Synthesis of Inverse Spinel Nickel Ferrite Like- Broccoli Nanoparticle and Thermodynamic Study of Photo-decolorization of Alkali Blue 4B dye

INTRODUCTION
The discharge of untreated wastewater containing dyes directly into natural water bodies is detrimental to the photosynthetic activity of aquatic ecosystems. Due to the presence of metals and aromatic compounds, it causes mutagenic or teratogenic effects on aquatic creatures including fish. In addition, the presence of dyes in the environment has minor to severe hazardous impacts on human health, such as carcinogenic, mutagenic, allergy, dermatitis, and renal disease consequences. It has been found that chromium-based dyes are often structurally complicated and carcinogenic to human health[1].There are several diverse families of dyes, like: anthraquinone, azo, triphenylmethane, indigo, triarylmethane have recently been studied and proved they are optically nonlinear under continuous and pulsed wave regimes. Triarylmethane is an important kind of synthetic dye and is widely used in antimicrobial, antifungal, and antitubercular activities[2]. Triarylmethane dyes are widely utilized in the textile and dying industries due to their vivid color and inexpensive cost. The primary structure of triarylmethane dyes is obtained from the existence of monomethine with three terminal aryl groups as chromophores and functionalization by hydroxyl, amino, or dimethyl amino auxochromic groups[3].One of type Triarylmethane dye is Akali blue 4B (acid blue 110)[4]. Due to the toxicity and hazard of dyes, there are a variety of ways to eliminate them in wastewater adsorption, photocatalysis, photodegradation, membrane filtration, oxidation, and irradiation[5]. In addition, nanoparticles of metal are used to decolorize colored effluent. The size and form of nanoparticles have a significant impact on decolorization and may be manipulated via a variety of physical and chemical methods. These particles have a propensity to agglomerate, reducing their activity[6].Spinel nanocrystals are thought to be two of the most important inorganic nanomaterial’s because of their electronic, optical, electrical, magnetic, and catalytic properties. A and B make up the AB2O4 structure of spinel, which represented the tetrahedral and the octahedral binary cation and trinity cation binding sites, respectively. The O stands for the oxygen anion location. The spinel ferrite metal is a widespread and vital exam for spinel materials. The general molecular formula applies is MFe2O4 (M = Ni, Zn, Mn, Co, or Mg) and has a face-centered-cubic (fcc) tight packing structure. The spinel ferrites compounds such as zinc ferrite, manganese ferrite, cobalt ferrite, and nickel ferrite. The metal ferrites have been widely investigated because of their inverse, normal, and mixed-use buildings form distinct architectural forms. The spinel crystal formations have a high electromagnetic conductivity, mechanical hardness, low coercively, high performance, outstanding chemical stability, and mild saturation magnetization. As a result, it is a strong candidate for the application, high-frequency soft magnets and low-loss materials[7]. Ni ferrite (NiFe2O4) is one of the inverse spiels. Half of the iron ions in Ni ferrite are in the tetrahedral sites (A-site), and the other half is in the oxygen sites. Ni ions are found in octahedral positions (B-site)[8]. The NiFe2O4 is one of the most significant spinel ferrites researched. It is an n-type semiconductor according to its stoichiometry. Depending on the particle size and shape, it shows a variety of magnetic characteristics, including paramagnetic, super paramagnetic, and ferromagnetic behavior. Furthermore, when its size is lowered to nanometers, it shows remarkable physical and chemical properties[9]. At room temperature, the UV–Vis diffuse spectra explain that the NiFe2O4 is an indirect band gap material with various  sintering temperatures from 1.78 to 2.72 eV [10,11]. Nickel ferrite is one of the most significant ferrous spinel alloys with a super-magnetic nature and a wide range of applications, such as gas sensors, liquid magnetism, catalysts, magnetic storage systems, photomagnetic materials, imaging devices, microwaves, magnetic cooling, as a microwave absorber, magnetic resonance, and, in biomedicine, for example, local drug delivery[12,13]. The magnetic properties of nanoparticles used in a variety of sectors are mostly determined by their size, shape, purity, and magnetic stability (e.g., blocked or unblocked state at a particular operating temperature). These particles should exist in a single domain state with a pure phase, high coercivity, and modest magnetism. Based on from an application standpoint, the super paramagnetic blocking temperature of the nanoparticles utilized for recording is rather low. For reliable data recording in these devices, their temperatures must be far above room temperature. In medical applications In place of traditional drug delivery systems, magnetic nanoparticles are utilized as drug carriers inside the body. Possibly ineffective delivery systems[9]. In the last few years, many physical and chemical methods have been used to synthesize nano-size ferrites. Since structural, magnetic and electrical properties of ferrites depends on their composition and microstructure and hence are strongly dependent on the synthesis method and synthesis conditions, some of these methods are Co-precipitation, Hydrothermal, Sol-gel, Thermal decomposition Method, Solvothermal Method, Sonochemical, Microwave-assisted Method, Microemulsion Method, Electrochemical, Mechanical milling Method [14]. This work focuses on the preparation and characterization and application of inverse spinal NiFe2O4 nanoparticle by precipitation process in the presence of the non- ionic surfactant (hexamine) and assisted by a solvothermal route. The XRD, SEM-EDX, FT-IR and Bg were studied of the as-prepped NiFe2O4 nanoparticle. In addition, the effect of pH and the thermodynamics function were estimated for the photo-decolorization of alkali blue 4B dye.

