Synthesize and Characterization of Mesoporous ZrFe2O4@SiO₂ Core-shell Nanocomposite Modified with APTES and TCPP

INTRODUCTION
Synthetic organic dyes which are widely used in various industries have become a significant source of water pollution. Methyl orange (MO) is a dye, which used in many industries, having various harmful effects on humans when released into the environment [1,2]. Therefore, removal of this dye from wastewater has been extensively studied [1,3].
In the past decade, magnetic nanoparticles have been considered to surface functionalization, such as a coating which could result in obtaining significant properties such as photocatalytic activity [4]. Although there are many kinds of materials available for coatings of the magnetic nanoparticles, such as metal oxide, noble metals and polymer materials, the silica is still considered to be the best candidate for surface functionalization because it is highly stable against degradation [5]. 
In the literature, there are only few studies on Zr-substituted magnetite [6-12]. To knowledge, the structural and magnetic properties of ZrFe₂O₄, models of the distribution of Fe²⁺, Fe³⁺ and Ti⁴⁺ ions in TiFe₂O₄ have been used because Zr is one of the family group of Ti [6,7,9-14].
There are several methods to prepare of ferrites, such as microwave [15], solvothermal [16], mechanochemical [17], sonochemical [18] and wet chemical methods [19]. It seems that a one-pot template-free solvothermal method in ethylene glycol (EG) as solvent and ammonium acetate as electrostatic stabilization to be the preferred method [16,20]. 
On ther other hands, covering the surface of ferrites nanoparticles by the silica shell, not only protected the magnetic cores against aggregating, but also made it easy to introduce new functional groups. To achive this gole, tetraethylorthosilicate (TEOS) is a proper material. It can be hydrolyzed to silica by ammonia (pH=10-12), which makes it an excellent system for uniform coating on nanomagnetic particles. [21]. 
Foretheremore, porphyrins are known as attractive compounds in chemical science because of its heterocyclic building blocks with a highly π-conjugated system They have considerable catalytic properties due to their interesting chemical and photochemical characteristics. The meso position of the porphyrin is one of the most reactive centers; the substituent effect on the electrochemical property of porphyrins that influence the efficiency of catalytic and electron transfer processes [22,23]. TCPP (5,10,15,20-meso-tetrakis(4-carboxyphenyl)porphyrin) is a kind of porphyrin, consists of a porphin ring and four benzoic acid groups. These macromolecules can be linked to magnetic materials [24], which can influence the electron transfer of porphyrins [25-30].
In this study, mesoporous ZrFe₂O₄ nanocauliflowers were synthesized as the core, then were coated by SiO2 as the shell, finally modified by TCPP. The prepared nanocomposite was applied as an environmentally friendly photocatalyst to remove of methyl orange (MO) under visible LED lamp light.

MATERIALS AND METHODS
Chemicals
In this study, zirconium chloride (ZrCl4), iron (ІІІ) chloride (FeCl3.6H2O), ethylene glycol (C2H6O2), ammonium acetate (NH4.OAC), ethanol (C2H5OH) 96% and 99%, tetraethylorthosilicate (TEOS), dry toluene (C7H8), (3-aminopropyl)triethoxisilan (APTES) and all the chemicals, solvents and reagents were purchased from Aldrich and Merck have been used without any further purification.

Synthesis of mesoporous ZrFe₂O₄ nanocauliflowers
First, 1 mmol (0.233 g) ZrCl4 and 2 mmol (0.541 g) FeCl3.6H2O were dissolved in 50 mL ethyleneglycol at vigorously stirred at 600 rpm for15 min to form a clear solution. Then 30 mmol (1.156 g) NH4.OAC as a protective agent, was added into the mixture solution under an ultrasonic bath for 30 min to form a dark yellow solution. In continuing, the condition in vigorously stirred at 600 rpm for 30 min at room temperature. Afterward, the mixture was sunk in a 50 mL Teflon-line stainless steel autoclave, fixed and kept in at 210 °C for 48 h. As a result, the black precipitate was collected by an extrnal magnetic field and washed with ethanol and distilled water several times. At last, products were dried in a vacuum system at 60 °C for 15 h [15].

