Photocatalytic activity of visible light active Sr-hexaferrite prepared by solid-state reaction and the pechini methods

In this work, strontium hexaferrite (SrFe12O19) was prepared using two different methods, the solid-state reaction and the sol–gel pechini methods. In each case, the structural properties and microstructural features were analyzed in order to evaluate their influence on the photocatalytic activity of the strontium hexaferrite. In addition, the magnetic properties of each sample were also investigated. The analysis of the photocatalytic activity was done using methylene blue as a test dye. The results show that the fabrication method significantly impacts how the photocatalytic activity occurs. Firstly, the bandgap energy of the sample obtained by the solid-state reaction method turned out to be smaller than that obtained by the sol–gel pechini method. This behavior was attributed to the structural differences shown between the two samples. On the other hand, particle size also has a significant effect on photochemical reactions. However, smaller particle sizes make it difficult for photons to transport in the system, resulting in reduced photocatalytic activity. In this case, better results were obtained from the sample obtained from the solid-state reaction method.


Introduction
Water is an important and limited resource, the consumption of which is constantly increasing due to population growth. Unfortunately, water pollution due to human activity is a global problem that threatens life and safety. In this context, the textile industry is one of the most polluting industries because of the use of organic dyes dissolved in large amounts of water [1][2][3]. Today, the development of effective, environmentally friendly, and cost-effective methods for the treatment of wastewater containing dyes is one of the main issues for sustainable human development [4]. Therefore, new and innovative alternatives compatible with water treatment and purification are currently being considered [5]. One of these alternatives involves the use of magnetic particles, which clean the water at a low cost with relatively high efficiency and can recover them from the water using an external magnetic field, allowing the particles' re-usability [6].
Strontium hexaferrite is a well-known iron-based magnetic compound widely used in the production of permanent magnets and high-frequency magnetic cores. This compound features a unique combination of structural, microstructural, and magnetic properties that results in high chemical and thermal stability. The hexaferrites also show photocatalytic activity against certain organic dyes because they can absorb a large Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. number of photons in the visible region and thus efficiently drive photochemical degradation reactions [7][8][9][10], with the additional advantage that magnetic catalysts can be removed from water by applying an external magnetic field [11].
In the photocatalytic process, the hexaferrite particles absorb energy from light and use this energy to initiate or accelerate a chemical reaction. After activation of the photosensitized hexaferrite, the generated electron-hole pair (e − /h + ) can be recombined to release energy in the form of heat or react as a donor or acceptor with the dyes adsorbed on the surface of the particles. Most organic dyes can be decomposed or mineralized by successive oxidation-reduction reactions to CO 2 and H 2 O since highly reactive species such as superoxide (·O 2 − ) and hydroxyl radicals (•OH -) are formed during the photocatalytic process. Also, the formation of the hydroxyl radical (highly reactive) with hydrogen peroxide (H 2 O 2 ) can be potentiated (Photo-Fenton) by its catalytic decomposition in the presence of iron ions (Fe 2+ or Fe 3+ ), thereby increasing the degradation efficiency [7,12]. Various studies have been carried out in recent years to investigate how strontium hexaferrite exposed to different conditions affects its photocatalytic activity. For example, table 1 shows the reported photocatalytic activity under different organic dyes of the strontium hexaferrite obtained from various fabrication methods. However, these results are hardly compared because the photocatalytic activity is determined by means of different experimental conditions, such as particle size, initial dye concentration, additives, etc Notwithstanding, it is observed that the fabrication method largely affects the photocatalytic efficiency of the material. There are no previous reports on the photocatalytic activity of strontium hexaferrite prepared by the solid-state reaction method. Only recently has the photocatalytic activity of barium hexaferrite, which belongs to the same structural group as strontium hexaferrite, been reported (see table 1) [13]. In addition, many studies have attempted to improve the photocatalytic activity by doping and substituting some cations of the hexaferrite structure [14,15].
In this work, we focus on evaluating the photocatalytic activity of strontium hexaferrite obtained by two different preparation methods: (i) the solid-state reaction method and (ii) the pechini sol-gel method. The former is the most common method of producing hexagonal ferrites due to its simplicity and low cost. However, this method requires high sintering temperatures and produces micron-sized particles. On the other hand, the pechini sol-gel method requires a lower sintering temperature and thus produces submicron particles. The magnetic and structural properties, and the microstructural characteristics, were evaluated in each case, and these properties were associated with the photocatalytic activity of the as-obtained hexaferrites, maintaining the same experimental conditions during the methyl blue degradation.
The hexaferrite precursors were weighed and mixed in stoichiometric ratios according to the chemical equations: until the evaporation of the system promoted a polyesterification reaction, obtaining a homogeneous resin in which the metal ions were uniformly distributed in the organic matrix. The obtained resin was pre-calcined at 380°C for 40 min and ground in an agate mortar. Finally, the obtained powders were calcined at 1100°C for 4 h to ensure the formation of the phase. For the strontium hexaferrite prepared by the solid-state reaction method, the precursors were weighed according to the stoichiometric chemical reaction (equation (2)), using 0.9024 g of iron oxide Fe 2 O 3 and 0.1390 g of strontium carbonate SrCO 3 . The powder was mixed with 50 ml of ethyl alcohol, grind in an agate mortar for 2 h, and dried at 50°C. Finally, the powder was sintered at 1250°C for 2 h and cooled slowly inside the oven. The samples were labeled as follows: SrM-SGP for the hexaferrite synthesized by the sol-gel pechini method and SrM-SSR for that synthesized by the solid-state reaction method.

