Photodegradation of Amido Black 10b Dye Under Visible Light Using Ni and Zn Ferrite Catalysts Prepared by a Simple Modified Sol–Gel Method

The pure α-Fe2O3, NiFe2O4, and ZnFe2O4 were prepared by a simple modified sol–gel method. The prepared catalysts were characterized by X-ray diffraction, transmission electron microscope, surface area, Zeta potential and optical techniques. The ferrite structure of samples is confirmed. The photocatalytic activity was evaluated toward Amido black 10b dye degradation under visible light at different pHs of 4, 8, and 10 for 90 min irradiation time. The photodegradation toward Amido black b10 dye reached maximum value at pH 8, and it reaches 92%, 89%, and 85% over ZnFe2O4, Fe2O3, and NiFe2O4 photocatalysts; respectively. The increased photoactivity of the ZnFe2O4 sample can also be attributed to its lower bandgap of 2 eV, the formation of the −OH-surface group. Since –OH can interact with the photoexcited holes that were originally formed on the catalyst surface, hydroxyl radicals are produced that have strong oxidizing properties. Whereas; the dye photodegradation is negligible in the case of Fe2O3, and NiFe2O4 catalysts at pH 10, due to the electrostatic repulsion between negatively charged catalyst surface and dye ions at high basic medium. While, in case of ZnFe2O4, the photodegradation reached only 40%.


Introduction
Dyes, along decades, have extensively used in wide spectrum of industries. The uncontrolled discharge of untreated hazardous dyes into fresh water causing massive environmental destruction to ecosystem and human health. Therefore, the dyes mineralization in aquatic system has become in a top priority. Photocatalysis technique is an ecofriendly effective method for dyes mineralization. In such technique, the advantageous semiconductors with large specific surface area, chemical stability, and high photocatalytic response are used as catalysts [1,2]. Iron oxide is a significant material due to their high catalytic activity, biocompatibility, low cost and nontoxicity [3][4][5][6]. Hematite, the most thermodynamically stable iron oxide phase, crystallizes in the rhombohedral morphology with a 2.1 eV bandgap [7] and hence absorb light in the visible range of the solar spectrum [8]. However, its B Radwa A. El-Salamony radwa2005@hotmail.com 1 Egyptian Petroleum Research Institute, 1A Ahmed El-Zomor St., Nasr City, Cairo 11727, Egypt photoactivity is limited by the fast electron-hole pairs recombination [9]. Various approaches such as doping α-Fe 2 O 3 with transition metals, which can overcome their limitations [10][11][12] have proposed. Therefore, coupling Fe 2 O 3 with a semiconductor that has a lower conduction band (CB) edge would delay the recombination of the photoexcited electrons with the positive hole. Doping of various metal ions such as Mg 2+ , Cu 2+ , Ni 2+ , Ce +2 , and Zn 2+ [13][14][15] at the Fe site in hematite influence the physical and photocatalytic properties. Where, the metal dopant serves as an ionized that likely improve the conductivity and the charge transport properties of the Fe 2 O 3 . The composite semiconductors generally possess a high photocatalytic activity as a result of their advantageous combined characteristics [16]. Numerous efforts have been made to improve the photocatalytic activity of both Fe 2 O 3 and ZnO [17]. Particularly, spinal ferrites was intensively considered as photocatalysts due to its unique properties and high magnetism which facilitate the catalyst separation by applying an external magnetic field [18]. Likely, the ZnFe 2 O 4 is a multi-active site with strong chemical stability. It also has a narrow bandgap (1.96 eV) which provide the advantageous photocatalysis in visible light range [19]. In addition, it is a cheap, ecofriendly, material with high thermal and mechanical strength and suitable for mass production [20]. ZnFe 2 O 4 is intensively applied as sensitizer to influence the semiconductor photocatalytic activity in visible range as a result of its high sensitivity [21]. So that, ZnFe 2 O 4 exhibited a high efficiency as photocatalyst for methyl orange and malachite green dyes mineralization [22]. Despite the ZnFe 2 O 4 abundant benefits, its activity is dramatically affected by its morphology and structure.
NiFe 2 O 4 is also an attractive semiconductor, due to its fabulous properties, that has been applied in many fields [23]. It is a promising photocatalyst in the visible range due to its small bandgap (2.19 eV) with a relative positioning conduction and valency bands and significant redox potential [24]. For example, NiFe 2 O 4 exhibited a high photocatalytic activity of 98% in methyl orange mineralization in aqueous media at visible range light luminance [25]. Meanwhile, the pure NiFe 2 O 4 as photocatalyst generally suffer a delayed electron excitation as well as fast electron/hole (e − /h + ) recombination [26]. So that, combination of ferrites with other materials such as Bi 24 O 31 Br 10 [27] and zeolites [28] were investigated to overcome the ferrites disadvantages, increase the surface area and their electronic structure.
Very recently, the nickel ferrite has possessed a super reactivity as photocatalyst for elimination of complete decolorization of the colored water [29], elimination of antibiotics [30], heavy metals [31] from waste water. Zinc ferrite has also exhibited a wide spectrum of applications with superior efficiency toward waste water neutralization [32].
In this study, the NiFe 2 O 4 and ZnFe 2 O 4 as photocatalysts were synthesized by modified sol-gel method. The effects of the nickel and zinc oxides doping on the structural, morphological, and optical properties of their corresponding ferrite were investigated using several analytical techniques. The photocatalytic activities of the prepared catalysts were evaluated based on the degradation of amido black b10 dye in an aqueous solution under visible light.

