Effective Photocatalytic Activity of Mixed Ni/Fe-Base Metal-Organic Framework under a Compact Fluorescent Daylight Lamp

Abstract

Mixed Ni/Fe-base metal-organic framework (Ni/Fe-MOF) with different molar ratios of Ni²⁺/Fe³⁺ have been successfully produced using an appropriate solvothermal router. Physicochemical properties of all samples were characterized using X-ray diffraction (XRD), Raman, field emission scanning electron microscopes (FE-SEM), fourier-transform infrared spectroscopy (FT-IR), N₂ adsorption-desorption analysis, X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS), and photoluminescence spectra (PL). The photocatalytic degradation performances of the photocatalysts were evaluated in the decomposition of rhodamine B (RhB) under a compact fluorescent daylight lamp. From XRD, IR, XPS, and Raman results, with the presence of mixed ion Fe³⁺ and Ni²⁺, MIL-88B (MIL standing for Materials of Institut Lavoisier) crystals based on the mixed metal Fe₂NiO cluster were formed, while MIL-53(Fe) was formed with the presence of single ion Fe³⁺. From UV-Vis DRS results, Ni/Fe-MOF samples exhibited the absorption spectrum up to the visible region, and then they showed the high photocatalytic activity under visible light irradiation. A Ni/Fe-MOF sample with a Ni²⁺/Fe³⁺ molar ratio of 0.3 showed the highest photocatalytic degradation capacity of RhB, superior to that of the MIL-53(Fe) sample. The obtained result could be explained as a consequence of the large surface area with large pore volumes and pore size by the Ni²⁺ incorporating into the MOF’s structure. In addition, a mixed metal Fe/Ni-based framework consisted of mixed-metal cluster Fe₂NiO with an electron transfer effect and may enhance the photocatalytic performance.

1. Introduction

Metal-organic frameworks (MOFs), a new class of high surface area and crystalline porous materials, assemble with metal clusters and organic bridging ligands [1]. These materials have received considerable attention in recent years due to their high resistance, high surface area, large pore volume, low density, and easily tunable framework. Among the MOFs, MIL-53(Fe) 88B (MIL standing for Materials of Institut Lavoisier) have attracted extensive interest for applications in gas storage [2,3], adsorption and separation of heavy metal [4], sensors [5], and in the biomedical field such as for drug delivery [6].

Recently, to eliminate organic dyes, many approaches have been suggested including adsorption [7,8,9,10] and photodegradation [11,12,13]. However, the latter is of interest because this process could decompose organic dyes to CO₂, H₂O, and harmless inorganics, while the adsorption process is only capable of removing dyes from water media. MIL-53(Fe) as a catalyst carrier or modification of MIL-53(Fe) as a catalyst for chemical reactions has received research attention [14]. MIL-53(Fe) has the chemical formula of Fe^III(OH)(O₂C–C₆H₄–CO₂)·H₂O, which consists of FeO₆ octahedral chains connected to benzene dicarboxylate (BDC) anions, forming a three-dimensional network with a large volume and high surface area [2,14,15]. The FeO₆ octahedral chains have the potential to act as a Lewis acid in many organic reactions [16]. Recently, MIL-53(Fe) with the potential use of FeO₆ octahedral chains has received much attention in photocatalytic degradation of many organic dyes, such as methylene blue [11,13,17], rhodamine B (RhB) [14,16,17], and p-nitrophenol [14], and has given good decomposition results. Therefore, this is a possible application direction of MIL-53(Fe) in the removal of organic dyes.

Fe-based MOFs materials have been reported as an effective photocatalyst for decomposition of organic dyes under visible light irradiation [18,19,20,21,22]. However, their photocatalytic performance is not as expected because of the fast recombination of photogenerated holes (h⁺) and electrons (e⁻), resulting in the lack of h⁺ for degradation dyes [13]. To address this, various approaches have been proposed to depress the recombination process. For example, inorganic oxidants (e.g., H₂O₂, KBrO₃, and (NH₄)₂S₂O₈), which act as electron acceptors, was introduced in the photocatalytic processes, significantly enhancing the photocatalytic effect of these materials. According to research by Yuan et al. [13], H₂O₂ is an efficient electron acceptor in the photocatalytic decomposition process of organic pigments by MIL-53(Fe) under visible light irradiation. Another approach that has been developed to enhance the photocatalytic performance of MiL-53(Fe) is the designed synthesis of composite photocatalysts containing MOFs materials such as CdS/MIL-53(Fe) [23], Ni-MOFs@GO [24], Fe₃O₄/MIL-53(Fe) [14], and Fe₂O₃/MIL-53(Fe) [25]. In addition, MIL-53(Fe) that has been doped or combined with one or more metals have also attracted much attention in recent years [26,27,28,29]. For this study, Qiao Sun et al. modified the MIL-53(Fe) by adding Mn, Co, and Ni metal into the framework of MIL-53(Fe) material, which exhibited excellent catalytic performance in liquid-phase degradation of phenol [30]. Various rare-earth or transition metals that modify MOFs structures have recently been reported such as three-dimensional Ln(III)–Zn(II) heterometallic coordination polymers [31], Fe substituted Cr MIL-101 [32], Ag-doped MOF-like organotitanium polymer (Ag@NH2-MOP(Ti)) [33], Ti-doped UiO-66 [34], Eu substituted Fe MIL-53 [35], and Zn-Ln coordination polymers (Ln = Nd, Pr, Sm, Eu, Tb, Dy) [36].

