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Article

Synthesis of Bi-Metallic-Sulphides/MOF-5@graphene Oxide Nanocomposites for the Removal of Hazardous Moxifloxacin

by
Aqsa Anum
1,
Muhammad Altaf Nazir
1,
Sobhy M. Ibrahim
2,
Syed Shoaib Ahmad Shah
3,
Asif A. Tahir
4,
Misbah Malik
1,
Muhammad Ahmad Wattoo
5 and
Aziz ur Rehman
1,*
1
Institute of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan
2
Department of Biochemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
3
Department of Chemistry, School of Natural Sciences, National University of Sciences and Technology, Islamabad 44000, Pakistan
4
Solar Energy Research Group, Environment and Sustainability Institute, Faculty of Environment Science and Economy, University of Exeter, Penryn Campus, Penryn TR10 9FE, UK
5
Department of Chemistry, The Quaid i Azam University Islamabad, Islamabad 15320, Pakistan
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(6), 984; https://doi.org/10.3390/catal13060984
Submission received: 15 May 2023 / Revised: 2 June 2023 / Accepted: 6 June 2023 / Published: 8 June 2023
(This article belongs to the Special Issue Advanced Catalytic Material for Water Treatment)
Browse Figures
Versions Notes

Abstract

:
The development of new and advanced materials for various environmental and energy applications is a prerequisite for the future. In this research, the removal of hazardous moxifloxacin (MOX) is accomplished by synthesizing new hybrids of MOF-5 i.e., Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni/Mo.S2/MOF-5 nanocomposites by using a metal-organic framework (MOF-5) and graphene oxide (GO) as a precursor. The introduction of NixMoxS2 facilitates the unique interfacial charge transfer at the heterojunction, demonstrating a significant improvement in the separation effectiveness of the photochemical electron-hole pairs. To evaluate equilibrium adsorption capacity, time, pH, and concentration of organic pollutants were used as experimental parameters. The adsorption kinetics data reveals pseudo-first-order (R2 = 0.965) kinetics when Ni/Mo.S2/MOF-5/GO photocatalyst was irradiated under light for 90 min against MOX degradation. This led to a narrow energy band gap (2.06 eV in Ni/Mo.S2/MOF-5/GO, compared to 2.30 eV in Ni/Mo.S2/MOF-5), as well as excellent photocatalytic activity in the photodegradation of moxifloxacin (MOX), listed in order: Ni/Mo.S2/MOF-5/GO (95%) > Ni.S2/MOF-5/GO (93%) > Mo.S2/MOF5/GO (90%) > Ni/Mo.S2/MOF-5 (86%) in concentrations up to 2.0 mgL−1, caused by the production of superoxide (O2•−) and hydroxide (OH•) radicals, which encouraged the effective photocatalytic activities of the heterostructure. After five successive tests demonstrating its excellent mechanical stability, the impressive recyclability results for the Ni/Mo.S2/MOF-5/GO revealed only a tiny variation in efficiency from 95% (for the first three runs) to 93% (in the fourth run) and 90% (in the fifth run). These findings show that the heterostructure of Ni/Mo.S2/MOF-5/GO is an effective heterojunction photocatalyst for the quick elimination of moxifloxacin (MOX) from aqueous media.

