Solvothermal -Assisted Cushy Precipitation Method of Like-Cauliflower Zinc Ferrite (ZnFe2O4) Nanoparticles Using Hexamethylenetetramine (Hexamine) As a Non-ionic Surfactant

INTRODUCTION
Nanotechnology is a fascinating field that works with particles that are smaller than 100 nanometers. The high surface-to-volume ratio of these particles is due to their small size. To put it another way, these particles contain a lot of active sites, which boosts their activity dramatically. In the literature, a variety of nanomaterials for various uses have been reported[1]. As magnetic materials, spinel ferrites are often used. Numerous metal oxides, mixed metal oxides, and ferrites have shown heightened sensitivity to certain gases and humidity. The spinel’s form is in a face-centered cubic (FCC) lattice with eight formula units per cubic unit cell, which corresponds to the space group Fd3m (O7h). (M2+)t [Fe23+]oO42- is the usual molecular formula of spinel ferrites, where M2+ and Fe3+ are the divalent and trivalent cations occupying tetrahedral (A) and octahedral (B) interstitial spaces of the FCC lattice formed by O2-ions. Spinel structures include both normal spinel and inverse spinel structures. As magnetic materials, spinel ferrites are often used. Numerous metal oxides, mixed metal oxides, and ferrites have shown heightened sensitivity to certain gases and humidity. The unfilled octahedral, tetrahedral, and tetrahedral holes may accommodate the analyzed spices and may function as an efficient sensor. Cobalt ferrite (CoFe2O4), Copper ferrite (CuFe2O4), and Nickel ferrite (NiFe2O4) are inverse spinel’s with ferric ions occupying tetrahedral (A) sites and ferric ions and the specific metal ions occupying octahedral (B) sites, as in [(Fe3+)A] [M2+ Fe23+] BO42-. ZnFe2O4 has a conventional spinel structure, with Zn2+ and Fe3+ occupying tetrahedral and octahedral sites, respectively [2]. Zinc ferrite is a normal spinel oxide with a cubic structure and considerable potential for use in gas sensors and other technological applications [3], magnetic and superparamagnetic materials[4], adsorbents[5], materials with optical and dielectric properties[6], antibacterial agent[7], catalysts and photocatalysts [8]. As a typical n-type semiconductor photocatalyst, spinel zinc ferrite (ZnFe2O4, ZFO) has a very narrow bandgap (1.9) eV, allowing ZFO to capture around 46% of the sun’s energy for the creation of photogenerated electrons (e-) and holes (h+). In the preceding decade, ZFOs were extensively employed as reusable magnetic photocatalysts for the removal of dyes, organic contaminants, microcystin-LR, and bacteria. Although ZFO has a broad light absorption range, its catalytic activity is poor due to the rapid recombination of photogenerated e- and h+. As reported by Deng and et al[9], ion doping will alter the coordination relationship between the iron (Fe) atom and oxygen atom in the ZFO lattice. Specifically, some formerly octahedral coordinated Fe atoms (FeO) are converted to tetrahedral coordinated Fe atoms (Fe T), and the transport of the e can be facilitated by the super exchange interaction. Dyeing businesses such as textiles and tanneries produce effluents containing dye that are hazardous and need treatment. In the last years, it has seen an increase in the use of advanced oxidation processes (AOPs) to remove organic contaminants from aqueous solutions. Among the AOPs, the heterogeneous Fenton reaction is one of the most interesting processes since it employs solid iron-based catalysts and their simple recovery after treatment by a magnetic field allows for their reuse. However, little research has been published on the utilization of zinc ferrite for the breakdown of organic contaminants via the heterogeneous Fenton process under visible light irradiation [10]. The synthesis of zinc ferrite oxide with the necessary physical properties for usage in a range of research fields has been a major focus. So, ZnFe2O4 ternary oxide has been produced in several methods, including precipitation followed by calcination, succinic acid-assisted hydrothermal route, electrospinning, surfactant-assisted hydrothermal synthesis, sol-gel technique followed by calcination, glycine combustion, solvothermal synthesis, sol-gel auto combustion reaction, polyethylene glycol-assisted route, and hydrothermal method in the presence of polyethylene glycol[11]. This work aims to prepare normal spinal ZnFe2O4 using the precipitation method in presence of non-ionic surfactant (hexamine). This synthesis method is assisted by the solvothermal method using ethanol at 180 oC at 90 min. The characterization of prepared ZnFe2O4 will measure using XRD, SEM-EDX, FT-IR and Bg. The normal spinal ZnFe2O4 will apply in the decolonization of dye.

MATERIALS AND METHODS
Materials
The powder of Ferric nitrate Fe(NO3)3.9H2O and Zinc nitrate Zn(NO3)2.9H2O were supplied by sigma cheml co. Ammonia solution (25 %), absolute ethanol (C2H5OH) and hexamethylenetetramine (hexamine) were purchased by Merck,  BDH and  Interchimiques SA, respectively. The alkali blue 4B dye was supplied from Merck and had information as flowing in Table 1.  