MATERIALS AND METHODS
Chemicals
The powdered ore of Ferric nitrate Fe(NO3)3.9H2O and Nickel nitrate Ni(NO3)2.9H2O were purchased by sigma cheml co. USA. Ammonia solution (25 percent) was supplied from Merck. Absolute ethanol (C2H5OH) and Hexamethylenetetramine (methenamine or hexamine) that includes a cage-like structure were outfitted from BDH and Interchimiques SA, respectively. The dye alkali blue 4B (C32H28N3NaO4S) was purchased by Merck. This dye is acidic type, molar mass = 573.64 g/mol, and the Structure as Fig. 1.

Procedure 
This procedure was monitored using two mixed solutions in a stoichiometric ratio (2:1) that was modified from the reference [15]. Based on Fig. 2, solution 1 contained 0.2 M of Ferric nitrate Fe(NO3)3.9H2O in 50 mL D.W under ultrasonic waves for 10 min. Solution 2  obvious contained 0.1 M of Nickel nitrate Ni(NO3)3.6H2O in 50 mL D.W under  ultrasonic waves for 10 min. Inside the ultrasonic bath, solution 2 was added step by step to solution 1 and mixed for 10 min. The final pH of this produced solution was measured to be about 3. An exact 0.5 g of hexamine (non-ionic surfactant) was added to solution as a capping agent, template and stabilizer, with continuous mixed using a magnetic stirrer at 60 oC for 10 min. After that, the pH of solution must be fixed using conc. ammonia solution that drop-wise added until raised to about 9.5 to 10 under stirring continued for 30 min at 60 oC. The brown solution was produced; this solution was filtered and washed with D.W and ethanol. In order to improve the properties of the brown precipitate, it was dispersive in 50 mL of Ethanol, then transported to a Teflon cup sealed in an autoclave hydrothermal instrument and heated in an oven at 180 oC for 90 min. After cooling the steles steal autoclave in air, the suspension solution was filtered and washed by 50 mL D.W then 30 mL of ethanol to ensure all the amounts of negative ions of precursor salts and humidity are removed. This precipitate was thrown to dry using silica gel inside a desiccator for overnight.
The chemical reactions of producing the inverse spinel NiFe2O4 were suggested using the reactions 1-2. 

  The suggested mechanism of using non –polar surfactant during the preparation of nickel ferrite can be depicted in Fig. 3. The step of addition of surfactant during precipitation method is vital to be capping agent and template in the same time that attitude to prevent formation of metal hydroxide, and produce homogenously growth for NiFe2O4 nanoparticle, respectively.  Moreover, the non-polar surfactant is also important to ensure the stabilization of formed nano-compound during the storage process.

Application of the Spinel NiFe2O4 used to decolorize alkali blue 4B dye solution
A homemade photo reactor was used to carry out the photoreaction in Fig. 4. This photoreactor is equipped with a 400-watt UV light that is put inside a wooden box as a reactor’s body to prevent the UV-A light risk. The light source was positioned directly above the Pyrex glass beaker (500 mL) to ensure a good focusing of it. Moreover, a magnetic stirrer, a Teflon bar, and used two different fans to fix and eliminate the generated heat from the lamp[17–20].
At a temperature 25 oC, exactly 0.025 g of spinel NiFe2O4 was added to 25 ppm from alkali blue 4B dye solution at pH 2.5. Without irradiation, the suspension solution was magnetically agitated for 15 minutes to allow for the establishment of an equilibrium adsorption condition[21-23]. Following the adsorption stage, the suspension was exposed to UV light, and about 5 mL of aliquots were collected at intervals time started from 5 minutes until 35 minutes. The collected suspensions were centrifuged twice at 6000 rpm for 20 minutes. The absorbance of the resulting filters was measured at 593 nm using a UV-Vis spectrophotometer (FAITHFUL model 721). The apparent rate constant for the first order equation [24-26] and the % efficiency of dye photo-decolourization [27-29] were calculated using equations 1 and 2.