1. Introduction

A family of synthetic antibacterial substances known as fluoroquinolones (FQs) is becoming popular for majority of the nations in Europe and is one of the most typical antibiotics found in the treatment of wastewater [1]. In the animal agricultural industry, tetracycline, as well as enrofloxacin are widely used antibiotics for both treatment of infectious diseases in livestock animals and promotion of livestock growth, however, they are also responsible for severe contamination of water bodies and soil [2]. Quinolones, especially second and third generation drugs, are used more frequently in human medicine [3]. A fluoroquinolone of the third generation, moxifloxacin (MOX), is commonly used to treat certain bacterial infections in humans and animals [4]. MOX’s propensity for genotoxicity [5] makes its release and buildup in the environment a significant risk to ecosystems and health. Thus, a method for decontaminating the FQ drug moxifloxacin (MOX) from wastewater that is both efficient and affordable needs to be developed. Various techniques in practice for the exclusion of antibiotics and other noxious pollutants from water include adsorption, photo-Fenton, photo-degradation, and advanced oxidation processes [6,7,8,9]. Environmental remediation can be accomplished using sunlight-irradiated photo-degradation and is considered the most economic, most efficient, and productive technique for the treatment of waste water using various materials such as GO-based composites, hexaferrites, porous silica-based composites, metal oxide-based catalysts etc. [10,11,12,13].
Among all the promising classes of advanced materials, metal-organic frameworks (MOFs) are well-structured crystalline coordination polymers with large surface areas, exceptional absorbability, and strong catalytic properties [14,15,16]. Moreover, the crystallite sizes, chemical compositions, atomic weights, and weight percentages of the resultant nanocomposites and the optimized crystal phases can be easily controlled by controlling the conditions, such as the temperature, dwell time, heating rate and gaseous atmosphere etc. MOFs have been extensively applied in a variety of applications such as catalysis, energy-storage, supercapacitors, oxidation reduction reactions, and the adsorption of contaminants from polluted water [17,18,19,20,21].
MOF-5, first published in 1999, is composed of ZnO4 units linked together by a 1,4- benzenedicarboxylate linker to form an extraordinary cubic lattice. The MOF-5, initially appearing as a cubic structure, came into prominence in 2002 [22]. As a result of using pristine MOFs containing heteroatoms or MOFs with metal/non-metal guests as precursors, it has been demonstrated that nanocomposites resulting from this procedure can be doped in situ with metals and/or heteroatoms such as N, C, S, and P to alter the semiconductor’s electronic structure, semi-conductive properties, and energy band positions in order to improve solar absorption and charge separation. A number of factors prevent pristine MOFs from being a good candidate for direct photocatalytic applications, including the weak coordination bonds between metal ions (oxo-clusters) and organic linkers, the wide band gaps, and poor semi-conductive properties (limited charge generation and transfer). It has been reported that MOFs need to be coupled with other substances for adsorption, such as meso-porous SiO2/(MOF), meso-porous Al2O3/(MOF), MOF/g-C3N4, and MOF/graphene [23] To enhance the photocatalytic performance of MOF derived nanocomposites, it is imperative to optimize the morphologies, compositions, electrical conductivity, and interfacial contacts [24,25,26]. GO, known to exist for about 150 years, is a carbon-based compound with many hydroxyl, epoxide, and carboxyl surface groups. By subjecting graphite to intense oxidizers, it is possible to produce graphene oxide (GO), which is composed of elements C, O, and H in varying proportions [13,27,28]. The concept of MOF-GO nanocomposites is based on specifications for the hydroxyl (OH) and epoxy functional groups of GO, which enable metal ions in MOFs to operate as composites. Therefore, GO can form nonporous, dense arrays of layers by fusing the complementary properties of two materials. Hydrothermal synthesis is a simple approach for synthesizing and increasing the mesoscopic regularity of a material by using a chemical process at low temperature and pressure. This study used a one-step hydrothermal technique to generate a hybrid of MOF-5 and GO. Recently, several novel findings on MOF-GO hybrids [29] have been reported employing a variety of MOFs, such as MOFs based on zinc (MOF-5) [30], zirconium MOF (UiO-66) [31], and iron (MIL-100) [32]. However, there is presently little study on the capacity of MOF-GO composites or their photocatalytic performance in the removal of noxious pollutants from waste water.
Molybdenum disulfide (Mo.S2) exhibits a six-party condensed structure that is similar to graphene and exhibits good optical and electrical transmission properties. In particular, Mo.S2 nanosheets can enhance the number of reaction sites and are utilized as a non-noble metal catalyst to increase photocatalytic yield [33]. Metal ions of molybdenum and nickel can improve the structure of ligands and MOFs, which ultimately regulates the photocatalytic activity of MOF compounds. In the present work, the fabrication of MOF-5/GO using Ni.S2 nanospheres was carried out via a solvothermal technique with Mo.S2 coated on the surface of Ni.S2 nanospheres, as outlined in the synthesis scheme. The high attraction created by homologous inorganic molecules between the nucleation sites was utilized in the construction of the Ni.S2/Mo.S2/MOF-5/GO nanocomposite.
This study focused on the development of an effective and efficient MOF-5/GO heterojunction for photocatalytic degradation of toxic antibiotics from simulated wastewaters. For this, the fabrication of MOF-5/GO was carried out to produce a series of nanocomposites: Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni/Mo.S2/MOF-5. It is expected that the newly synthesized nanocomposites would expand the surface area and boost the amount of photogenerated electron-hole pairs, hence improving the photocatalytic activity of MOF-5/GO hybrids as they are formed. Photodegradation of pollutants, formation of reactive species such as superoxide and hydroxyl radicals, and formation of holes are the steps that lead to the transformation of organic pollutants into H2O and CO2. In our research, several factors are evaluated to determine how well the photodegradation process works [34], including the wavelength (UV, UV-Vis), intensity of incident light, pH value of the medium in the reactor, concentration of the photocatalysts and the organic pollutants, reaction time, and dynamics of charge carriers. We hope that researchers will be motivated by this work to investigate and develop new photocatalysts for the purification of water containing MOX and other noxious pollutants.

2. Results and Discussion

2.1. XRD Analysis

XRD patterns of Ni.Sx/Mo.Sx/MOF-5/GO are shown in Figure 1a–d while the MOF-5 is represented as (♣) in spectras. These graphs confirm the successful synthesis of nanocomposites Pristine MOF-5(♣) displays crystalline peaks at 2θ values 6.80°, 9.64°, 13.55°, 15.29°, 20.60°, 22.41°, 24.69°, 31.30°, and 32.12° for crystal planes 200, 220, 400, 420, 531, 533, 551, and 911 respectively [35].As seen in Figure 1a, GO has a distinctive, strong peak at 2θ = 26.5° that is attributed to 002. The MOF-5/GO exhibits diffraction peaks at 2θ = 10.12°, 15.61°, 18.55°, 20.58°, 25.95°, 27.27°, 28.39°, 30.31°, 31.12°, 32.34°, 34.06°, and 43.80° [36]. These peaks are somewhat displaced, but they do not alter the crystalline structure of the material. From the Ni/Mo.S2/MOF-5, Ni.S2/MOF-5/GO, and Ni-Mo.S2/MOF-5/GO catalysts, peaks attributed to Ni.S2 can be found at 2θ = 32.0° (200) and 45.6° (220), while Mo.S2 presence was confirmed (PDF#75-1539) with indexes at 2θ = 35.25° (103) and 54.5° (106). The spectra also comprised a few peaks of Ni from NiS2 at 62.2° and 75.01°, with indexes (220) and (311) along with peaks of Mo from Mo.S2 at 49.32° and 71.71° which correspond to (200) and (101), respectively. Furthermore, different structural parameters of the prepared nanocomposites determined from XRD analysis are presented in Table 1.