Synthesis of Spinel ZnFe2O4 nanocrystals
This procedure was modified from reference [12], and then performed using two precursor salt solutions: solution 1 was contained 0.2 M of Ferric nitrate Fe(NO3)3.9H2O  that prepared from dissolved  4.04 g in 50 mL D.W under ultrasonic waves for 10 min. Solution 2  was contained 0.1 M of Zinc nitrate Zn(NO3)3.6H2O that prepared from dissolved 1.45 g 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 pH of the produced solution was measured to be about 3, after that, 0.5 g of Hexamethylene tetramine called also (methenamine or hexamine) as a non-ionic surfactant with a cage-like structure, was added as a capping agent and enhanced the growth of NP shape under continuous mixed using a magnetic stirrer at 60 oC for 10 min.
The pH of the solution must be fixed using conc. ammonia solution that drop-wise added, and reached to be about 9.5 to 10 under stirring continued for 30 min at 60 oC. The dark brown solution was produced; this solution was filtered and washed with D.W then ethanol. This precipitate was dispersive in 50 mL from Ethanol, then transported to a Teflon cup sealed in an autoclave of hydrothermal instrument and heated in the oven at 180 oC for 90 min. After cooling the steles steal autoclave in air, the solution was filtered and washed by 50 mL D.W to ensure all surfactant is removed, then 30 mL of ethanol to remove all humidity After that the dark reddish brown precipitate was thrown to dry using silica gel inside a desiccator overnight to get Zinc ferrite as powder. All of the steps for the preparation of zinc ferrite are shown in Fig.  1. 
The chemical reactions of spinel ZnFe2O4 formed were suggested using the following equations.

Zn(𝑁𝑂3)2. 6𝐻2𝑂 + 2Fe(𝑁𝑂3)3. 9𝐻2𝑂 + 8𝑁𝐻4𝑂𝐻 →  Zn(𝑂𝐻)2. Fe2(𝑂𝐻)6 + 8𝑁𝐻4𝑁𝑂3 + 24𝐻2𝑂                   (1)

Zn(𝑂𝐻)2.Fe2(𝑂𝐻)6 →  Zn𝑂. Fe2𝑂3  or  ZnFe2𝑂4 + 4𝐻2𝑂                                                                                 (2)              

Application of the Spinel ZnFe2O4 used to decolorize alkali blue 4B dye solution
A homemade photoreactor was used to carry out the photoreaction in Fig.  2. This photoreactor is equipped with a 400-watt UV light ( light intensity equal to 1.87 x 10-7 ensien.sec-1) that used a wooden box as a reactor’s body to prevent the UV-A light risk[13], with a magnetic stirrer, a Pyrex glass beaker (500 mL), a Teflon bar, and used two different fans to depress the heat and fixed the temperature of photoreactor [14-16]. 
The dark reaction is essential for the establishment to reach the reaction for the equilibrium adsorption condition[17-20]. This step was performed by magnetically agitating 25 ppm of alkali blue 4B dye with n g from spinel ZnFe2O4 at pH 2.5 for 15 minutes. Following the adsorption stage, the suspension was exposed to UV light, and about 5 mL of aliquots were collected at 5 minute intervals until 30 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).  Equations 3 and 4 were used to get the apparent first order rate constant of this photoreaction and the efficiency of decolonization[21-26] respectively. 

RESULT AND DISCUSSION  
Structure Properties 
According to Fig.  3, XRD analysis was used to determine the structure of prepared spinal ZnFe2O4 ranging from 0o to 80o using a Lab X XRD 6000-Shimadzu. The analysis of XRD result was the ZnFe2O4 peaks were embedded in an amorphous matrix that was due to no calcination. The results give low and broad peaks with diffraction patterns at 2θ of 29. 9o, 35.30o, 36.40o 42.78o, 51.8o, 55.05o and 62.2o, that ascribed to the reflection of (220), (311), (222), (400),(331), (422), and (440) planes of the spinel ZnFe2O4, respectively. These peaks are in agreement with results reported in references [27,28]. These results are in agreement with the FT-IR analysis that supported the prepared of normal spinel ZnFe2O4, which is discussed later.The mean crystallite size for spinal ZnFe2O4 was calculated using the Scherer’s equation(5) [29-33], and found to be equal 16.98 nm. This value proved the prepared spinel  is nanomaterial and amorphous [34,35]. 

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 zinc ferrite surfaces display in Fig.  4.  SEM reveals that the sample shows a compressed order of homogenous nanoparticles with an almost spherical agglomerated as a like-cauliflower nanoparticle shape. This growth of Zn ferrite nanoparticle depends on the properties of surfactant which leads to the form of this reasonable shape. Most particles are assembled, hence, the particle size is more than the mean crystal size [32, 34], and equal to 27.78 nm, this behaviour indicates the ZnFe2O4 nanoparticles are polycrystalline in their structure. 

