Enhanced Toxic Dye Degradation Using Single Crystal Zn-doped Fe3O4 Octahedrons

In the present work, single crystalline octahedral Zn-doped Fe3O4 was prepared through a simple hydrothermal method and then analyzed. The possible mechanism for the formation of Zn-doped Fe3O4 octahedrons is discussed. Their photo-Fenton activity in photodegradation of Rhodamine B under visible light was sequentially investigated. The physicochemical properties of prepared ferrites were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The analysis of the results showed that synthesized products are uniform Zn-doped Fe3O4 octahedrons with high crystallinity. The experimental results manifested that Ostwald ripening processes and N2H4∙H2O amount plays an important role in the formation of the octahedrons. Also, octahedral Zn-doped Fe3O4 exhibited high photocatalytic capacity for dye degradation. • Zn-doped Fe3O4 octahedral-shaped were prepared by solvothermal method. • Ostwald ripening and N2H4.H2O play the important role in the formation of octahedral photocatalyst. • The catalysts exhibit high photo-Fenton activity. • Rhodamine degradation showed no significant change during four successive cycles.


Introduction
Dyes are widely used in several industrial activities such as the textile industry, plastic production, printing, leather manufacturing, and so on (Bari et al. 2022;Hu et al. 2019). The presence of dye pollutants in the aquatic environment has become a great environmental concern. Such dyes exhibit good resistance to UV or solar light irradiation and microbial attack, and sometimes may produce more hazardous intermediates during the degradation process. Using semiconductor-based photo-Fenton catalysis for degradation of these toxic organic compounds has received great attention in recent years. Magnetically high photocatalysts have been considered as promising materials for the degradation of hazardous organic pollutants of many categories because of their high performance, magnetic recyclability and reusability, and the low energy consumption (Alani et al. 2022).
Spinel oxide nanoparticles have attracted much more attention due to their unique electrical conductivity, interesting magnetic properties and high photocatalytic efficiency (Abdel Maksoud et al. 2018). Among spinel oxide, magnetite with an inverse spinel structure has been widely applied as effective candidate for a variety of fields, such as catalytic, photo-magnetic, biomedical, and sensors and highfrequency device applications (Aydin et al. 2019). However, in many applications, the magnetization of Fe 3 O 4 is significantly declined to reduced size from bulk to nano, which is caused by the surface effects and magnetic order disorganization (Beketova et al. 2020). Thus, doping Fe 3 O 4 nano-particles with divalent cation is regarded as a promising approach to solve the drawbacks of Fe 3 O 4 nanoparticles, such as a low relatively magneticity, poor dispersibility, low adsorption ability and limited catalytic capacity (Anandhi et al. 2020;Khalid et al. 2022). Introducing metal ions Zn 2+ into Fe 3 O 4 can cause more effective doping with Fe 3 O 4 due to the same ionic radii of Zn and Fe.
Among various synthesis methods for spinel ferrites, hydrothermal or solvothermal synthesis has been considered as one of the most effective and economic approaches for its relatively low cost, short process time, homogeneity, reproducibility, energy efficiency, environmental friendliness, and simplicity (Yang et al. 2020). Up to now, Zn-doped Fe 3 O 4 with different morphologies, i.e., sphere, hollow, wires, plate, nanocubic, have been studied by many researchers. For example, Zn-doped Fe 3 O 4 with cluster-shaped or hollow spheres were prepared through a simple one-pot hydrothermal method and showed significant photo-Fenton activity (Cen and Nan 2018). Zinc-doped Fe 3 O 4 nanoclusters have been prepared via a hydrothermal method and used for detecting pathogenic bacteria in milk (Anjana et al. 2018).
Many reports have demonstrated that the photocatalytic activity of photocatalysts is enhanced due to their exposed crystal facets, crystallinity, morphology, size particles (Cheng et al. 2022). Unfortunately, crystal Zn-doped Fe 3 O 4 of octahedral shape has been rarely reported for dye photo-Fenton degradation. Moreover, most of these studies are focused on the synthesis of Zn-doped Fe 3 O 4 particles with different shapes but are limited to expose the morphology revolution and the possible formation mechanism.
In this paper, the single crystal Zn-doped Fe 3 O 4 octahedral-shaped were synthesized through a simple hydrothermal route. The role of the Oswald ripening process  and N 2 H 4 .H 2 O amount in the formation processes of Zn-doped Fe 3 O 4 octahedrons were investigated. Furthermore, the employment of the prepared octahedrons for the photo-Fenton rhodamine B (RhB) degradation under visible light is also addressed. The study results reveal Zn-doped Fe 3 O 4 octahedral-shaped exhibiting high photocatalytic activity and excellent stability. The development of a facile and economic method for the synthesis of single Zn-doped Fe 3 O 4 octahedral-shaped would greatly promote their applications in environmental processes and other fields.

Materials
All chemicals (purchased from Sinopharm Chemical Reagent Co., Ltd) were of analytical reagent grade and further used without additional purification or treatment. Deionized water was used as the solvent throughout the experiment.