2.2. FTIR Analysis

The FTIR graphs of synthesized nanocomposites are depicted in Figure 1e–h. FTIR spectra of Ni/Mo.S2/MOF-5/GO show significant distinctive bands of GO at 3416, 1657, 1490, 1218, 1040, and 860 cm−1 [37]. The typical peaks in GO include the stretching vibrations of the OH at 3416 cm−1, C-O at 1490 cm−1 (carbonyl or carboxylic group), C=O at 1657 cm−1 (aromatic stretching frequency), and C-O at 1218 cm−1 (epoxy). The epoxy or peroxide functional group showed an absorption band at 860 cm−1, while the C-O that belongs to alkoxy shows a stretching vibration band at 1040 cm−1.
The MOF-5 shows two distinct bands at 1691 and 1599 cm−1 attributed to C=O and C-O stretching vibrations of the aromatic ring of terephthalic acid. However, distinctive peaks attributed to the aromatic ring of terephthalic acid in MOF-5 were observed in the vicinity of 1200–600 cm−1. The band that appeared at 769 cm−1 corresponds to C-H stretching vibrations. The inset Figure 1e shows an absorption band at 476 cm−1 attributed to the Zn-O stretching [38]. The FTIR spectra of MOF-5/GO nanocomposites [39] show that GO [40] had no prominent effect on MOF-5 properties. Meanwhile, the peak at 2850 cm−1 demonstrates the existence of uncondensed NH3. A few wide peaks at 3200 and 3650 cm−1 are due to OH and NH bonds, as displayed in Figure 1e,f, and another band at 2920 cm−1 is linked to adsorbed water molecules.

2.3. Morphology and Structure Analysis

The morphology of the synthesized nanocomposite materials was examined through SEM analysis. The SEM pictures in Figure 2a demonstrate that GO is incorporated into the MOF-5 framework without significantly changing its shape. Images taken by scanning electron microscope showed that the breakdown of the nanocomposite made from MOF-5 enabled the creation of flaky carbon nanostructures with graphene oxide acting as their support. Due to the presence of Na2S in the synthesis of molybdenum disulfide, which generated a few irregular nanoparticles, the altered morphology of GO/MOF-5 indicated the adherence of the metallic sulphides Mo.S2 and Ni on the surface of GO/MOF-5. However, the Ni/Mo.S2-modified MOF-5/GO has changed the shape of the tiny ball structure into a flaky structure, as seen in Figure 2b; thus, Ni/Mo.S2/MOF-5/GO was successfully developed. Additionally, EDS was performed to reveal the elemental composition of newly synthesized nanocomposites derived from MOF-5/GO. Nanocomposites provided valid evidence for elements like C, O, N, Cl, Zn, Ni, and Mo.

2.4. Band Gap Turning Analysis

Using UV-visible spectroscopy, the band gap energies and absorbance spectra of the newly synthesized photocatalysts namely, Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni/Mo.S2/MOF-5 were analyzed, as shown in Figure 3a. Their Tauc plots, as presented in Figure 3b, were drawn using the standard approach, and the following equation was used to determine the corresponding energies of these absorption peaks [41].
Eg = hc/λabsorption.
where, Eg = optical band gap, c = velocity of light, h = Planck constant, and λ = wavelength.
The proposed photocatalytic nanocomposite Ni/Mo.S2/MOF-5/GO is a heterojunction between MOF-5/GO and bi-metallic sulphides (Ni.S2, Mo.S2). There was a significant increase in absorption due to the reduction in the energy band gap, from 2.30 eV to 2.06 eV, shown by Ni-Mo.S2/MOF-5. The highest light absorption is demonstrated by Ni/Mo.S2/MOF-5/GO, which leads to the highest photocatalytic activity. Moreover, enrichment in light absorption intensity also showed a viable increase in the number of photo-induced (e−/h+) pairs [42,43]. As a result, more active species are detected as (O2•−) and (OH•) radicals. This ultimately led to enhanced photocatalytic activity of Ni/Mo.S2/MOF-5/GO [44].