Catalyst's Synthesis
The materials used in this work, the Fe(NO 3

Catalyst Characterization
The formed crystalline phases and structure were investigated by the XRD analysis using the X'PertPRO PANalytical instrument. The apparatus equipped with a Ni filter with Kα radiation (λ = 0.15406 nm). The lattice size of all the prepared samples was calculated according to Scherrer equation. The textural properties of the prepared catalysts were obtained by BET analysis using NOVA 3200 apparatus, USA, instrument. Before measurements, the catalysts were degassed at 120°C for 3 h. The S BET was calculated from the adsorption isotherm while pore size distributions (PSD) were calculated from desorption isotherms.
The sample morphologies were conducted by transmission electron microscope using the JOEL microscope (JEM-200CX, Japan) instrument worked at 200 kV. Before the analysis, the samples were suspended in ethanol and sonicated for 30 min. a few droplets were then placed on the cupper grid and left to dry for 10 min then investigated. Fourier-transform infrared investigations were performed using Nicolet (Is-10 model, USA) spectrophotometer via the KBr technique. While, UV-reflectance was analyzed using a UV-spectrophotometer (V-570, JASCO, Tokyo, Japan). The direct bandgap energies for samples were estimated from the Tauc plot method via the following Equations: where E g is the bandgap energy (eV), h is Planck's constant, hν is the photon energy (eV), A is a proportional constant, and α is the absorption coefficient. The absorbance or Kubelka-Munk function of the reflectance (f (R)) is proportional to the absorption coefficient (α), and n is 0.5 and 2.0 for direct transition semiconductor and indirect transition semiconductor, respectively. Photoluminescence (PL) analysis was measured at room temperature using the Spectrofluorometer model (JASCO FP-6500, Japan). ZetaSizer Nano Series (HT), Malvern Instruments UK, the instrument was used to determine the hydrodynamic particle size distribution of the prepared

Photocatalytic Activity
The amido black b10 azo dye (C 22 H 14 N 6 Na 2 O 9 S 2 ) supplied by Fisher Scientific had the characteristics of molecular mass 616.487 g/mole; λ max = 584 nm and pH 8. Scheme 1 represents the chemical structure of Amido black 10 b dye. All photocatalytic experiments were conducted in a cylindrical glass vessel with an air pump to continuously defuse the air through the suspension. The experiments were carried out with 1 g/L suspended photocatalyst/dye solution. Typically, 0.1 g catalyst suspended in a 100 ml aqueous solution of 25 mg/L Amido black b10 dye under three different pH of 4, 8, 10 were investigated. The experiments were performed under continuous stirring in the presence of 100W Tungsten visible lamp (> 400 nm). Before irradiation, the suspension was equilibrated under darkness for 30 min to reach the adsorption equilibrium. Aliquots of 2 mL were collected after interval times of illumination and then filtered through 0.22 mm filter. The filtrate was determined by a JENWAY-6505 UV-visible spectrophotometer at λ max of 584 nm.