Samples characterization
Samples were characterized by x-ray diffraction (XRD) using an Inel diffractometer (Equinox 2000) with a cobalt x-ray tube with K α radiation (1.789 Å) within a 2θ angular range of 20 to 80°operated at 30 KV and 20 mA. Rietveld analysis of crystalline structure phase purity, and crystallite size was performed using the MAUD program. The initial structural model used for the refinement was based on the SrFe 12 O 19 phase (COD-ID: 1006000) hexagonal, P6 3 /mmc space group and lattice parameters a = b =5.8844 Å, and c = 23.0500 Å [22]. Optical properties were analyzed using a Perkin Elmer (Lambda XLS) spectrophotometer. The collected study solutions were placed in 1 cm×1 cm x 5 cm quartz cells and the spectra were recorded at room temperature in the wavelength range of 200 to 800 nm. The arrangement of particles and their morphological characteristics were analyzed with a Hitachi S-570 transmission electron microscope (TEM) working at 100 kV and with a Jeol 1200 scanning electron microscope (SEM) with backscattered electrons working at 120 kV. The magnetic properties were achieved using a Princeton AGM magnetometer Micromag 2900 with a maximum applied field of 11 kOe.

Photodegradation study
The photocatalytic activity of strontium hexaferrite was evaluated by measuring the degradation of methylene blue (MB) in an aqueous solution. MB is a heterocyclic aromatic compound with molecular formula C 16 H 18 ClN 3 S•3H 2 O (98.5%, Meyer) and 373.90 g/mol molecular weight. Figure 1 shows the chemical structure of the dye MB. The photocatalytic studies were performed under visible light generated by three LED lamps (4.5 W, 400 lumens MR16 − 9290018741 Philips), one located in the upper part and the other two in a perpendicular position to the system, with an emission wavelength of 400-750 nm and a maximum peak at 460 nm. During the photocatalytic study, 150 ml of an aqueous solution containing 100, 200, and 300 mg l −1 of the photocatalyst and a pollutant concentration range of 3-5 mg l −1 was used. Air bubbling was used to maintain the homogenization of the reaction system. During the study, the suspension was stirred in the dark for 30 min to establish the adsorption-desorption equilibrium after the first sample (time zero). Then, 2 ml of the suspension was subtracted and centrifuged at 4000 rpm for 15 min to remove the solid residue that could interfere during UV-vis measurements. Following this process, the samples for the photocatalytic study were collected each 0.5-1 h for a period of 8 h under the same operating conditions. The concentration of MB in the system liquid was measured using a UV-Vis spectrophotometer monitoring the absorption band at 664 nm (maximum absorption in the visible light region) at ambient conditions [12]. The photodegradation efficiency of MB is given by equation (3) Degradation here: C 0 = Initial dye concentration. C 1 = Dye concentration measured after a certain time. Figure 2 shows the experimental x-ray patterns (dotted-black lines) of the strontium hexaferrite for each one of the fabrication methods. The maximum reflections (blue marks) are indicated at the bottom of each pattern, and the Rietveld refinement adjustment (continuous-red lines) also was indicated in figure 2. Table 2 shows the structural parameters obtained from the Rietveld refinement using the MAUD program [23]. The quality of the Rietveld fit was evaluated from the Chi-squared value χ 2 = (R wp /R exp ) 2 . Chi-squared values between 1 and 2 and the well-adjust observed between the experimental and the calculated patterns indicate a good Rietveld refinement [24]. According to the results, no second phases are present in either of the synthesized samples. The crystallite size also had significant differences regarding the fabrication route. The crystallite sizes of the SrM-SSP sample will be in the medium particle size range due to its nanometric nature. Also, it is observed a higher crystallinity in the SrM-SSR sample due to the high sintering temperature used for its obtainment. Figure 3(a) shows the micrograph obtained with a transmission electron microscope for the strontium hexaferrite nanoparticles obtained with the Pechini method. In this case, the particles show rounded bords with an average particle size of 94.4 nm. Although the nanoparticles reach an equilibrium by forming ∼330 nm agglomerates. Figure 3(b) shows the micrograph obtained by scanning electron microscopy of the strontium hexaferrite prepared by the solid-state reaction method, obtaining an average particle size of 1.18 μm. These particles show a flattened morphology with straight edges. The larger particle size obtained by the ceramic method in contrast to that obtained by the Pechini method is mainly due to the initial size of the precursors and the higher sintering temperature of 1250°C necessary to reach the M-type structure.