Synthesis of ZrFe₂O₄@SiO₂ -NH₂
ZrFe₂O₄@SiO₂ nanocomposite was obtained by the Stober method [15]. To synthesis of ZrFe₂O₄@SiO₂ -NH₂, 0.5 g (1.54 mmol) ZrFe₂O₄@SiO₂ was dispersed in 10 mL dry toluene and 0.5 mL (2.5 mmol) APTES was added under N₂ atmosphere. Subsequently, the mixture was refluxed for 18 h. The obtained product was washed by hot toluene and dried in a vacuum oven [31].

Synthesis of ZrFe₂O₄@SiO₂ -NH-TCPP 
Synthesis of ZrFe₂O₄@SiO₂ -NH-TCCP was carried out by mixing and ZrFe₂O₄@SiO₂ -NH₂ nanocomposite in the ratio of 1:20. Therefore, 0.01 g TCCP and 0.2 g ZrFe₂O₄@SiO₂ -NH₂ were added to 150 mL DMF and refluxed for 8 hour by a magnetic stirrer. The obtained product was washed with distilled water and dried in a vacuum oven [32].

Equipment and characterization
The functional groups of the materials were recorded by Fourier transform infrared spectroscopy (FT-IR) on a Shimadzu FTIR 8400S spectrophotometer with KBr pellet. X-ray diffraction (XRD) analysis was performed on Philips Pw 1730 X-ray diffractometer with Cu Kα radiation. The morphology of the synthesized samples was observed by a Tescan Mira3 field emission scanning electron microscopy (FE-SEM). The elemental constituents of samples were obtained by energy-dispersive X-ray spectroscopy (EDS) using VEGAII, Tescan, Czech Republic instruments. The nitrogen (N2) adsorption and desorption equipment, Brunauer-Emmett-Teller (BET) surface areas and Barrett-Joyner-Halenda (BJH) pore sizes were calculated on an ASAP 2020 (Micromeritics Ins. Corp.) at liquid nitrogen. Also vibrating sample magnetometer (VSM, MDKB, Magnetic Daghigh Kavir Co. Iran) was employed to measure the magnetic behavior of nanomagnetic particles at room temperature. Shimadzu (UV-2550) spectrophotometer obtained the diffuse reflectance spectroscopy (DRS) spectra. Spectrofluorophotometer (RF-6000, Shimadzu) was used to characterize the optical properties of material.

Photocatalytic experiments under visible light irradiation
The photocatalytic activity of the prepared samples was evaluated by degradation of MO solutions. In a typical process, the catalytic reaction was carried out in a 100 mL photoreactor, which contains 50 mL of MO dye solutions with a concentration of 10 mg L-1 (pH=3) and 5 mg of catalyst. Irradiation was provided using a 10 W LED lamps as the light sources for one hour. All photocatalytic tests were carried out at the same conditions. The concentration of MO dye was monitored using a UV-Vis spectrophotometer (Shimadzu UV-1700) at 464 nm.