XRD Analysis
The samples crystallography and patterns are investigated by XRD and illustrated in Fig. 1. The pure iron oxide sample ( Fig. 1a) showed 2θ of 24.05°, 32.94°, 35.53°which are the main characteristic peaks of the (012), (104), (110) plans of rhombohedral hematite α-Fe 2 O 3 structure [34,35]. The recorded peaks have well-defined positions with high sharpness and intensity that implies the prepared samples high crystallinity. In addition, the absence of any other patterns corresponding to other iron oxide phases reflects the sample high purity.  [36].
The average crystallite size was calculated using the Debye-Scherrer equation [39] via the FWHM of the main peaks corresponds to (104)  small size of about 29 nm. Meanwhile, the α-Fe 2 O 3 crystallite was enlarged by almost two folds when by Ni doping while slightly increased in the case of Zn doped hematite. The lattice enlargement may be explained via the undistributed cations, where Fe 3+ is allocated in a large octahedral structure while the Ni 2+ or Zn 2+ is located in a small tetrahedral shape as in the normal case of the bulk ferrite materials formation at high temperature [40,41]. This observation indicated that the

BET Results
The N 2 adsorption-desorption isotherms and particle size distribution curves are measured to determine the catalysts textural properties and illustrated in Fig. 2A, B, respectively. Figure 2A clearly indicates that all samples exhibited BET isotherms of Type IV according to IUPAC classification [42]. Furthermore, all samples' isotherms had hysteresis loops but in a different manner. Whereas, the pure Fe 2 O 3 and ZnFe 2 O 4 samples exhibited an H4 hysteresis loop according to the IUPAC system which indicated the presence of mixed pore structure [43]. Meanwhile, For the NiFe 2 O 4 sample, the H3 hysteresis loop was obtained [44]. This hysteresis type normally belongs to the ink bottle flexible aggregates with plate-like has given rise to slit-shaped pores. Also, the hysteresis indicated the ink bottle with a wide neck and small volume-shaped pores. The obtained isotherms imply the existence of pores within the mesoporous range in all samples. For all the prepared samples, the presence of the isotherm plateau in desorption curve for the H3 hysteresis loops confirms the presence of large mesopores in the sample. The pore size distribution (PSD) (Fig. 2B-a)

FTIR Results
The FTIR spectrum of pure Fe 2 O 3 (Fig. 3a) showed two closer intense peaks around 458.7, 550 cm −1 and a small peak at 1621 cm −1 . Those recorded peaks attributed to the stretching vibration of the tetrahedral Fe-O group in Fe 2 O 3 nanoparticles [45,46]. The spectrum of NiFe 2 O 4 (Fig. 3b) has shown an additional intense peak at 420 cm −1 which ascribed to the Ni-O stretching vibration. Also, a small shoulder at 846 cm −1 and a wide peak at 1380 cm −1 assigned to the stretching vibration in metal-O bond in a tetrahedral structure. The detected peaks have confirmed the NiO and Fe 2 O 3 coexistence and also the formation of NiFe 2 O 4 spinal form which runs with the XRD results (see Fig. 1b).

TEM Analysis
The electron microscope image of α-Fe 2 O 3 sample (Fig. 4a) revealed the dominant formation of regular crystalline rhombohedral structure. The figure also revealed a small nanoscale particle with narrow particle size distribution in a range of 26-35 nm and slight agglomeration degree. NiFe 2 O 4 micrograph (Fig. 4b) showed the formation of some large and irregular hexagonal-shaped particles accompanied with penalty of irregular cubic-shaped small particles. The large existed particles in Fig. 4b had an average particle size of 70 nm while the cubic small particles had particle size distribution ranged from 14 to 18 nm. The ZnFe 2 O 4 sample (Fig. 4c) was assembled by a much smaller particle size and much aggregates tiny amount than the NiFe 2 O 4 sample. The figure demonstrated the irregular shape fabricated ZnFe 2 O 4 particles. It is worth to mention that the observed porosity in electron microscope images can be due to the formed particles inter distance raised by evaporation of organic solvent during calcination. The observed porous nature that would enhances the photocatalytic activity are in good agreement with the X-ray diffraction and surface area results.

