared in as-synthesized ZIFs, indicating the deprotonation of the N-H groups of the Hmim ligands upon coordination with metal ions [11].A shift of the C-H bending signal from 1117 cm -1 to 1143 cm -1 was observed (Fig. 2), which can be assigned for the transformation of imidazole to imidazolate [13].
All those information indicated that ligand molecules deprotonated to form coordination bonds with metal ions.The morphology and particle size of ZIFs samples were illustrated via SEM images (Fig. 3).
All samples showed regular morphology and Photocatalytic activity tests: In a typical photodegradation experiment, 50 mg of the photocatalyst powder was dispersed in a quartz beaker containing 50 mL of an aqueous solution of indigo carmine (C0 = 50 ppm).Before irradiation, the suspension was stirred magnetically in the dark for a couple of hours to obtain an absorption-desorption equilibrium condition.A solar simulator (150 W short arc lamp) was used as a light source.At selected time intervals, aliquots of the suspension were collected, centrifuged, and filtered by membranes to remove the catalyst particulates.The concentration of the rest (C) of the solution was evaluated using a UV-Vis spectrophotometer (Jasco-V-670).
The existence of characteristic peaks for single-metal ZIF-8 on XRD patterns confirmed the successful synthesis of ZIF-8(Zn)[11,12].The characteristic peaks of ZIF-8 still appeared on XRD patterns of bimetallic ZIFs, demonstrating that the zeolite structure of ZIF-8 still remained under the participation of the second metal ion (Ni 2+ , Co 2+ ).For all ZIFs, the XRD patterns exhibited sharp peaks with no appearance of additional phases, suggesting that the obtained products are highly purity and crystallinity.The coordination of the organic ligand with the metals was investigated by IR spectra.The spectra of Hmim showed a strong and broad band from 3400 to 2200 cm -1 which is assigned for the N-H•••N hydrogen bond established between