RESULTS AND DISCUSSION
FT-IR spectra of the prepared samples
FT-IR spectra of ZrFe₂O₄, ZrFe₂O₄@SiO₂, ZrFe₂O₄@SiO₂ -TCPP, and ZrFe₂O₄@SiO₂ -NH-TCPP are shown in Fig. 1. Peaks at 585 and 455 cm-1 belong to Fe-O and Zr-O bands, respectively. The peaks at 3444, 1645 and 1398 cm^-1 point out the presence of O-H stretching bending and deforming vibration of adsorbed water [33]. At spectrum of ZrFe₂O₄@SiO₂ , the new sharp peaks at 1090, 800 and 470 cm^-1 are assigned to the asymmetry stretching, symmetric stretching and bending vibrations of Si-O of the silica shell on the ZrFe₂O₄ nanocauliflowers, respectively [21]. 
The ZrFe₂O₄@SiO₂ -NH₂ spectrum, the broad peak around 3400 cm-1 corresponds to N-H stretching modes, and the peak at 1060 cm-1 indicate the existence of a terminal –NH₂ of APTES, however, this peak has been protected by stretching vibration of Si-O of silica. The peaks that appeared at 835 cm^-1 could be assigned to the bending vibrant of C-H from APTES [21].
In spectra of ZrFe₂O₄@SiO₂ -TCPP and ZrFe₂O₄@SiO₂ -NH-TCPP, the presence of the peak at 3200 cm^-1 approve N-H bending vibration band of pyrrole ring, and at 2900 cm^-1 show the C-H stretching vibration of aryl group which corresponds to TCPP. Referring to the FT-IR spectrum of pure TCPP, a sharp C=O stretching peak can be observed at approximately 1700 cm^-1 [34].

Structural characterization of the prepared samples
Fig. 2 shows the XRD patterns of ZrFe₂O₄, ZrFe₂O₄@SiO₂ , ZrFe₂O₄@SiO₂-TCPP and ZrFe₂O₄@SiO₂ -NH-TCPP. It confirms that all the samples were formed in an inverse spinel structure similar to that of magnetite (space group Fd3m, No. 227, with standard card JCPDS No. 01-088-0315). According to the Debye–Scherrer formula, the crystalline size of the sample is given by:

Where β is the full-width at half-maximum (FWHM) value of XRD diffraction lines, the wavelength λ = 0.154056 nm and θ is the half diffraction angle of 2θ. The size of crystalline of ZrFe₂O₄ nanocauliflowers was found 44.6 mm, which calculated by taking the average of the sizes at its peaks. The intensity of the peaks decreased after immobilizing of silica on the surface of ferrite particles. In this pattern, no extra peaks for other phases were detected, and no redundant reaction had occurred between the core and shell. 
The crystallite size of ZrFe₂O₄, ZrFe₂O₄@SiO₂, ZrFe₂O₄@SiO₂ -TCPP and ZrFe₂O₄@SiO₂ -NH-TCPP based on Scherrer equation was 41.3, 36.9, 34.8 and 26.5 respectively, indicated that after coating of ferrite with SiO₂ and modifying by APTES and TCPP, the crystallite size of nanostructures has been decreased.

Morphological characterization of the prepared samples
The microstructure of the samples can be identified by scanning electron microscope (SEM) tool. Fig. 3 shows the morphology of high resolution and nanosize is the most regular shape of the particle. ZrFe₂O₄ particles with an average size of 35 nm were agglomerated to form nanocauliflowers with an average size of 150 nm. Ethylene glycol and ammonium acetate play essential roles in the self-assembly of the nanocrystals and fabrication of monodisperse fine metal oxides (Fig. 4). SEM images of prepared nanocomposites indicate that after SiO₂ coating and functionalizing (with APTES, and TCPP), the average size of nanoparticles decreased to 32, 29, 25, and 23 nm for ZrFe₂O₄, ZrFe₂O₄@SiO₂ , ZrFe₂O₄@SiO₂ -TCPP, and ZrFe₂O₄@SiO₂ -NH-TCPP, respectively.
Fig. 5 shows the typical TEM images of the ZrFe₂O₄@SiO₂ -NH-TCPP nanocomposite, which indicates a well-defined core-shell mesoporous nanostructure. The amount of thickness the shell layer, containing silica and porphyrin is about 10-9 
nm.