In this work, we report the synthesis of Ni/Fe-MOF with different Ni²⁺/Fe³⁺ molar ratios using the solvothermal route and their application for the degradation of RhB solution under visible light irradiation using a 40 W compact fluorescent lamp. To illustrate our method for the synthesis of Ni/Fe-MOF, we have selected the preparation of the MIL-53(Fe) structure, which consists of FeO₆ octahedral chains connected to BDC anions. Thanks to the presence of Ni²⁺ ions in the reaction solution, MIL-88B crystals were formed with neutral mixed-metal clusters (Fe₂NiO) connected via BDC anions. This structure is similar to the MIL-88B structure consisting of the trinuclear oxo-centered iron cluster (Fe₃O) [27,28]. However, our bimetallic metal MOF products were expected to exhibit an excellent adsorption capacity and photocatalytic activity in comparison to the original single metal MOFs. The advantage of selecting MOF material containing Fe and Ni is due to the low cost, non-toxicity, and natural abundance of these two transition metal oxides. In addition, the MOF material is also capable of improving the separation efficiency of electron–hole pairs when Ni is incorporated into the structure of materials [37,38]. The structure, morphology, and optical properties of the obtained photocatalysts have been characterized using X-ray diffraction (XRD), Raman, field emission scanning electron microscopes and energy-dispersive X-ray spectrometer (FE-SEM/EDS), fourier-transform infrared spectroscopy (FT-IR), N₂ adsorption-desorption analysis, X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS), photoluminescence (PL) spectra and nitrogen physisorption measurements (BET). Besides, to obtain the optimal reaction conditions for the RhB photodecomposition, the effect of the initial RhB concentration and pH on the degradation of RhB was also investigated in detail.

2. Results and Discussion

2.1. Physical Properties of MIL-53(Fe) and Ni-Doped MIL-53(Fe)

2.1.1. XRD Analysis

Figure 1 presents the XRD diffraction patterns of the MIL-53(Fe) and Ni/Fe-MOF samples isolated from dimethylformamide (DMF) and H₂O. In patterns of MIL-53(Fe) samples (Figure 1A, curve a), the main diffraction peaks that appeared at 2θ of 9.1°, 9.4°, 14.1°, 16.5°, and 18.8° are similar to those previously reported for MIL-53(Fe) isolated from DMF [2,11,39]. In patterns of Ni/Fe-MOF samples (Figure 1A, curves b–e), the main diffraction peaks that appeared around 2θ of 7.3°, 8.9°, 9.3°, 9.9°, 16.8°, 18.7°, 17.7°, 20.1°, and 21.9° are similar to those previously reported for MIL-88B isolated from DMF. Notably, the diffraction peak at a 2θ of 7.3° observed in the XRD patterns of Ni/Fe-MOF samples increased in intensity as the molar ratio of Ni²⁺/Fe³⁺ increased from 0.1 to 0.7. With the presence of Ni²⁺ in the reaction solution, MIL-88B crystals were made up and the crystallinity of the material increased. This observation might be attributed to the fact that the structure formation of Ni/Fe-MOF was significantly influenced by the presence of Ni²⁺ in the reaction solution. In addition, no other diffraction peak associated with nickel oxides, iron oxides, or other impurities could be detected, demonstrating the high purity of the samples.

XRD patterns of the MIL-53(Fe) and Ni/Fe-MOF samples isolated from H₂O (Figure 1B) showed the rugged background and weak intensities; however, the main diffraction peaks still maintained the same structure as in Reference [4]. The difference in XRD patterns of samples isolated from DMF and water may attribute to the breathing behavior of MIL-53(Fe) and MIL-88B, which has been well documented by Alhanami et al. [15]. Moreover, MIL-53(Fe)·H₂O sample essentially shows a noncrystalline phase similar to those for MIL-53(Fe)·DMF. They can be explained by the effect of the synthesis temperature on the structure formation of MIL-53(Fe). Pu et al. demonstrated that iron ion and H₂BDC could not coordinate successfully under a low temperature (100 °C), and therefore the MIL-53(Fe) crystal structure could not fully develop [40]. However, the Ni/Fe-MOF samples still show a high crystalline phase under low synthesis temperatures. Again, these results indicate that the presence of a mixed metal ion (Ni²⁺ and Fe³⁺ ion) did have a significant influence on the formation of Ni/Fe-MOF crystal structure, in which a Ni²⁺ and Fe³⁺ ion can coordinate with H₂BDC to form MIL-88B crystals instead of MIL-53(Fe) crystals.