RESULT AND DISCUSSION  
Structure properties 
The XRD analysis of the structure of prepared spinal NiFe2O4 with ranging from 0o to 80o using a Lab X XRD 6000-Shimadzu is represented in Fig. 5. The XRD data of NiFe3O4 synthesized powder exhibited the three essential not sharp peaks, with diffraction patterns at 2θ = 35.77o, 43.51o, 63.04o that correspond to the planes of (311), (400), and (440), respectively (Card No.: 00-003-0875). In contrast to diffraction patterns of perfect crystals[30, 31]. These results are in agreement with another study for preparing the NiFe2O4 nanoparticle before calcinated it [15]. 
The mean crystallite size (L) in nm was calculated for spinal NiFe2O4 powder using the Scherer’s equation(3)[32-37], and found to be 20.13 nm. It seems that the produced size of the prepared spinal NiFe2O4 powder is nanomaterial (less 100 nm)[38,39]. This result enhanced the good bending between Ni2+ and Fe3+ by the metallic bonds in crystal lattices of spinal NiFe2O4 nanoparticle, which due to the nearly ionic radii that equal to 0.59 Å and 0.609 Å, respectively[40].  

  Where k, denotes the shape constant, λ is Cu’s wavelength used as a source of x-ray, 2θ is a Bragg diffraction angle, and (FWHM) is meaning a full width at half maximum intensity.

Morphology of studied photo catalyst surfaces 
SEM analysis
 It was used to determine the morphology of the sample surface using (FESEM FEI Nova Nano SEM 450). The SEM images of spinel Nickel Ferrite surfaces display in Fig. 6.  The SEM analysis reveals that the sample shows a compressed order of homogenous nanoparticles with an almost spherical shape that agglomerated to form a like-broccoli nanoparticle shape. Most particles are assembled, hence, the particle size is more valuable than the mean crystal size[41, 42] and equal to 30.67 nm. This behavior may be attributed to the small dimensions of nanoparticles and has a high density [15]. The NiFe2O4 nanoparticle pieces of evidence it is a polycrystalline structure. 

EDX analysis 
It was used to validate the sample’s components. As seen in Fig. 7, the nickel ferrite illustrates the Ni, Fe and O elements uniformity distribution in the polycrystalline layer without any impurities. Moreover, the wt % of Fe is found to be double than  the wt % of Ni, this is good agreement with the stoichiometry preparation ratio (2:1) that is used in the preparation procedure[15], and reported in references[43,44].

Optical Properties 
FTIR Analysis
The location of the ions in the crystal structure and their modes of vibration are revealed by FTIR spectra, revealing the various arrangement locations on the structural characteristics of composites. In this study, the FTIR spectra of the produced sample were analyzed using Shimadzu 8400S Japan equipment throughout the range of 250–4000 cm-1 using a CsI disk. 
As seen in Fig. 8 the FT-IR spectra of all spinels, and especially ferrites includes  two primary wide metal oxygen bands are seen below about 600 cm-1 beyond tetrahedral and octahedral sites[45]. The tetrahedral is short bond therefore it lies in a high frequency vibration site. Nickel ferrite is having an inverse spinel structure group, therefore, the highest-frequency band, seen at ν1 = (563.23-651.93) cm-1, is attributed to tetrahedral site vibrations of (Fe-O) tetra, while at the lowest-frequency band, seen at ν2= (362.63-428.21) cm-1, is attributed to octahedral site vibrations of (Ni-O)octa..These results are in good agreement with the reported for similar Metal ferrite [45-48]. The bands at 3456.55 cm-1 and 1620 cm-1 correspond to the manner of stretching mode of H2O and OH molecules as a broad band and bending mode of H2O molecules, respectively [45,47]. The use a hexamine as a non- polar surfactant  as a capping agent and stabilizer for Ni ferrite leads to appear the bands at 1367 cm-1 and the band at 1118 cm-1 may be beyond the C-H bending vibrations and C-N stretching vibration [49]. 

The band gaps energy  
The optical energy bandgaps (Eg) value in eV revealed information about the ability of this material to investigate as a photocatalyst[50,51].  The bandgap can be determined for all photocatalyst samples using the Tauc equation[52,53] equation 4,  which is based on the absorption coefficient (α) in equation 5 that depended on the thickness (t) and the absorbance (A).