Elemental analysis of ZrFe₂O₄ and ZrFe₂O₄@SiO₂
To identify the elemental composition of mesoporous ZrFe₂O₄ nanocauliflowers and ZrFe₂O₄@SiO₂ , an energy-dispersive X-ray spectroscopy (EDS) was used. As can be seen in Fig. 6, EDS pattern of ZrFe₂O₄ nanocauliflowers shows the peaks of Fe, Zr, Si and O elements, and approves the presence of the iron and zirconium elements in the molar ratio of 2:1. Chemical characterization of the ZrFe₂O₄ sample showed that it was composed of iron, zirconium, oxygen and silicon elements, provide the existence of SiO₂ on surface of ZrFe₂O₄. Furthermore, the molar ratio of SiO2 to ZrFe2O4 was 10:1, which confirmed by XRF analysis of samples.

Nitrogen adsorption and desorption isotherms of ZrFe₂O₄ and ZrFe₂O₄@SiO₂ -NH-MnTCPP
The specific surface area and pore volume data of the mesoporous ZrFe₂O₄ nanocauliflowers and ZrFe₂O₄@SiO₂-NH-TCPP composite were characterized using the N2 adsorption-desorption method, with a typical isotherm shown in Fig. 7. The isotherm demonstrates a type IV isotherm. Data obtained by BET also showed large surface areas for the ZrFe₂O₄ and ZrFe₂O₄@SiO₂ -NH-TCPP composite (431 and 978 m2g-1, respectively). These values are relatively large compared to other reported mesoporous ferrite structures [35]. The very uniform pore sizes of the ZrFe₂O₄ and ZrFe₂O₄@SiO₂ -NH-TCPP were found to be 5.8635, and 4.5345 nm, respectively. The decreasing particle size of composite rather than pure ferrite can lead to the decrease of pore size and increase of surface area and pore volume of the nanocomposite. The pore volume of ZrFe₂O₄ and ZrFe₂O₄@SiO₂ -NH-TCPP composite was calculated 0.5812 and 0.9692 cm³/g, respectively. The ZrFe₂O₄@SiO₂ -NH-TCPP composite showed higher textural mesoporosity, volume that facilitates the achievement in its area of the scaffold-confined mesoporosity, and modifying its catalytic properties.

Magnetic properties of the prepared samples
The magnetic properties of mesoporous ZrFe₂O₄ nanocauliflowers were measured at room temperature by using VSM in an external magnetic field ranging from -8 kOe to 8 kOe and compared with the coated material (SiO₂ and TCPP) as the shell which is synthesized, ZrFe₂O₄@SiO₂, ZrFe₂O₄@SiO₂ -TCPP and ZrFe₂O₄@SiO₂ -NH-TCPP (Fig. 8).
The saturation magnetization (Ms) value, extracted from the corresponding hysteresis loop, from the uncoated ferrite sample at 300 K, is 18.3 emu/g. Ms decreased for the coated samples as expected to be 12.4, 5.7, 4.1 and 2.8 emu/g for ZrFe₂O₄, ZrFe₂O₄@SiO₂, ZrFe₂O₄@SiO₂ -TCPP and ZrFe₂O₄@SiO₂ -NH-TCPP, respectively, demonstrates that SiO₂ and TCPP have been successfully immobilized on the surface of ZrFe₂O₄ nanocauliflowers. By coating the silica on ferrite, the Fe ions tend to bond with silica (Fe-O-Si), so the magnetic moment of Fe ions would get diminished [36]. The same reason, the presence of TCPP causes for a decrease in the magnetic properties of the nanomagnetic particles. However their magnetic properties are still significant for the separation of these particles (as a catalyst) by using an external magnetic field. 
The coercivity (Hc) values are little increased in the range of 50 Oe for the bare magnetite nanocauliflowers to 100, 100, 150 and 150 Oe for ZrFe₂O₄@SiO₂, ZrFe₂O₄@SiO₂ -NH-TCPP and ZrFe₂O₄@SiO₂ -TCPP, respectively. This increasing can be occurred due to the shrinking of ferrite nanoparticles after immobilizing of SiO2 and functionalizing with TCPP. In a single-domain particle, all the spins are aligned in the same direction, and the particle is uniformly magnetized. Because there are no domain walls to move, the magnetization will be reversed through spin rotation rather than through the motion of domain walls, leads to large coercivity of the nanoparticles [37]. Another possible explanation for this result could be as follows: The magnetic moments of magnetic nanoparticles are pined by the SiO2 and TCPP, so that a higher magnetic field is required to align the single domain nanoparticles in the field direction [34].