EDX analysis 
It was used to validate the sample’s components. As seen in Fig.  5, the catalyst zinc ferrite consists  Zn, Fe and O, without any impurities, this is good agreement those reported in references[36-38].

FT-IR spectrum of ZnFe2O4
Fig.  6 explains the FT-IR spectrum of ZnFe2O4 powder. Two resemble bands 590 cm-1 and 655 cm-1 are observed as a characteristic of the normal spinel ZnFe2O4.  The low-frequency band (590 cm-1) is an assignment to the stretching vibration of the chemical bond of (Fe3+ – O2-) in octahedral sites, while the higher frequency band (655 cm-1) is attributed to the stretching vibration of the chemical bond of (Zn2+ –O2-) in the tetrahedral sites. These results are in agreement with that previously reported, the range of wave numbers for the interaction of metal-oxygen in crystal lattices are obtained for octahedral vibration mode and tetrahedral vibration mode at 474-636 cm-1   and (550-750) cm-1, respectively [39,40]. Moreover, the peaks at 3390 cm-1 and 1620 cm-1 are beyond the stretching vibration and bending mode of water molecular[27]. The clear bands in the range of (1330 – 1346) cm-1 and the band at 1215 cm-1 can be associated with the C-H bending vibrations and C-N stretching vibration [41,42] in the used surfactant structure( Hexamine).

Optical Property of studied photocatalyst          
The band gaps energy 
The optical energy bandgaps (Eg in eV) were determined for all photocatalyst samples using the Tauc equation[41-45], as seen in equations 6 and 7.

  Where , h, , k, t, A, and m are absorption coefficient, Plank’s constant the light, frequency, optical constant, thickness, the absorbance and constant value equal to ½ or 2 for direct and indirect transitions, respectively.
The equation 4 plotted in Fig.  7 and demonstrated that the band gap of prepared spinel ZnFe2O4 nanoparticle is an indirect band gap[27] and equal to 2.15 eV, this value is in agreement with the value reported in reference [46].

Photo-decolorization of Alkali blue 4B dye 
After determining the effectiveness of the prepared spinel ZnFe2O4 Nanopartical depended on measuring the optical properties like band gap, this spinel was applied in photo-decolorization of Alkali blue 4B dye. And that is by taking several weights from the photo catalyst and knowing the ideal weight for the removal, then applying the specified weight over a range of temperatures to find the thermodynamics functions. Several weights of the prepared spinel ZnFe2O4 Nanopartical (0.015, 0.025, 0.035, 0.05) g were taken. The rate of reaction and efficiency rises with increasing the dose, as shown in Figs. 8 and 9. That attitude raises the active site surface, which leads to increase the interaction between the dye and hydroxyl radicals on the photocatalyst surface [45]. The appropriate weight occurred 0.025g with the efficiency equal to 97.3. On the other hand, the increment in spinel ZnFe2O4 Nanopartical dose of more 0.025 g causes declining in efficiency, that due to increase the in the turbidity of the solution, which depresses of penetration of light through the solution this effect is called the screen effect [25, 47,48].
After choosing the ideal weight of the photocatalyst and applying the reaction over a range of temperatures (15, 20, 25, 35) oC, it was found that the best temperature for the reaction is 25 oC.  Based on figs. 10 and 11, the Arrhenius equation plot [48-51],and Eyring-Polanyi equation plot [52,53] were done to calculate activation energy and thermodynamic functions listed in Table 2. From the results, it was found that the photoreaction is endothermic, non-spontaneous, and less random and has low activation energy. The positive values of ∆H # and ∆G # are in agreement with data published in references [47,53,54],this conduct is beyond incrementing the solvation of transition state between dye molecules and hydroxyl radical species[45,55,56].

CONCLUSION
The main conclusions from this manuscript can be summarized by:
Preparation of normal spinel ZnFe2O4 as nanoparticles like cauliflower using precipitation method-assisted hydrothermal method in the presence of ethanol and capping agent (hexamine) as non-polar surfactant.
The normal spinel ZnFe2O4 nanoparticle is proved using XRD analysis and SEM-EDX.
The FT-IR analysis has supported the synthesis of normal spinel ZnFe2O4 nanoparticle depending on octahedral and tetrahedral sites for Metqal interaction Oxygen in lattices.
The normal spinel ZnFe2O4 nanoparticle can be used as a photocatalyst depending on the calculated band gap, which has been an indirect type.
The photoactivity of normal spinel ZnFe2O4 nanoparticle increases with increased dose then decreases.
The photoreaction is obeyed pseudo-first order kinetic with low activation energy and endothermic reaction.

ACKNOWLEDGMENTS
The Authors gratefully acknowledge all personnel who support 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.