Zeta Potential
The behavior of nanoparticles and their efficiency to decline the dispersion rate of stable and durable very small particles in aqueous solutions is highly affected by the zeta potential values. Where the repulsion rate between the similar and/or neighbor particles in aquatic solution is normally decided by zeta potential property. The dispersed tiny particles generally need a high zeta potential value to form a stable solution. While the particles with low zeta potential tend to accumulate and form large masses [48,49]. On the other hand, very stable suspended particles, which have a zeta potential of ± 30 mV, creates an electrostatic divergence between particles and prevent accumulation [49]. Measuring the zeta potentials at different pH values provides the nanoparticles isoelectric point pH pzc . The pH pzc is effectively determined the pH value whereas the adsorbent can adsorb both negatively or positively charged particles. When the pH of the solution is less than pH PZC , the surface charge of the adsorbent is positive, while its surface charge is negative when the pH of the solution is greater than pH PZC [50].
In this work, the zeta potentials of the prepared catalysts were measured in water to determine their isoelectric point and the results are illustrated in Fig. 5. The Figure showed that the obtained pH pzc values were 5.01, 5.31, and 5.62 for the NiFe 2 O 4 , Fe 2 O 3, and ZnFe 2 O 4 samples respectively. The obtained pH pzc value of the ZnFe 2 O 4 in this work is slightly lower than the result reported in Ref. [51] which indicates the positive load in the Zinc ferrite structure. Since the Amido 10b molecules are negatively charged and the ZnFe 2 O 4 surface positive load, higher dye absorptivity and hence the higher photocatalytic degradation would be achieved. Depending on the surface area data and the obtained pH pzc values, it is expected that the ZnFe 2 O 4 sample would possess a higher photocatalytic activity.

Optical Properties
The optical properties of the prepared photocatalysts were studied by UV-visible diffuse reflectance spectroscopy and The indirect and a direct bandgap were indicated from Tauc plots that showed in Fig. 7, while the bandgap energy is given in Table 2. Authors in [54] have reported that α-  were reported within the range of 1.38-2.09 eV [55,56]. While, the direct bandgap between 1.95 and 2.35 eV can be distinguished [57]. The Fe 3+ absorption in the visible region is generally attributed to the 3d-3d spin forbidden transition excitation (indirect transition). Where, the direct bandgap of α-Fe 2 O 3 is about 1.8 eV, which is consistent with reported literature [58]. Herein, the observed bandgap energy of the sample has a direct bandgap occurring in conjunction with an indirect bandgap [59].
The indirect bandgap of α-Fe 2 O 3 is 1.9, in agreement with the typical literature value from 1.9-2.2 eV, depending on the crystalline status and methods of preparation [60]. As in NiFe 2 O 4 , the bandgap obtained from the indirect gap is 1.9-3.1 eV and that from direct bandgap is 1.92-1.9 eV. The slightly reduce in direct bandgap is attributed to the slightly affected conduction band edge of α-Fe 2 O 3 by the incorporation of nickel in the lattice [61]. This result confirms the formation of NiFe 2 O 4 spinal ferrite as indicated from the XRD and FTIR analysis. In the case of ZnFe 2 O 4 catalyst, the obtained indirect bandgap is 1.9-2 eV and the determined direct bandgap is 1.92-2 eV. The increase in bandgap energy usually occurred in case of higher loaded percent of Ni [62],  Zn [63], and Ho [64] more than in this work. This confirms the much higher surface Ni than in bulk; as represented in the results of transmission electron microscope and surface area analysis. This behavior of NiO is in-line with results reported by Liu [34]. In general; the results suggest that electron/hole pairs can be generated, although the particle is illuminated with long wavelength visible light.
The trapping efficiency, migration, and charge carriers transfer are important factors that generally determined from the photoluminescence technique. However; the intensity of the photoluminescence spectrum is directly proportional with the electron-hole recombination rate [63]. Photoluminescence characterization under a wavelength excitation of 400 nm, corresponding to the visible light used in the photodegradation experiment, is shown in Fig. 8