2.5. Photocatalytic Activity of MOF/GO

The well-known fluoroquinolone antibiotic, moxifloxacin was picked as a representative emerging hazard to study how effectively the synthesized materials function as photocatalysts when exposed to visible light. The MOX absorption spectra exhibit maximum absorption at approximately 288–300 nm in the presence of various photocatalysts when exposed to unfiltered sunlight, and it was observed that the MOX peak steadily reduced with time in the presence of all catalysts. In order to examine the effectiveness of the photocatalysis of nanocomposites composed of Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni/Mo.S2/MOF-5 under sunlight illumination for 90 min, moxifloxacin (MOX) was subjected to photocatalytic degradation. As illustrated in Figure 4a–d, Ni/Mo.S2/MOF-5/GO demonstrated 95% degradation efficiency, which, when compared to the other composite materials, indicates a narrower charge zone results in the formation of the best possible heterojunction, since all the composites that were synthesized were determined to be photocatalytically active for moxifloxacin (MOX). Electron hole recombination can be prevented by graphene’s ability to operate as an electron receiver, taking in photoexcited electrons from semiconductors [45,46]. The absorption of visible light cannot be increased by graphene [47]. Visible light-absorbing graphene is required as a co-catalyst for MOF [48,49,50].
The influence of Ni.Sx/Mo.Sx/MOF-5/GO catalyst dosage, MOX concentration, and pH on MOX degradation by all catalysts as a function of time under sunlight irradiation was also checked; results are summarized in the supporting file as (Figure S1).

2.6. Kinetics Studies of the Photodegradation of MOX

The increase in photocatalyst surface active sites, which in turn leads to an increase in the generation of species responsible for MOX photodegradation, including (e−/h+), (O2•−), and (OH•), is the reason for accelerated degradation [51]. Although the additional catalyst causes the particles to assemble, which inhibits light from interacting with the catalyst and leads to the successive absorption of photons by the photocatalyst surface, the percentage degradation starts to decline above the optimal catalyst loading limit for each catalyst, and ultimately dampens the performance.
As a result, the activity of degradation is increased, and electron-hole pair recombination is decreased. The quantitatively given photodegradation reaction kinetics were studied using the linear fit model and were calculated by the following equations:
Ct = C0 e−kt
and
−ln (Ct /C0) = kt
where Ct = concentration at time t, C0 = initial concentration, e = base e, k = rate constant of decline/day, t = time, and ln = natural log. By plotting −ln (Ct/C0) on the y-axis and irradiation time (t) on the x-axis, a straight line is obtained, as shown in Figure 5a, where the k values of samples Ni/Mo.S2/MOF-5/GO (0.044 min−1), Ni.S2/MOF-5/GO (0.043 min−1), Mo.S2/MOF-5/GO (0.041 min−1), and Ni/Mo.S2/MOF-5 (0.033 min−1) were calculated from the slope.
The calculated values of k (rate constant) for different photocatalysts used for MOX degradation are presented in Table 2. The rate constant’s R2 values for the synthesized composites were 0.992, 0.981, 0.986 and 0.965, which supported the pseudo-first order kinetics and continuous photodegradation of MOX.

2.7. Detection of the Reactive Species

Designing a potential charge transfer pathway for the degradation of organic pollutants requires careful consideration of the primary reactive species. In order to acquire (O2•−), (OH•), and h+, a number of scavengers: benzoquinone (BQ), isopropyl alcohol (IPA) and ammonium oxalate (AO), respectively, were tested during the degradation of MOX [52,53]. Different scavengers exhibit distinct influences on the photodegradation pathway, as may be seen experimentally. The degradation capacity was significantly decreased when BQ and IPA were introduced to this reaction, as shown in Figure 5b, indicating that photo-generated (O2•−) and (OH•) radicals were essential for the degrading process. Moreover, the addition of AO affected the catalyst’s ability to degrade, suggesting that the h+ species were not the main active oxidative species in the photocatalytic degradation of MOX. Furthermore, the significant impact of the presence of (O2•−) on MOX removal was confirmed by BQ addition, and a significant decrease in the MOX photocatalytic ability was observed. However, the remarkable quenching phenomena was seen in the sample with the addition of IPA, showing that the grown Ni/Mo.S2/MOF-5/GO nanocomposite acts as the top reactive species during MOX removal [54]. The MOX degradation followed the given order of reactive species as h+ < O2•− < OH• [55,56], which is well demonstrated in the graphical representation in Figure 5b.

2.8. Effect of Photo-Catalyst on the Photodegradation of MOX

The effect of nanocomposites dosage on MOX degradation with regard to time is depicted in Figure 5c. The order of photodegradation of MOX was followed by all of the synthesized photocatalysts, which are all determined to be active. This demonstrates an increase in degradation efficiency, as the catalyst dose is improved for all Ni/Mo.S2/MOF-5/GO (95%) > Ni.S2/MOF-5/GO (93%) > Mo.S2/MOF5/GO (90%) > Ni/Mo.S2/MOF-5 (86%) up to 2.0 mgL−1 (Table 3).

2.9. Stability and Reusability MOF-5/GO Nanocomposites

As a result of photodegradation, the photocatalyst’s activity decreased under continuous exposure. Hence, before using the composites in real applications, it is essential to critically consider their stability and reusability [61]. Figure 5d illustrates the effectiveness of the produced Ni/Mo.S2/MOF-5/GO nanocatalyst’s photocatalytic degradation of MOX over five consecutive cycles. To verify the stability of the used catalysts after cycling, the Fourier transform infrared (FTIR) spectra were taken and no significant change was observed in synthesized nanocomposites, as shown in Figure S3 (Supporting Information).
In the first three cycles it remained constant; however, a minor decline in adsorption percentage was seen in the final two cycles. The MOX molecule may have moved inside the MOF-5/GO nanocomposite’s internal cavities, where it could not be readily removed, which may be the reason for the slight decline in the adsorption efficiency of nanocomposites. Nevertheless, faster adsorption, combined with high reversibility as shown by these data, reflects its utility and value for MOX adsorption in practical applications.