2.1.2. FT-IR Spectra

FTIR spectroscopic studies were performed for all samples in the wave range of 400–4000 cm⁻¹, as shown in Figure 2. As shown in Figure 2A,C, strong vibrational bands around 1657, 1601, 1391, 1017, and 749 cm⁻¹, which are attributed to υ(C=O), υas(OCO), υs(OCO), υ(C–O), and δ(C–H) vibrations confirms the presence the bridge coordination mode of metal carboxylates in the MOF structures [4,25,30]. No band at 1700 cm⁻¹ was found, implying no free H₂BDC [27]. The band characteristics of DMF (1657 cm⁻¹) and H₂O (3387 cm⁻¹) were present in the samples MIL-53(Fe)·DMF, Ni/Fe-MOF·DMF, MIL-53(Fe)·H₂O and Ni/Fe-MOF-x·H₂O, respectively [27].

At lower frequencies (Figure 2B), vibrational bands around 750 cm⁻¹, 690 cm⁻¹, and 660 cm⁻¹ represent the C–H vibration, C=C stretch, OH bend, and OCO bend, respectively, were found, implying the presence of the vibrations of the organic ligand BDC [27]. Figure 2B also shows that the strong band at 547 cm⁻¹ in all samples could be attributed to Fe–O vibrations or Ni–O vibrations [41]. The band around 625 cm⁻¹ belongs to the Fe₃O vibration, which was observed in MIL-53(Fe) and Ni-Ni/Fe-MOF-0.1 samples. The weak band around 720 cm⁻¹ is related to the Fe₂NiO vibration, which was observed in Ni/Fe-MOF-x samples [27]. These results reaffirmed that Ni²⁺ and Fe³⁺ ions can coordinate with H₂BDC to form MIL-88B crystals.

2.1.3. Raman Spectra

Samples were analyzed using Raman spectroscopy using an excitation wavelength at 633 nm and spectra recorded at a wavenumber range of 100–900 cm⁻¹, as shown in Figure 3. According to previous studies, the BDC bridge in MOFs has Raman-active modes: the symmetric vibration modes (vs. (COO)) and asymmetric vibration (vas (COO)) of the carboxylate group (1445 cm⁻¹ and 1501 cm⁻¹), the vibration of the C–C bond between the benzene ring and the carboxylate group (1140 cm⁻¹), and the external plane deformation of the C–H link (865 cm⁻¹ and 630 cm⁻¹) [28]. As seen in Figure 3, the presence of a BDC linker was also observed in all samples, and no Raman signals corresponding to nickel oxides, iron oxides, or other impurities were found on any of the samples, which is consistent with the results of the XRD patterns. Notably, the Raman signal corresponding to the symmetric vibration (vs. (OCO)) of the carboxylate group showed a shift to a lower wavenumber and the peak split into two peaks corresponding to an increase of the Ni²⁺/Fe³⁺ molar ratio. This result was due to the change in the charge distribution in the organic bridge when they were coordinated with different metal ions (Figure 3B). Ionic Ni²⁺ has a smaller nuclear charge and a larger ionic radius than Fe³⁺ ($r_{N{i}^{2+}}=0.69 \mathsf{\AA }$ and $r_{F{e}^{3+}}=0.55 \mathsf{\AA }$) [42]. Therefore, Ni²⁺ creates a weaker coordinated link with the OCO group on the organic bridge than Fe³⁺, thus the symmetric vibration (vs. (OCO)) of the carboxylate group when forming coordinated bonds with Ni²⁺ moves to a lower wavenumber than Fe³⁺ [43]. This result is commensurate with the XRD and IR results for Ni/Fe MOF.

2.1.4. FE-SEM/EDS Analysis

Figure 4 displays SEM images and EDS spectra of the as-prepared MOF samples. As shown in Figure 4, the morphologies and shapes of MOF samples varied according to the molar ratio of Ni²⁺/Fe³⁺. MIL-53(Fe) sample mostly had amorphous nanoparticles (Figure 4(a1,a2)), which is in good agreement with the results of XRD patterns with a poor crystallinity. When the molar ratio of Ni²⁺/Fe³⁺ was set to 0.1, the crystals of Ni/Fe-MOF-0.1 were not homogeneous with different shapes and sizes (Figure 4(b1,b2)). A mixture of octahedral and hexagonal bipyramidal shapes, and nanoparticles, were perceived when the molar ratio of Ni²⁺/Fe³⁺ (0.3–0.7) was increased further. However, these octahedral and hexagonal bipyramidal shapes collapsed with cracks on the crystal surface. These results, along with the XRD, IR, and Raman results above, indicate that a mixed-metal Ni/Fe-MOF was successfully synthesized using the solvothermal method.

Moreover, to confirm the molar ratio of Ni²⁺/Fe³⁺ in the Ni/Fe-MOF samples in comparison to the theoretical value, EDS was also conducted. The result from the EDS spectrum of the obtained MIL-53(Fe) sample (Figure 4(a3)) showed the coexistence of C, O, Fe, and Cl. The presence of Cl may have been due to the FeCl₃ precursor, further confirming that the MIL-53(Fe) crystal structure could not fully develop at a low temperature (100 °C). The EDS spectra of the Ni/Fe-MOF samples (Figure 4(b3,c3,d3,e3)) revealed that these samples contained C, O, Fe, and Ni. However, the existence of Cl was still observed in the Ni/Fe-MOF-0.1 sample. The molar ratio of Ni²⁺/Fe³⁺ of Ni/Fe-MOF-0.1, Ni/Fe-MOF-0.3, Ni/Fe-MOF-0.5, and Ni/Fe-MOF-0.7, obtained using EDS analysis, was 0.16, 0.30, 0.48, and 0.66, respectively. In addition, the map of Fe, O, C, and Ni is shown in Figure S1, which indicates that they were uniformly distributed over the MOF surface.