 
Where h, , k, and m  are Plank’s constant the light, frequency, optical constant and constant value equal to ½ or 2 for direct and indirect transitions, respectively.
Equation 4 is plotted in Fig. 9, and demonstrated that the band gap of as- prepared inverse spinel NiFe2O4 nanoparticle is an indirect band gap [54,55], and equal to 2.05 eV. 

Photo-decolorization of Alkali blue 4B dye 
The effectiveness of the prepared inverse spinel NiFe2O4 nanoparticle was estimated, based on the value of the band gap, so, this spinel was applied in a photochromic decolorization of Alkali blue 4B dye. 

The influencing of initial pH of dye solution on the photo-decolorization process
This manner was done with the pH ranging equal to 2.5, 4, 5.5, and 7. The initial pH of this dye was found to be 2.5. In Figs. 10 and 11, it was observed that the percentage of dye photo-decolorization and photo-decolorization rate increase with the increasing pH ranging from 2.5 to 7. That is due to the increase in an attractive force between the dye molecule and catalyst surface because of dye is having acidic nature. Moreover,  the percentage of hydroxyl radicals is also increased in the solution , these causes lead to enhance the photocatalytic activity of the decolorization process [56,57].The pH value was not performed with an increase of more than 7 that attitude to the change in the color of the dye solution, which referred to the change in dye structure.

The influencing of temperature on the photo-decolorization of dye
This effect was performed at range of temperatures (15, 20, 25, 35) oC using 25 ppm of dye with 0.01 g of inverse spinel nickel ferrite. It was found that the best temperature for the reaction is 25 oC. The Arrhenius equation plot (equation 6)[58,59] and the Ering-Pollani equation plot (equation 7)[ [60-63] were done in Fig. 12 and Fig. 13, to find the activation energy (Ea), enthalpy (ΔH#)and entropy (ΔS#), respectively. Where: kB is Boltzman constant (1.380649 × 10-23 m2 kg s-2 K-1) and h is Planck constant (6.626x10–34 Js), A is frequency factor, R is gas constant ( 8.314 J. mol-1. K1-).
   

The Gibbs energy (equation 8) was calculated depending on the values of ΔS# and ΔH# [64,56].

           
The calculated activation energy and thermodynamic functions were listed in Table 1. From the results, it was found that the photoreaction is endothermic, non-spontaneous and less random. These results are in agreement with the reported references [64-66]. The negative value of entropy (-0.240 kJ mol-1) confirms that the transition state generated is low structural freedom compared to the reactant (dye molecule) through the decolonization process of this dye, which enhanced the real of the decolorization process is irreversible [57, 59]. The activation energy was a small value and equal to10.878 kJ mol -1 that can be beyond the diffusion process in this photoreaction is controlled when its value less than 29 kJ mol-1[57]. Also, the reaction on the spinel surface can happen in multiple chain steps, one is a fast step that appears when the bond on the octahedral Ni-O site is formed, and another is a slow due to form the bond on the tetrahedral Fe-O site, which needed to high energy to bind.   

CONCLUSION
The vital results of this paper may be included:
The inverse spinel NiFe2O4 as nanoparticles like- broccoli was successfully prepared using the precipitation method that assisted with a hydrothermal method.
 The non-polar surfactant (hexamine) was used in the preparation of inverse spinel NiFe2O4 and acts as a template, capping agent and stabilizer.
Using XRD analysis and SEM-EDX analysis, the inverse spinel NiFe2O4 nanoparticles are confirmed.
According to FT-IR spectrum, the positions of the octahedral Ni-O site and the tetrahedral Fe-O site was determined, and proved the inverse spinel NiFe2O4 nanoparticle were formed.
The bandgap of the inverse spinel NiFe2O4 nanoparticle was found to be indirect type.
The photoactivity in using the inverse spinel nanoparticle NiFe2O4 nanoparticles with alkali blue 4B dye depended on pH of solution and the best result can occurred in a neutral medium. 
The photoreaction is a fast and obeyed pseudo-first order kinetic with low activation energy. 
The low absolute values of Ea, ΔH#, and ΔS# for the reaction estimated the prepared catalyst is having high catalytic activity to generate electron-hole pairs that lead to hydroxyl radical in aqueous solution.

ACKNOWLEDGMENTS
The authors would like to thank everyone who helped with this manuscript in the University of Kerbala’s faculty of science- department of chemistry and department of physics.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.