Magnetic properties
The magnetic properties of the strontium hexaferrites obtained from each one of the fabrication methods (SSR and SGP) were obtained from their magnetization curves. The hysteresis loops in figure 4 were obtained using a maximum applied field of 11 kOe. The SrM-SSR sample shows the highest magnetization saturation (M s ), while the SrM-SGP sample shows a significant increase in the coercive force (H c ). This behavior is attributable to the fabrication method. The higher sintering temperature required to obtain the strontium hexaferrite by the SSR method increases its crystallinity and enhances its magnetic saturation due to the high magnetocrystalline anisotropy featured in these types of compounds. On the other hand, the increase in the coercive field showed by the SrM-SGP sample is due to the used chemical reagents requiring a lower sintering temperature giving small particles size. Besides, the magnetic properties could influence the dispersion and stability of the strontium hexaferrite particles in the studied solution. This is because aggregation or the formation of aggregates due to magnetic interactions can reduce the effective surface area and thus decrease the photocatalytic efficiency.

Evaluation of optical properties
To determine the behavior of the catalytic activity of strontium hexaferrite, the band gap (E g ) in the visible region must be calculated. The photocatalysts were evaluated based on their characteristic spectra measured by absorbance. In figure 5, the absorption spectra of the SrM-SGP and SrM-SSR samples are presented as a function of the wavelength. The sample SrM-SGP showed a maximum absorption band at 274 nm, while in the sample SrM-SSR this band shifts to 279 nm.  On the other hand, the band gap energy values of the samples were determined from the data obtained by the UV-vis technique using the indirect transition model by Tauc-plot, equation (4) [25][26][27].
Where, α is the absorbance, hu is the photon energy (eV), A is a constant, and E g the indirect band gap (eV). However, the equation for estimation of h E u = is given by equation (5) h where E is photon energy (eV), h is Planck's constant = 4.136 × 10 −15 (eV·s), c is the light velocity = 2.998 × 10 17 (nm s −1 ), and λ is the wavelength (nm). The band gap energy (E g ) would be the intersection to (E) 1/2 = 0 [28]. Figure 6 shows the Tauc calculation to obtain the following band gap energy values recorded for the samples SrM-SGP (2.5 eV) and SrM-SSR (2.1 eV), respectively, which are the minimum energies required to activate the samples [7,21,27,29,30]. The unique confinement of the samples is responsible for the variation in band gap energy. This depends on the synthesis conditions of the method used to fabricate the hexaferrite samples. According to the values we obtained for the forbidden energy gap, the catalysts can be photo-catalytically active in the visible region.
Photocatalysis consists of the interaction of electromagnetic radiation with the material, in particular: visible light -strontium hexaferrite. Electromagnetic radiation provides sufficient energy to promote a photon to move an electron from its valence band (VB) to the conduction band (CB), creating an electron-hole pair. The difference between these energy levels is known as the previously calculated band gap (E g ), and the energy of the photon must be equal to or greater than the value of E g . The electron-hole pair is responsible for the oxidation or reduction of the chemical species on the hexaferrite surface [31].