2.1.5. XPS Spectra

To analyze the chemical states of Ni and Fe in the Ni/Fe MOF structure, XPS spectroscopy was carried out. As illustrated in Figure 5A, the wide-scan XPS spectra of MIL-53(Fe)·H₂O possesses the characteristic peaks of C, O, Fe, and Cl, while Ni/Fe-MOF-0.3.H₂O contained C, O, Fe, and Ni. Based on the XPS analysis, the Ni/Fe-MOF-0.3 had a surface molar ratio of Ni²⁺/Fe³⁺ of 0.26, which approximates the EDS results above. Besides, N was not detected in either sample, indicating that the DMF solvent was sufficiently eliminated from the MOFs.

Figure 5B shows the C 1s XPS spectra of MIL-53(Fe)·H₂O and Ni/Fe-MOF-0.3·H₂O samples. Both spectra were fitted into three peaks at a binding energy (BE) of 285.01, 288.9, and 291.7 eV, which could be assigned to the carbon components on the phenyl and the carboxylate groups of the BDC linkers [30,40,44,45,46]. The O 1s XPS spectra (Figure 5C) could also be fitted into three peaks, which are (i) the peak at 533.8 eV corresponding to the O components on C=O/H₂O, (ii) the peak at 532.3 eV attributed to the O components on the BDC linkers, and (iii) the peak at 530. 5 eV was assigned to the O components on the Fe–O bonds (for MIL-53(Fe) sample) or Fe₂NiO clusters (for Ni/Fe-MOF-0.3 sample). These results further confirmed the coordination between the metal ion (Ni²⁺ and/or Fe³⁺) and BDC linkers, which is commensurate with the XRD, IR, and Raman results above.

The Fe 2p high-resolution XPS spectrum of MIL-53(Fe) sample (Figure 5D) displays two main peaks that were indexed to Fe 2p1/2 (712.4 eV) and Fe2p3/2 (726.1 eV). The splitting energy of the 2p doublet was 13.7 eV, implying that the valence state of Fe was +3 [4,23,44]. Similarly, the valence state of Fe in the Ni/Fe MOF structure was also +3 because the splitting energy between Fe 2p1/2 (712.9 eV) and Fe 2p3/2 (726.2 eV) was 13.3 eV. To further confirm the valence state of Fe in both of these samples, the Fe 2p3/2 peak was fitted into six peaks including Gupta and Sen (GS) multiples, surface structures, and shake-up-related satellites [28,47,48]. The fitting results, as shown in Figures S6 and S7, were indexed well with Fe³⁺ GS multiplets, which indicated that the valence state of Fe in the MIL-53(Fe) and Ni/Fe MOF structure was +3. In the high-resolution XPS spectrum of Ni 2p (Figure 4e), we observed the BE of the Ni 2p3/2 (857.2 eV) and Ni 2p1/2 (874.8 eV) core-level peaks with the doublet separation of 17.6 eV, implying that the valence state of Ni was +2 [49,50].

2.1.6. N₂ Adsorption/Desorption

The specific surface area and porous structure of MIL-53(Fe) and Ni/Fe-MOF crystals isolated from DMF and H₂O were determined using N₂ adsorption–desorption isotherms at 77 K. The N₂ adsorption–desorption isotherms, as shown in Figure 6A, displayed an intermediate mode between type I and type IV, which was associated with mesoporous and microporous materials, respectively [51]. The Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore width of MIL-53(Fe) and Ni/Fe-MOF-0.3 samples are shown in

Table 1. Specific surface area and porosity of MiL-53(Fe) and Ni/Fe-MOF samples.

Samples	Specific Surface Area (m2/g)	Micropore Volume (×10−3 cm3/g)	Mesopore Volume (×10−3 cm3/g)	Average Pore Width (nm)
MIL-53(Fe)·DMF	300	128	97	13
MIL-53(Fe)·H2O	158	65	59	11
Ni/Fe-MOF-0.3·DMF	480	212	128	8
Ni/Fe-MOF-0.3·H2O	247	94	271	13