2.10. Enhancement Mechanism of Photocatalytic Activity

The enhancement in the photocatalytic mechanism of the Ni/Mo.S2/MOF-5/GO composite is illustrated in Figure 6, based on the aforementioned experimental results. The results demonstrate that adding Ni-Mo.S2 and GO to the carbon-based/semiconductor/MOF composite improves the photocatalytic activity of MOF-5. Within the MOF-5/GO substrate, Ni.S2 and Mo.S2 clusters may act as quantum fragments, surrounded by terephthalate ligands. When used as antennas, these organic bridging ligands may generate visible light and then efficiently transmit the energy to the Ni.S2 and Mo.S2 clusters. The creation of an (e−/h+) pair on the catalyst surface governs the fundamental workings demonstrated in experiments on photocatalytic MOX degradation. Additionally, following GO alteration, the Ni/Mo.S2/MOF-5/GO composite showed a stronger positive valance band potential, which would increase its capacity for photocatalytic oxidation in the degradation of MOX. The literature states that the Zn-based MOF-5 conduction band potential (ECB) and valence band potential (EVB) are around 1.01 and 2.87 eV vs. NHE, respectively [62]. When MOF-5 gets energized by photons, the electron in the VB state is excited to the CB state. This can be accomplished by making a hole in the VB, causing the photogenerated electrons on the CB MOF-5 to simultaneously move quickly to the GO surface, resulting in effective carrier splitting and increased photocatalytic activity. In this way, the GO sheet will serve as an excellent electron acceptor, and the electron-hole recombination will be interrupted [63,64]. Consequently, because of its two-dimensional pi-conjugate structure, graphene oxide is a great conductor [65]. The electrons on the CB reduce O2 to generate superoxide radicals (O2•−). However, the standard redox potential of (H2O/OH•) = 2.8 eV vs. NHE) was slightly lower than the VB of the catalyst, allowing the holes in the VB of Ni/Mo.S2/MOF-5 to quickly oxidize water to create more (OH•), increasing the photodegradation efficiency.
MOF-5, GO + hν→ e−CB [MOF-5, GO] + h+VB [MOF-5, GO]
e−CB [MOF-5, GO] + O2 → MOF-5, GO + O2•−
h+ VB [MOF-5] + OH−→ h+ VB[MOF-5] + OH•
h+, O2•−, OH• + MOX→ CO2 + H2O
As a result of the hydroxyl (OH) group substitution in aqueous solution scavenging the holes in the valance band, an innovative route for the generation of remarkably volatile radicals (OH•) is created. This OH• radical dominates the reaction and makes it simpler for reactants to diffuse to the active sites, as the catalyst oxidizes the MOX. The effectiveness of pollutant degradation is dependent on how many active sites are available on the catalyst surface. The hydroxyl, superoxide radicals and holes generated could directly oxidize the MOX molecules to form the degradation by-products CO2 and H2O. However, we observed in the radical scavenging experiments that the contribution of the holes and superoxide radicals was less than that of the hydroxyl.

3. Experimental Methods

3.1. Materials

The supplier of the antibiotic moxifloxacin (BAY12-80369 MOX 98%) was Bayer (Berlin, Germany). For our experimental work, natural graphite flakes were purchased from Sigma Aldrich (Saint Louis, United States). Daejung Chemicals (Gyeonggi, Korea) provided the sulfuric acid (H2SO4 98.0% purity), anhydrous ethanol (EtOH, 99.8%), potassium permanganate (KMnO4 99.8%), zinc nitrate hexahydrate [Zn(NO3)2·6H2O, 99.0%], ethylene glycol (99.9%), and N,N-dimethylformamide (DMF). Without additional purification, terephthalic-acid (1,4-benzenedicarboxylic acid, H2BDC, 99.9%), hydrochloric-acid (HCl 36%), phosphoric-acid (H3PO4 87%) and hydrogen peroxide (H2O2 35%) by Duksan Chemical (Ansan, Korea) were all employed as analytical-grade reagents. All studies were conducted using deionized (DI) water.

3.2. Analysis and Characterization

The as-synthesized samples’ physicochemical properties were investigated using various tools and methods, such as an X-ray powder diffractometer (XRD)(9JDX-3532 JEOL, Tokyo, Japan), An Alpha-Bruker Fourier transform infrared spectroscopy (FTIR(Mannheim, Germany)Scanning electron microscopy (SEM)(JSM5910 JEOL, Tokyo, Japan), and a UV-Vis spectrophotometer(Cecil CE 7400, Waltham, USA). To sonicate the reaction solution, an ultrasonic bath (E30 H Elmasonic) (Elma Ultrasonic, Singen, Germany)was used.