Photocatalysis study
The photocatalytic activity of SrM was studied by the degradation of the MB dye during an adsorption time of 8 h, exposed to light in the visible range. In this case, it is estimated that between 10%-20% of the photocatalytic efficiency results from the direct interaction between MB and light. Figure 7 shows the degradation profiles of MB due to the SrM-SGP and the SrM-SSR samples. The gradual decrease in the absorbance band at 664 nm indicates the MB photodegradation as the reaction time increases. Also, the bands observed at 664 and 292 nm did not suffer any shift, and no other bands emerged during the process, indicating that only the photochemical reaction was generated [32]. The results of the degradation tests performed with high catalyst concentrations of 200 mg l −1 and 300 mg l −1 showed lower catalytic activity due to high particle conglomeration. This high concentration of SrM particles does not allow the light to cross into the system and reduces the photocatalytic efficiency. The evolution of the degradation in the experiments with the highest amount of photocatalyst was relatively low, which is due to the low generation of •OHin the reaction. On the other hand, the photocatalytic activity performed with lower particle concentrations showed an increment for the same experimental setup. Therefore, particles of strontium hexaferrite at a concentration of 100 mg l −1 had good dispersion in the photocatalytic system, which enhanced the photocatalytic activity. Also, table 3 shows that the SrM-SSR photocatalyst has the highest degradation efficiency due to its smaller band gap than the one exhibited by the SrM-SGP sample. Figure 8 shows the degradation kinetics studied by six experiments depending on the concentration of the pollutant and the concentration of the photocatalyst. The concentration of MB dye varied from 3 to 5 mg l −1 to study the effect on the degradation rate. The sample with the highest degradation of 82% was reached by the SrM-SSR sample. On the other hand, with increasing the dye concentration a decrease in degradation efficiency was observed. This behavior is attributed to the surface saturation of the photocatalyst, and consequently, there  is less incidence of photons on the surface of the particles. That condition avoids the hydroxyl radical's formation and the degradation rate decreases. The degradation kinetics were studied based on the concentration of the hexaferrite and the MB concentration. The degradation rates were calculated with equation (6) v k SrFe O MB 6 x y 12 19 [ ][ ] () = where x and y correspond to each species of study.
The degradation rate had zero-order kinetics that is, the rate does not depend on the concentration of the reactants. The MB degradation occurs through the surface of the photocatalyst, so the particle's surface size is a critical parameter controlling the degradation rate. The rate constant k for a reaction zero-order was determined in 0.001 mol l −1 ·min using equation (7) v k MB 7 [ ] ( ) = Figure 9 shows how an increase in catalyst concentration causes a decrease in degradation rate. As the amount of catalyst exceeds a critical value, it was found that the degradation activity decreases at higher concentrations due to the obstruction of photon penetration by the opaque of the suspension, thus reducing the number of photons reaching the active sites of the catalyst [33]. The experimental data were adjusted using linear behaviors, and the slopes of these curves indicate the degradation rate of the MB. Figure 9 also shows the  behavior of the MB degradation rate as a function of the amount of strontium hexaferrite particles obtained from the SSR and the SGP methods. The hexaferrite obtained by the SSR method showed the highest ability to photodegrade the MB dye. These results also showed that a hexaferrite concentration of 100 mg l −1 was most effective than using higher amounts, and all the samples fabricated by the ceramic method showed a higher degradation rate. Although this behavior can be affected by factors such as particle size, morphology, and particle agglomeration, the higher hexaferrite crystallinity and density also appear to play an important role in accelerating the photodegradation rate. Although the nanometric size of the nanoparticles can be useful to improve the photocatalytic activity, the particle agglomeration and darkness caused by the nanoparticle's dispersion are serious problems to be considered for its application.
The MB degradation mechanism is represented by equations (8) to (15). The electrons of the valence band are excited and have so much energy that they are hurled into the conduction band. This jump produces photoinduced holes in the valence band that begin to interact and then react with water molecules on the surface to produce highly reactive radicals such as hydroxyl (OH•). In this sense, the photogenerated electrons in the conduction band react with oxygen molecules to generate the radical O 2 Figure 10 shows how a magnetic field can separate the hexaferrite powders from the water after the photocatalytic reaction is complete. In this case, the photocatalyst is easily attracted by the ferrite magnet, and the powder can be recovered for later reuse. This property demonstrates the great ability of these materials in the field of water remediation. The highest degradation efficiency was obtained by the hexaferrite powder fabricated with the ceramic method due to its morphological characteristics and homogeneous overall distribution allowing high contact surface with the dye. On the other hand, the nanoparticles obtained from the Pechini method tend to agglomerate, decreasing their photocatalytic activity. In general, the use of these materials at the micron scale is very attractive and novel because they are easily extracted and reused from the aquatic environment, besides they show high photocatalytic efficiency.

Conclusion
This work showed the influence of the fabrication route on the photocatalytic efficiency of strontium hexaferrite. Here, the method for obtaining the hexaferrite determines the structural and morphological features that affect its band gap and behavior in the photocatalytic system. Two fabrication methods (Solid-State Reaction and Sol-Gel Pechini) were used to fabricate pure strontium hexaferrite. The hexaferrite particles obtained by the SSR method have micron-scale sizes but higher crystallinity and density. On the contrary, the particles obtained by the SGP method are submicron, with low crystallinity and density. These factors play a key role in determining the photocatalytic efficiency of the strontium hexaferrite. Specifically, the strontium hexaferrite obtained by the solid-state reaction method showed a lower bandgap energy, although its particle size was higher. However, both parameters are beneficial for improving the photocatalytic efficiency, as a smaller particle size darkens the photocatalytic system, and reduces the efficiency of the photocatalytic reaction. These results help determine the best photocatalyst both in terms of band gap energy and how it performs in the photocatalytic system.