. The MIL-53(Fe)·H₂O, MIL-53(Fe)·DMF, Ni/Fe-MOF-0.3.H₂O, and Ni/Fe-MOF-0.3.DMF had specific surface areas of 158, 300, 247, and 480 m²/g, respectively (Table 1). The mesopore size distribution curve of samples calculated using the Barrett–Joyner–Halenda (BJH) model is shown in Figure 6B. The MIL-53(Fe)·H₂O and MIL-53(Fe)·DMF sample was non-porous, whereas Ni/Fe-MOF-0.3·H₂O, and Ni/Fe-MOF-0.3·DMF showed a pore size centered at about 3.8 nm and 21.4 nm, respectively. Therefore, compared with MIL-53(Fe), Ni/Fe-MOF-0.3 showed a higher value in the specific surface areas. In addition, the higher surface area and micropore volume for samples isolated from DMF, as compared with samples isolated from H₂O, was due to the reversible breathing behavior of these materials, which was dependent on the molecule present inside their pores, where the pores were opened in the presence of DMF and closed in the presence of H₂O [27,28,52]. The formation of porous material for Ni/Fe-MOF-0.3 could be explained by the formation of Fe₂NiO cluster in the Ni/Fe-MOF structure, which could affect the reversible breathing behavior of these materials. MIL-88B(Fe) crystals with trinuclear metal clusters were known as non-porous materials due to the need for compensating the anion inside their porous system [28,53]. Do and coworkers demonstrated that MOF structure with the presence of Fe₂NiO cluster as nodes in the MIL-88B framework avoids the compensating anion [27,28], which results in the formation of porous material for Ni/Fe-MOF-0.3. In addition, the cracks on the crystal surface of Ni/Fe-MOF-0.3 (Figure 4) could also partly create the characteristics of microporous or mesoporous materials for this sample.

2.1.7. UV-Vis Spectra

The light absorption properties of the material were studied through the UV-Vis-DRS spectra. The UV-Vis-DRS spectrum of the material is shown in Figure 7. For washing samples with DMF (Figure 7A), MIL-53(Fe)·DMF gave strong absorption bands in the wavelength range of 200 to 400 nm. The strong absorption bands at 256 to 310 nm could be due to the transfer of the charge from the oxygen center of the organic bridge to the metal center in the octahedral FeO₆ structure [17,54]. The band at 350 to 500 nm was due to the shift of d–d (6A1g → 4A1g + 4Eg (G)) of Fe³⁺ in the MIL-53(Fe) structure [14,27]. The main absorption edge (λ, nm) of the MIL-53(Fe)·DMF was 478 nm, corresponding to the bandgap energy E_g = 2.59 eV (E_g = 1240/λ). This result is in accordance with previous reports [44,55]. When the MIL-53(Fe) was modified with Ni, the material have the decreased absorption in the wavelength range from 200 to 500 nm, and the absorption spectrum extended in the range from 250 to 800 nm, so it was difficult to determine the absorption of the material accurately. When the material was washed with water (Figure 7B), the modified material had an increased absorption in the wavelength range from 200 to 400 nm, and the absorption intensity was higher and broader in the visible light region as compared to the modified sample washed with DMF. As the material was washed with water, there was a structural change between the large pore and the narrow pore caused by the “breathing” effect when the material absorbed the water molecules inside the pore. This phase transformation of the structure led to a change in the electronic structure [56], and subsequently, a change in the absorption spectrum of the material and decreasing E_g. For Ni/Fe-MOF-0.1·H₂O, Ni/Fe-MOF-0.3·H₂O, and Ni/Fe-MOF-0.5·H₂O samples, the absorption intensity in the visible light region and the absorption band of the material shifted to a wavelength longer than for MIL-53(Fe)·H₂O. As absorption in the visible light increased, the visible light energy could be used more efficiently, thus contributing to the increased photocatalytic efficiency of the material. The absorption edges of MIL-53(Fe)·H₂O, Ni/Fe-MOF-0.1·H₂O, Ni/Fe-MOF-0.3·H₂O, Ni/Fe-MOF-0.5·H₂O, and Ni/Fe-MOF-0.7·H₂O were 504, 553, 532, 513, and 516 nm (Figure S2), corresponding to the optical bandgap of 2.46, 2.24, 2.33, 2.42, and 2.40 eV, respectively. These results provided a potential photoreactivity of MIL-53(Fe) and Ni/Fe-MOF samples in the visible light range.

2.1.8. PL Spectroscopy

PL spectra of MIL-53(Fe) and Ni/Fe-MOF samples were recorded at room temperature and are shown in Figure 8. When the MIL-53(Fe) sample was excited by a 320 nm laser, its emission spectrum showed a strong emission range of 350 to 500 nm and a weak emission range of 570 to 750 nm. In comparison, the intensity of Ni/Fe-MOF samples was significantly lower than that of the MIL-53(Fe) sample because of the presence of the Ni₂FeO cluster in the structure of the Ni/Fe-MOF crystal. These results demonstrated that electron–hole recombination could be inhibited in the Ni/Fe-MOF, resulting in the improvement of photocatalytic performance. PL spectra, along with the UV-Vis DRS result, could satisfy the prerequisite for visible-light photocatalysis.