3.3. Zn-Based MOF (MOF-5) Preparation

The DMF technique was used to synthesize the MOF-5, in line with the report, after modification. In a single-step experiment, a homogeneous solution was made by vigorously stirring 0.6 g of Zn(NO3)2·6H2O and 0.12 g of terephthalic acid (H2BDC) with 20 mL of ethylene glycol for an hour. An autoclave composed of stainless steel and coated with Teflon was used to heat the solution for eight hours at 170 °C. The product was obtained by centrifugation, cooling at room temperature, and repeated washes with DMF and ethanol. The solvent was removed by dissolving the creamy white precipitates in pure chloroform solvent for 24 h at room temperature. The final product was then collected and washed with ethanol before being dried at 70 °C.

3.4. Synthesis of Graphene Oxide (GO)

A modified Hummers method was applied to manufacture GO from graphite powder [66]. A flask equivalent to 50 mL of 98 percent H2SO4 was placed in an ice-bath. The flask was then filled with 2.0 g of graphite and vigorously agitated. After that, 6.0 g of KMnO4 was gradually added to the flask while the temperature of the reaction was sustained under 25 °C in an ice bucket for approximately half an hour. The reaction mixture was then heated in a water-bath up to 35 °C, stirred for roughly 45 min, and then added to the flask. The reaction temperature was further raised to 90 °C after adding 50 mL of water to the flask containing the reaction mixture, and the mixture was finally stirred for 30 min.

3.5. Synthesis of Metal Organic Framework-5/Graphene-Oxide (MOF-5/GO)

With multiple changes to a typical MOF-5 [67], the MOF-5/GO nanocomposite was synthesized by adapting the single-step solvothermal method. Initially, 0.6 g of Zn (NO3)2·6H2O and 0.12 g of terephthalic acid H2BDC were dispersed in 60 mL of DMF to obtain a final clean solution. To create a homogenized DMF emulsion of graphene oxide, 0.36 g of GO was sonicated into a solution of 10 mL of DMF for 120 min [68]. Following the addition of the GO suspension, the previously clean solution was stirred for an additional 30 min, before being transferred to a 100 mL stainless-steel autoclave lined with Teflon and heated to 120 °C for 48 h. After that, a vacuum drying process employing a desiccator fitted with a heater at 80 °C for 12 h was used to produce the resulting solid sample, which was cleaned with DMF and CHCl3. The hybrid materials were named MOF-5-GO in their first state of synthesis.

3.6. Fabrication of Ni/MoS2/MOF-5-GO Nanocomposite

A hydrothermal approach was employed to fabricate the MOF-5/GO composite. In 50 mL of deionized water, 0.3 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 0.5816 g of nickel nitrate hexahydrate (Ni(NO3)2·6H2O) were mixed. This mixture was homogenized by using a sonicator for 30 min. Following that, 0.15 g of sodium sulphide nonahydrate was dissolved in 100 mL of DI water to create a 0.006 molar aqueous solution of sodium sulphide nonahydrate (Na2S·9H2O); 12 mL of this was pipetted out and added to the above solution. In a further step, 0.25 g of freshly prepared MOF-5/GO was added and it was stirred once more for an hour. The mixture was then transferred to a sealed Teflon vessel and incubated for 21 h at 90 °C. After lowering to room temperature, the catalyst was centrifuged (4500 rpm) for 15 min. The obtained product was a Ni/Mo.S2/MOF-5/GO composite rinsed with deionized water and anhydrous ethanol. Following that, the precipitates were desiccated for 8 h at 25 °C. The acquired product was finally grounded and stored. By adapting the scheme proposed above in Figure 7a,b the photocatalyst composites Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Ni/Mo.S2/MOF-5 were also successfully synthesized.

3.7. Photocatalytic Experiments

MOF-5-based Ni/Mo.S2/MOF-5/GO photocatalysts used to degrade moxifloxacin molecules out of wastewater were investigated. In a typical experiment, without adding any further chemicals, the aqueous solution of MOX was created and used as a sample of wastewater. Prior to the desired dilution (10 ppm), the stock solution of MOX (1000 ppm) was prepared by dissolving 0.1 g of MOX in 100 mL of distilled water. In tests involving photocatalytic degradation, 10 mg of photocatalyst was introduced to a photoreactor containing 50 mL of MOX solution atpH 5. The solution was stirred magnetically in a dark room for 30 min, then placed in sunlight for 90 min to observe the adsorption-desorption equilibrium. Under dark conditions, there was no degradation of MOX observed; however, the photocatalytic efficiency for MOX degradation was significantly enhanced up to 90 min when Ni/Mo.S2/MOF-5/GO composite was irradiated with unfiltered sunlight.
During the process, aliquots of 2 mL were withdrawn from the parent solution after 10 min intervals, separated through centrifugation, and the intensity of characteristic electronic absorption band was noted using a UV-Vis spectrophotometer. The degradation of MOX was recorded by observing the reduction in the UV-Vis absorption spectra (max = 290 nm) as a function of exposure time. The following equation was used to calculate MOX (%R) removal efficiency:
% R = (C0 − C)/C0 × 100
where, C0 (mg L−1) is the primary concentration of MOX and C (mg L−1) is the secondary concentration of MOX after photocatalytic decomposition in response to a series of catalysts at different time intervals under UV irradiation [69,70].