Optical properties of the prepares samples
The optical properties of the synthesized materials were studied by the UV–Vis diffuse reflectance spectrophotometer (DRS). The obtained curves are shown in Fig. 9A. ZrFe₂O₄@SiO₂ exhibited slightly weaker absorption in the range of 200–800 nm than ZrFe₂O₄ nanocauliflowers, probably resulting from the light reflection and refraction of SiO2 [38]. In the ZrFe₂O₄@SiO₂-NH-TCPP spectrum, the peak observed at 440 nm can be corresponded to the soret band of porphyrin that confirms immobilizing of porphyrin on the surface of ZrFe₂O₄@SiO₂ nanocomposite. It can be seen that a red shift in ZrFe₂O₄@SiO₂ -NH-TCPP nanocomposites was obtained in comparison with pure porphyrin. This means that the ZrFe₂O₄@SiO₂-NH-TCPP nanocomposite is sensitive to visible light, which accounts for the photoactivity under visible light irradiation. It is generally accepted that the red shift of the absorption band can increase photon numbers by extending the energy range of photoexcitation, which can be absorbed by the catalyst and utilized for the photocatalytic reaction.
The bandgap energy can be approximately calculated from 1.4-1.7 eV (Fig. 9B). The results of the experiments confirm that modifying the surface of the composite with porphyrin play a crucial role in enhancing the photocatalytic performance of ZrFe₂O₄@SiO₂ nanocomposite for MO degradation.

Fluorescence spectra of the prepared samples
The fluorescence spectra of the prepared samples, shown in Fig. 10, give more detailed information. The emission spectra of the all prepared samples have shown emission peak at 430 nm (excitation peaks at 400 nm), but TCPP has illustrated two peaks at 663 and 733 nm with significant quenching after immunizing on the surface of nanocomposite (ZrFe₂O₄@SiO₂ -TCPP, and ZrFe₂O₄@SiO₂ -NH-TCPP) when compared to the original porphyrin. Such quenching of emissions is predicted that can be attributed to the binding of the porphyrin to the nanocomposite surface and transferring photogenerated electrons of TCPP on nanocomposite.

Photocatalytic activity of the prepared samples
The photocatalytic activity of the prepared samples was investigated by performing experiments on the degradation of 10 mg L⁻¹ methyl orange. The photodegradation results were shown in Fig. 11. As can be seen from this figure, the excellent catalytic activity of photocatalysts with mesoporous structure is assigned to the mesoporous network structure. The higher surface area in ZrFe₂O₄@SiO₂ -NH-TCPP is beneficial to raise the number of active sites of these photocatalysts, so exhibited the highest activity to degrade methyl orange.

CONCLUSION
In this work, core–shell structured ZrFe₂O₄@SiO₂ nanocauliflowers were fabricated and modified with 3-aminopropyltriethoxysilane as a linker, and tetrakis(4-carboxyphenyl)porphyrin as a sensitizer agent. Based on the XRD, FT-IR, SEM, TEM, EDS, BET, VSM, DRS and fluorescence analysis results, as-prepared nanocomposite can be extended to synthesize mesoporous texture magnetic nanocomposites with large specific surface areas and narrow size distribution. Study on photocatalytic activity of the prepared samples for removal of methyl orange from aqua solutions showed that the final fabricated nanocomposite (ZrFe₂O₄@SiO₂ -NH-TCPP) could degrade this pollutant about 100% under only 10 W LED lamp irradiation. The obtaine results indicated that the synthesized nanocomposite not only shows excellent photocatalytic activity for removal of MO, but also can be separated easily by an external magnetic field.

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