2.2. Photocatalytic Activities

2.2.1. RhB Removal by MIL-53(Fe) and Ni-MIL-53(Fe)

The photocatalytic activities of MIL-53(Fe) and Ni/Fe-MOF-x photocatalysts were evaluated in the liquid-phase photodegradation of RhB dye under visible light irradiation. Figure 9 displays the changes of RhB concentrations via adsorption and photocatalytic degradation under different experimental conditions. As shown in Figure 9, a negligible degradation of RhB concentrations was observed in the several blank runs including RhB/H₂O₂/Dark, RhB/H₂O₂/Light, and RhB/Dark systems, proving the stability property of RhB under visible light irradiation of compact fluorescent light. Also, as shown in Figure 9A, after 180 min adsorption (in the dark), 16% and 51% RhB were removed in the presence of MIL-53(Fe) (MIL-53(Fe)/Dark system) and Ni/Fe-MOF-0.3 (Ni/Fe-MOF-0.3/Dark system), respectively. The higher adsorption capacity of the Ni/Fe-MOF-0.3 sample was due to its higher surface area (247 m²/g for Ni/Fe-MOF-0.3 and 158 m²/g for MIL-53(Fe)). In addition, there was no significant difference in the removal of RhB concentration in the two adsorption experiments with the presence of H₂O₂ (MIL-53(Fe)/H₂O₂/Dark and Ni/Fe-MOF-0.3/H₂O₂/Dark systems and the absence of H₂O₂ (MIL-53(Fe)/Dark and Ni/Fe-MOF-0.3/Dark systems. Therefore, our photocatalytic experiments do display the presence of a Fenton reaction.

Under visible light irradiation, the presence of MIL-53(Fe) could enhance the degradation efficiency of RhB up to 81.46% using a photolysis process in MIL-53(Fe)/Light/H₂O₂ catalytic system (Figure 9B). For the Ni/Fe-MOF-0.3/Light/H₂O₂ catalytic system, the degradation efficiency of RhB was remarkably enhanced where about 91.14% RhB removal was achieved (Figure 9B). The higher photocatalytic activity of the Ni/Fe-MOF-0.3 sample as compared with MIL-53(Fe) could also be indicated by the change of the UV-Vis absorption spectra of the solution in the course of the RhB degradation (Figure 9C,D). As seen in Figure 9C,D, the primary absorption band, which could be attributed to RhB, shifted from 554 to 500 nm in a step-wise manner. This change could be reasonably assigned to the removal of ethyl groups one by one in this reaction, which is in good agreement with the previous literature. The photodegradation of RhB over MIL-53(Fe) and Ni/Fe-MOF-0.3 photocatalysts approximately followed a pseudo-first-order kinetics model: ln(C_o/C) = k_obst [57,58,59]. The presence of Ni/Fe-MOF-0.3 promoted the photodegradation rate; the rate constants were 8.88 × 10⁻³ min⁻¹ for MIL-53(Fe) and 11.15 × 10⁻³ min⁻¹ for Ni/Fe-MOF-0.3.

To investigate the role of H₂O₂ on the photocatalytic performance of MIL-53(Fe) and Ni/Fe-MOF photocatalysts, the photocatalytic processes with the presence and absence of H₂O₂ were conducted in parallel (Figure 9B). After 180 min of irradiation, the degradation rate of RhB over MIL-53(Fe)/Light/H₂O₂ and MIL-53(Fe)/Light process was 81.46% and 27.60%, respectively. Only MIL-53(Fe) with the absence of H₂O₂ exhibits the low efficiency of RhB photodegradation due to the fast electron-hole recombination, which is in good agreement with the previous literature [13,17]. For the MIL-53(Fe)/H₂O₂/Light process, H₂O₂ acted as an electron accepter, resulting in the suppression of charge recombination; therefore, the rate for RhB decomposition could be significantly enhanced, as was demonstrated by Du et al. [13]. Similarly, Ai et al. also showed that the enhancement of MI-53(Fe) photocatalytic performance could be due to the synergistic effects of the combination of MIL-53(Fe) and H₂O₂ under visible light irradiation [17]. Interestingly, the effect of H₂O₂ on the photocatalytic performance of the Ni/Fe-MOF photocatalyst showed a considerable difference. The Ni/Fe-MOF sample could degrade more than 90% of the initial RhB content regardless of the presence or absence of H₂O₂.

The superior catalytic performance of the Ni/Fe-MOF sample could be explained by the formation of the mixed metal cluster Fe₂NiO in the Ni/Fe-MOF framework. According to recent reports, the Fe-based framework (MIL-101, MIL-100, MIL-88, and MOF-235), containing single metal cluster Fe₃-µ₃-oxo clusters with small particle sizes, are proposed as a visible light photocatalyst [44,60,61,62,63]. The reaction mechanism of these materials have been reported based on semiconductor theory and previous reports [61,62,63,64]. Particularly, when the surface of MOFs material absorbs photons (E_photons ≥ E_g), the electrons (e⁻) in the valence band (VB) will be excited to the conduction band (CB), leaving the holes (h⁺) in the VB. These photogenerated e⁻–h⁺ pairs may be further involved in the following three processes: (i) successfully migration to the surface of MOFs, (ii) being captured by the defect sites in bulk and/or on the surface region of semiconductor, and (iii) recombining and releasing the energy in the form of heat or a photon. Then, the h⁺ can accept electrons and induce water molecules to generate hydroxyl radicals (•OH), which exhibit a high oxidation ability to decompose the organic dyes. However, there is a recombination of excessive electrons and holes, resulting in the restricted photocatalytic activity of this material. In our study, mixed a metal Fe/Ni-based framework that consists of a mixed-metal cluster Fe₂NiO with electron transfer effect may enhance the photocatalytic performance [45,61,65].