4. Conclusions

In summary, we have effectively synthesized a unique hybrid photocatalyst using several bimetallic sulphide combinations and characterized it by XRD, SEM, EDS, FTIR, and UV-Vis analysis. In this article, experimental data confirm that metal organic frameworks with specific surface areas such as MOF-5 can be synthesized by combining double sulfides of bi-metals (nickel and molybdenum). There is a substantial difference in photocatalytic activity between GO modified with sulfides of a single metal (molybdenum or nickel) and GO modified with sulfides of both metals. The hybrid composite Ni/Mo.S2/MOF-5/GO displayed excellent visible light-driven photocatalytic activity in contrast to the other hybrids, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni-Mo.S2/MOF-5. This was due to the improved photocatalytic performance related to the development of the best junction of the Ni/Mo.S2/MOF-5/GO composite. When photodegradation occurs, GO, with its outstanding conductivity, and MOF, with its specific adsorption action, are both involved. The separation and resettlement of photo-generated carriers benefit from this synergistic action. Consequently, Ni/Mo.S2/MOF-5/GO has good recycling performance, as demonstrated by the cyclic experiment, and its photodegrading efficiency is estimated to be 95%. The improved photocatalytic activity of the Ni/Mo.S2/MOF-5/GO hybrid was attributable to a significant rise in the overall number of photogenerated electron/hole pairs (e−/h+), and the active species (OH•) and (O2•−). Results of scavenger tests confirmed that the OH• and O2•− radicals were vital to the photodegradation of MOX. The h+ radical, on the other hand, appeared to be a relatively inactive species for degradation. Based on this research, it can be concluded that the modification of the bimetallic sulphide MOF-5 by using GO (electron acceptor) was effective and can serve as an inspiration for the development of other MOF-5/GO composite photocatalysts to increase their applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13060984/s1, Figure S1: Optimization of reaction parameters using Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, Ni/Mo.S2/MOF-5 (a) MOX concentration, (b) photocatalyst dosage (c) Effect of pH of on degradation of moxifloxacin (d) MOX degradation by all catalysts as a function of time under sunlight irradiation; Figure S2: Fourier transform infrared (FTIR) spectra of sample (a) Ni/Mo.S2/MOF-5/GO, (b) NiS2/MOF-5/GO, (c) Mo.S2/MOF-5/GO, (d)Ni/Mo.S2/MOF-5 nanocomposites of the used catalysts after cycling to verify their structural stability.

Author Contributions

A.A. and M.M. performed the experiments and prepared the original draft. M.A.N. and S.S.A.S. performed data correction, data curation, data interpretation, editing, and revision of the draft. M.A.W. helped with the manuscript preparation and data interpretation. S.M.I. and A.A.T. performed the spectroscopic analysis. A.u.R. initiated the idea and supervised the project. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Researchers Supporting Project number (RSP2023R100), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

All relevant data may be provided upon request.

Acknowledgments

This work was supported by Researchers Supporting Project number (RSP2023R100), King Saud University, Riyadh, Saudi Arabia. The authors are highly grateful to Ph.D. analytical Lab, Institute of Chemistry The Islamia University Of Bahawalpur for providing all the possible access to analysis and experimental work.

Conflicts of Interest

The authors declare no conflict of interest or personal relationships that could have appeared to have influenced the work reported in this paper.