Photocatalytic Activity
Photodegradation of amido black 10B dye was carried out using α-Fe 2 O 3 , NiFe 2 O 4, and ZnFe 2 O 4 photocatalysts under visible light at different pH 4, 8, and 10 for 90 min; as shown in Fig. 9. Initially, the experiment was conducted in dark for 30 min to reach the adsorption/desorption equilibrium before catalyst illumination. In all cases; the photodegradation is in parallel with the adsorption affinity of the photocatalysts and the doped metal increased the adsorption capacity of Fe 2 O 3 . Photocatalytic activity is maximum for natural dye pH 8, good for pH 4, and minimum for pH 10 as shown in Fig. 9.
At pH 4, Amido black 10b dye photodegradation was 91%, 88.3%, and 88% for ZnFe 2 O 4 , Fe 2 O 3 , and NiFe 2 O 4 photocatalysts, respectively. This result is matching the zeta potential value of the photocatalysts; whereas; the catalyst exhibited the highest zeta potential to increase the dispersion and stability of the catalyst particles in water and charge transfer which effective positively on adsorption and photocatalytic activity [65]. The electrostatic interaction of dye molecules with a positively charged catalyst particle surface may be the cause of the photocatalytic performance at pH 4 [66].
At pH 8; the photodegradation of dye has reached the maximum of 92%, 89%, and 85% for ZnFe 2 O 4 , α-Fe 2 O 3 and NiFe 2 O 4 photocatalysts; respectively. The highest photoactivity of ZnFe 2 O 4 sample can also attributed to the low bandgap of 2 eV in comparison to 3 eV assigned to NiFe 2 O 4 photocatalyst. The creation of superficial − OH group in ZnFe 2 O 4 , as confirmed by FTIR, is crucial property. Since, − OH can react with the photoexcited holes originated on the catalyst surface, producing hydroxyl radicals, that acts as powerful oxidants. In general, in a neutral medium; when the photocatalysts are exposed to visible light the electron transfer takes place from valance band to conduction band (e − cb ) and a hole is created in the valence band (h + vb ). Hydroxyl ions combine with a positive hole to produce · OH. Thus, · OH radical is attacking the double bond in the dye molecules which resulting its mineralization [67].
The oxygen present in the dye solution functions as an electron scavenger. This oxygen traps the electrons from the conduction band and forms superoxide radical anion O 2 ·− species. This prevents the recombination of positive holes in the valance band and the electrons in the conduction band. Further O 2 − is reacting with chemisorbed H + on conduction band to produce peroxide radicals HO 2 · active radical as illustrated in Eqs. 1-5 Scheme 2.
The Amido black 10b dye photodegradation is negligible in the case of Fe 2 O 3 , and NiFe 2 O 4 catalysts at pH 10, due to the electrostatic repulsion between negatively charged catalyst surface and dye ions [66] at high basic medium. While, in case of ZnFe 2 O 4 , the photodegradation reached only 40%.
The durability of the three catalysts was studied under the determined optimum conditions along 5 h. During the study, an aliquot was firstly collected each 30 min for 3 h. Then a liquid sample was separated every one hour for further 2 h. The results in Fig. 10a imply that the loss in photodegradation affinity was 8%, 5% and 3% over the Fe 2 O 3 , NiFe 2 O 4 and ZnFe 2 O 4 , respectively. These observations prove the catalysts super durability for Amido black 10b dye removal.
For more convenience, the catalysts reusability were investigated along 10 reaction cycles. Figure 10b shows that catalytic activity along the investigated cycles were in order of ZnFe 2  Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. This work was supported by the laboratories of the Egyptian Petroleum Research Institute (EPRI).

Data Availability
The original data will be available upon request to the corresponding author.

Conflict of interest
The authors declare that they have no competing interests.
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