Besides, a mixed metal Fe/Ni-based framework that consists of the mixed-metal cluster Fe₂NiO possesses large pores and a high surface area, as compared with a single metal Fe-based framework; therefore, Ni/Fe-MOF exhibited a high adsorption capacity of RhB and high photocatalytic activity in RhB degradation. XRD patterns of Ni/Fe-MOF-0.3 before and after reactions were shown in Figure S3 (SI file). As shown in Figure S3, there was no apparent difference in the crystal structure. This result indicated that the crystal structure of the material did not change after the photocatalytic reaction.

2.2.2. Effect of Initial Dye Concentration, Initial Solution pH, and the Molar Ratio of Ni²⁺/Fe³⁺ on the Degradation of RhB

The effect of initial dye concentration on the degradation of RhB over the Ni/Fe-MOF-0.3/Light/H₂O₂ system was evaluated (Figure 10A). As shown in Figure 10A, the degradation efficiency of RhB was slightly decreased when increasing the initial dye concentration from 1 × 10⁻⁵ to 4 × 10⁻⁵ M. This was mainly because of the increase of the dye molecules around the active sites leading to inhibiting the penetration of light to the surface of the catalyst [66].

The effect of the initial pH on the degradation of RhB on the degradation of RhB over Ni/Fe-MOF/Light/H₂O₂ system was also investigated. The pH of the initial solution was selected as follows: 3, 5 (acidic), 7 (neutral), and 9 (basic). At different pH conditions, the Ni/Fe-MOF-0.3 remained most effective when it came to removing RhB. The RhB removal efficiency peaked at the solution pH of 5 and decreased with increasing pH thereafter (Figure 10B). This result could be explained by the fact that when the pH exceeded the isoelectric point of the material, they were negatively charged. In addition, the RhB used in this experiment was a cationic color such that the material would absorb the color gradually from pH 5 to 9. As the adsorption increased, the color molecules would shield the catalytic surface, which prevented light from irradiating on the catalyst surface, thus decreasing photocatalytic activity and reducing color removal. The pH at the isoelectric point or point of zero charge-pzc of the material was an important parameter for evaluation of the acidity/basicity and the surface charge of the adsorbent in solution. The determination of pHzpc was carried out according to our previously published study [67,68,69], as follows: Photocatalysts (20 mg) was added to flasks containing 100 mL of KCl 0.1 M at different initial pH values (pH_i = 2, 4, 6, 8, 10, and 12). The solutions were shaken in the shaker for 24 h, and then solids were removed from the mixture by centrifugation at 4000 rpm for 15 min. The final pH of the solution (pH_f) is measured using a pH meter. The curve was plotted via pH_f against the pH_i, and the pHpzc was calculated at pH_i = pH_f. As shown in Figure 11A,B, the pH_pzc values of the MIL-53(Fe) and Ni/Fe-MOF-0.3 were approximately equal and were within the pH range of 4.1–4.2.

The degradation results of the different molar ratios of Ni²⁺/Fe³⁺ in the samples are shown in Figure 10C, where the best performance was obtained with the Ni/Fe-MOF-0.3 sample, followed by the Ni/Fe-MOF-0.1 and Ni/Fe-MOF-0.7 samples. The Ni/Fe-MOF-0.5 sample showed the lowest catalytic activity among all the Ni/Fe-MOF catalysts. This result indicated that the different molar ratio of Ni²⁺/Fe³⁺ had a significant impact on the photocatalytic performance of Ni/Fe-MOF samples, which may be conducive to the structure and morphology formation of Ni/Fe-MOF.

3. Experimental

3.1. Materials

1,4-Benzenedioic acid (H₂BDC, 98%) and RhB (≥95%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Iron(III) chloride hexahydrate (FeCl₃·6H₂O, 99%), nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99%), N,N-dimethylformamide (DMF, 99%), ethanol, and hydrogen peroxide (H₂O₂, 30%) were obtained from Xilong Chemical Co., Ltd. (Guangzhou, China). All reagents were used as received without further purification.