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Figure 1. The XRD spectra of (a) Ni/Mo.S2/MOF-5(♣)/GO, (b) Ni.S2/MOF-5(♣)/GO, (c) Mo.S2/MOF-5(♣)/GO, and (d) Ni/Mo.S2/MOF-5(♣) and Fourier transform infrared (FTIR) spectra of sample (e) Ni/Mo.S2/MOF-5/GO, (f) Ni.S2/MOF-5/GO, (g) Mo.S2/MOF-5/GO, (h) Ni/Mo.S2/MOF-5 nanocomposites.
Figure 1. The XRD spectra of (a) Ni/Mo.S2/MOF-5(♣)/GO, (b) Ni.S2/MOF-5(♣)/GO, (c) Mo.S2/MOF-5(♣)/GO, and (d) Ni/Mo.S2/MOF-5(♣) and Fourier transform infrared (FTIR) spectra of sample (e) Ni/Mo.S2/MOF-5/GO, (f) Ni.S2/MOF-5/GO, (g) Mo.S2/MOF-5/GO, (h) Ni/Mo.S2/MOF-5 nanocomposites.
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Figure 2. SEM images of (a) MOF-5/GO (b) Ni/Mo.S2/MOF-5/GO.
Figure 2. SEM images of (a) MOF-5/GO (b) Ni/Mo.S2/MOF-5/GO.
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Figure 3. (a) Absorbance spectra and (b) Tauc’s plot of Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, Ni/Mo.S2/MOF-5 nanocomposites.
Figure 3. (a) Absorbance spectra and (b) Tauc’s plot of Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, Ni/Mo.S2/MOF-5 nanocomposites.
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Figure 4. Absorbance spectra of moxifloxacin (MOX) (a) Ni/Mo.S2/MOF-5/GO, (b) Mo.S2/MOF-5/GO (c) Ni.S2/MOF-5/GO, (d) Ni/Mo.S2/MOF-5 nanocomposites.
Figure 4. Absorbance spectra of moxifloxacin (MOX) (a) Ni/Mo.S2/MOF-5/GO, (b) Mo.S2/MOF-5/GO (c) Ni.S2/MOF-5/GO, (d) Ni/Mo.S2/MOF-5 nanocomposites.
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Figure 5. (a) Pseudo first-order kinetic model fitting, (b) Scavenger test, (c) MOX % degradation, and (d) Reusability studies for moxifloxacin (MOX) with Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni/Mo.S2/MOF-5 photocatalysts.
Figure 5. (a) Pseudo first-order kinetic model fitting, (b) Scavenger test, (c) MOX % degradation, and (d) Reusability studies for moxifloxacin (MOX) with Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, and Ni/Mo.S2/MOF-5 photocatalysts.
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Figure 6. Schematic illustration of the enhanced photocatalytic mechanism of nanocomposite Ni-Mo.S2/MOF-5/GO.
Figure 6. Schematic illustration of the enhanced photocatalytic mechanism of nanocomposite Ni-Mo.S2/MOF-5/GO.
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Figure 7. Schematic illustration for the synthesis of (a) MOF-5 (b) graphene oxide/MOF-5.
Figure 7. Schematic illustration for the synthesis of (a) MOF-5 (b) graphene oxide/MOF-5.
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Table
Table 1. Geometric parameters of Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, Ni/Mo.S2/MOF-5 nanocomposite determined from XRD analysis.
Table 1. Geometric parameters of Ni/Mo.S2/MOF-5/GO, Ni.S2/MOF-5/GO, Mo.S2/MOF-5/GO, Ni/Mo.S2/MOF-5 nanocomposite determined from XRD analysis.
Nanocompositesa (A°)c (A°)Volume (A°)3Crystallite Size (nm)Strain ε (Nm−2)Dislocation Density
(m−2)		Lattice Spacing
(A°)
Ni/Mo.S2/MOF-5/GO3.5615.673251.732424.08661.524 × 10−61.760 × 10−63.78
Ni.S2/MOF-5/GO2.44384.607124.643725.24361.495 × 10−61.569 × 10−62.47
Mo.S2/MOF-5/GO2.2174.47618.86725.89731.456 × 10−61.479 × 10−62.59
Ni/Mo.S2/MOF-53.13555.429146.924124.1151.478 × 10−61.659 × 10−63.42
Table
Table 2. Kinetic rate constant (k); Degradation efficiency, percent; and R2 values of grown nanocomposites.
Table 2. Kinetic rate constant (k); Degradation efficiency, percent; and R2 values of grown nanocomposites.
SampleDyeDegradation Efficiency, %Rate Constant, K (min−1)R2Band Gap (±0.1 eV)
Ni/Mo.S2/MOF-5MOX860.0330.9922.30
Mo.S2/MOF-5/GOMOX900.0410.9812.27
Ni.S2/MOF-5/GOMOX930.0430.9862.22
Ni/Mo.S2/MOF-5/GOMOX950.0440.9652.06
Table
Table 3. Comparison with various studies for degradation of moxifloxacin.
Table 3. Comparison with various studies for degradation of moxifloxacin.
CatalystSynthesis MethodIrradiation Time (min)Light Source% DegradationRef.
Bio-FeMnCoO xBio-synthesis20---95.2[57]
CNABA hybridsPhysico-chemical50300 W xenon lamp65[58]
Porous sulfurized iron-based compositeCo-precipitation30XL (300 W)97[59]
CaBiFe85%Ti15%O3
Nanoparticle		Co-precipitation60UV-visible light89.16[60]
Ni/Mo.S2/MOF-5/GOSolvo-thermal90Sunlight
(280–320 nm)		95Current study
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Anum, A.; Nazir, M.A.; Ibrahim, S.M.; Shah, S.S.A.; Tahir, A.A.; Malik, M.; Wattoo, M.A.; Rehman, A.u. Synthesis of Bi-Metallic-Sulphides/MOF-5@graphene Oxide Nanocomposites for the Removal of Hazardous Moxifloxacin. Catalysts 2023, 13, 984. https://doi.org/10.3390/catal13060984

AMA Style

Anum A, Nazir MA, Ibrahim SM, Shah SSA, Tahir AA, Malik M, Wattoo MA, Rehman Au. Synthesis of Bi-Metallic-Sulphides/MOF-5@graphene Oxide Nanocomposites for the Removal of Hazardous Moxifloxacin. Catalysts. 2023; 13(6):984. https://doi.org/10.3390/catal13060984

Chicago/Turabian Style

Anum, Aqsa, Muhammad Altaf Nazir, Sobhy M. Ibrahim, Syed Shoaib Ahmad Shah, Asif A. Tahir, Misbah Malik, Muhammad Ahmad Wattoo, and Aziz ur Rehman. 2023. "Synthesis of Bi-Metallic-Sulphides/MOF-5@graphene Oxide Nanocomposites for the Removal of Hazardous Moxifloxacin" Catalysts 13, no. 6: 984. https://doi.org/10.3390/catal13060984

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