3.2. Preparation of Catalysts

Ni/Fe-MOF samples were synthesized using a solvothermal router similar to MIL-53(Fe), according to the previous literature [39]. In a typical synthesis, 9 mmol of H₂BDC, 6 mmol of FeCl₃·6H₂O, and a certain amount of Ni(NO₃)₂·6H₂O were dissolved in 60 mL DMF. The obtained mixture was vigorously stirred for 30 min before being transferred into a 100 mL hydrothermal synthesis autoclave reactor 304 stainless steel high-pressure digestion tank with PTFE lining (Baoshishan Co., Ltd., Shanghai, China). The autoclave was heated at 100 °C in an oven (Memmert UN110, Schwabach, Germany) with a heating rate of 5 °C/min for three days. After being cooled to room temperature in air, the remaining H₂BDC was removed using a distillation method with DMF solvent for 24 h at 100 °C with a heating rate of 5 °C/min. The obtained suspension was centrifuged at 6000 rpm for 30 min, and the orange precipitates located at the bottom of the tube were washed with DMF (three times) and water (three times), respectively. Finally, the product was dried for 24 h at 60 °C. The obtained MOFs samples with corresponding Ni concentration were denoted as Ni/Fe-MOF-x (x is the molar ratio of Ni²⁺/Fe³⁺, and was chosen as 0, 0.1, 0.3, 0.5, and 0.7). The specific description is shown in Table S1 and the flow chart of the synthesis method is described in Figure S4. The sample was washed with DMF and water to obtain Ni/Fe-MOF-x·DMF and Ni/Fe-MOF-x·H₂O, respectively. For comparison, MIL(53) also was prepared using a similar method above without the presence of Ni(NO₃)₂·6H₂O in the reaction solution mixture.

3.3. Catalyst Characterization

Powder X-ray diffraction (XRD) patterns were conducted on a D8 Advance Bruker powder diffractometer with a Cu Kα source (λ = 0.15405) at a scan rate of 0.04°/s with 2θ = 2 to 30°. The surface morphologies and particle size of Ni/Fe-MOF samples were observed using field emission scanning electron microscope (FESEM, JEOL JSM-7600F, Peabody, MA, USA) equipped with an energy dispersive X-ray spectroscope (EDS, Oxford instruments 50 mm² X-Max, Abingdon, UK). FT-IR spectra were recorded on an EQUINOX 55 spectrometer (Bruker, Germany) using the KBr pellet technique. Raman spectroscopy was carried out on the HORIBA Jobin Yvon spectrometer with a laser beam of 633 nm. To examine the existence of Ni and Fe in the samples, X-ray photoelectron spectra (XPS) of the samples was measured using MultiLab 2000 spectrometer (Thermo VG Scientific, Waltham, MA, USA). The optical absorption characteristics of the photocatalysts were determined using ultraviolet-visible (UV/Vis) diffuse reflectance spectroscopy (UV/Vis DRS, Shimazu UV-2450, Kyoto, Japan) in the range 200–900 cm⁻¹. PL spectroscopy was performed using a Hitachi F4500 Fluorescence Spectrometer (Schaumburg, IL, USA) with the Xe Lamp Power range (700–900 V) at room temperature. The specific surface area and pore distribution of MIL-53(Fe) and Ni/Fe-MOFs were determined using the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) method, respectively (TriStar 3000 V6.07, Micromeritics instrument corporation, Norcross, GA, USA). The samples were kept at 200 °C for 5 h to degas. The pH value was measured using a pH meter (Consort-C1010, Turnhout, Belgium) at room temperature.

3.4. Photocatalytic Test

The photocatalytic activities of Ni/Fe-MOF photocatalysts were evaluated using the photodegradation of RhB under visible light irradiation with a 40 W compact fluorescent lamp (Philips) in the open air and at room temperature (Figure S5). The intensity and wavelength of the light source was 4400 lm and >400 nm, respectively (Figure S6 and Table S2). Therefore, it was suggested that the photocatalytic processes in our experiments were mainly due to the action of the visible light range [70,71,72]. In each run, a mixture of RhB aqueous solution (3.10⁻⁵ mol/L, 100 mL), the given catalyst (20 mg), and H₂O₂ (10⁻⁵ mol/L) was magnetically stirred in the presence or absence of light. Five milliliters of the suspension was withdrawn at the same intervals and immediately centrifuged to separate the photocatalyst particles for 15 min. The concentration of RhB was analyzed using a UV-visible spectrophotometer (Model Evolution 60S, Thermo Fisher Scientific, Waltham, MA, USA) at a maximum absorbance wavelength of λ = 554 nm. In addition, the effect of parameters including initial dye concentration and initial solution pH on the photodegradation of RhB over Ni/Fe-MOF photocatalysts was also investigated. pH levels of 3, 5, 7, and 9 were selected, whereas the concentrations of RhB were increased from 1.10⁻⁵ M to 4.10⁻⁵ M.

4. Conclusions

In summary, we have successfully prepared mixed Ni/Fe-base MOF with different molar ratios of Ni²⁺/Fe³⁺ via a direct solvothermal approach. The structure characterization results from XRD, Raman, XPS, and FT-IR confirmed that with the presence of mixed ionic Fe³⁺ and Ni²⁺, MIL-88B crystals based on the mixed metal Fe₂NiO cluster was formed, while MIL-53 (Fe) was formed with the presence of a single ion Fe³⁺. The photocatalytic performance of the obtained photocatalysts was evaluated in the decolorization of RhB dye. The results indicated that the obtained Ni/Fe-MOF samples exhibited high photocatalytic activity in comparison to MIL-53(Fe). The degradation rate of Ni/Fe-MOF-0.3 could reach the highest (91.14%) after 180 min of visible light irradiation. These results suggest that the Ni/Fe-MOF, which consist mixed-metal cluster Fe₂NiO with electron transfer effects, might enhance the photocatalytic performance.