Octahedral Zn-doped Fe 3 O 4 Synthesis
In a typical procedure, 3 mmol of ZnCl 2 .4H 2 O and 5 mmol of FeSO 4 .7H 2 O were dissolved in 60 mL of ethylene glycol (EG). Successfully, a 5 mmol of N 2 H 4 .H 2 O was then introduced to solution under continuous stirring for 60 min. Then, the mixture solution was transferred into an 80 mL Teflon-lined stainless steel autoclave and treated at 180 °C for 24 h. After naturally cooled at room temperature, the resulting product was collected by a magnet, rinsed with ethanol and deionized water several times and dried at 80 °C for 12 h. Undoped Fe 3 O 4 was prepared similarly to the preparation of Zn-doped Fe 3 O 4 except for addition of zinc chloride.

Characterization
The X-ray diffraction (XRD) method was conducted on a D/MAX 2250 V diffractometer (Rigaku, Japan), by monochromatized Cu Kα (λ = 0.15418 nm) radiation under 40 kV and 100 mA and scanning over the range of 15°-80°. The morphologies of prepared samples were characterized by a field-emission scanning electron microscope (FESEM, JEOL, JSM-6700F). The microstructure and crystalline orientation were identified using a transmission electron microscope, and high-resolution transmission electron microscopy (HRTEM) with a JEOL JEM-2010 HR electron microscope. UV-vis diffuse reflectance spectra of the samples were obtained on a UV-vis spectrophotometer (Hitachi U-3010). The PL spectra of the product were measured by a transient fluorescence spectrometer (Shimadzu RF-5301PC).

Photo-Fenton Degradation Tests
To evaluate the photocatalytic activity of the sample, the reaction was set up by the photocatalytic decolorization of RhB under visible light. The light irradiation source consists of a 500 W Xe lamp with a 420 nm cutoff filter. A mount of 0.05 g Zn-doped Fe 3 O 4 was introduced into 50 mL RhB solution (10 -5 mol/L), pH = 5, under stirring for ensuring the establishment of adsorption-desorption equilibrium between the photocatalyst and RhB. Then, the solution was irradiated with continuous stirring and 3 mL of suspension was collected after every 10 min. The RhB degradation effect was measured by checking the absorbance at 553 nm during the photodegradation process with a Hitachi U-3010 UV-vis spectrophotometer.

Characterization of Zn-doped Fe 3 O 4 Octahedrons
The XRD patterns of the pure Fe 3 O 4 and octahedral Zn-doped Fe 3 O 4 samples are exhibited in Fig. 1. As shown in Fig. 1a, both the diffraction peaks of Zn-doped Fe 3 O 4 and pure Fe 3 O 4 are almost the same. Both patterns show the reflection peaks ascribed to the crystal planes (220, 311, 400, 422, 511 and 440), which matches well with the standard patterns of cubic spinel structure (JCPDS 77-1545). Observing the patterns, no impurity diffraction peaks of ZnO appeared, suggesting that crystalline ZnO was not formed during the synthesis process. However, it can be seen that the 311 crystal plane of Zn-doped Fe 3 O 4 slightly shifted to a smaller angle and the corresponding lattice constant changed from 0.8386 nm to 0.8402 nm (as depicted in Fig. 1b). This change could be ascribed to the substitution of a small amount of Fe 2+ (ion radius = 0.61 nm) and Fe 3+ (ion radius = 0.49 nm) in the magnetite. The phenomena could relate to Zn 2+ with a larger ion radius of 0.74 nm. The results indicate that Zn has been successfully doped into the crystal structure of magnetite. Herein Fe 3+ ion and a small amount Fe 2+ ion occupied in tetrahedral and octahedral sites could be substituted by Zn 2+ ion (Dhiman et al. 2022;Saha et al. 2019).
Raman spectra analysis was conducted to further investigate the phase purity and degree of crystallization for the prepared Zn-doped Fe 3 O 4 . Raman spectroscopy was conducted at excited 633 nm with 20% and 40% power, as shown in Fig. 2. The Raman spectra of the samples excited at 633 nm with 20% power are depicted at Fig. 2a. The band appeared at 670 cm −1 related to the characteristic peak of magnetite, which agrees with a symmetric Fe-O breathing A 1g mode. It is clearly seen that no characteristic signal of ZnO is observed, which corresponds with the results of XRD. Figure 2b presents the Raman spectra of the synthesized product excited at 633 nm with 40% power. It is worth mentioning that the significant phase changes appeared at the surface of the samples when the samples were excited at 633 nm with 40% power. Both pure Fe 3 O 4 and Zn-doped Fe 3 O 4 exhibit A1 g + E g + 2F 2g four Raman active modes. Similar results have been reported in other studies ). The modes above 600 cm −1 with A1 symmetry belong to the vibrational motion of oxygen at tetrahedral sites. The peaks below 600 cm −1 are ascribed to the vibrational bonds at octahedral sites. The peaks at 220 cm −1 and 486 cm −1 are attributed to the A 1g modes. The peaks observed at 286, 397, 604, 656, 722 cm −1 are in accordance with the E g modes. There is no band ZnO in the Raman spectra that usually appear at about 436 cm −1 . However, a shifting of all peaks is observed with the increase in Zn ion substitution indicating the Zn-doped into Fe 3 O 4 successfully (Dhiman et al. 2021).

Morphology and Crystalline Structure of Obtained Mesocrystal
The morphology of the prepared Zn-doped Fe 3 O 4 was observed by SEM, TEM, selectedarea electron diffraction (SAED) technique, as shown in Fig. 3. From Fig. 3a, it is clearly seen that the product consists of the uniform octahedrons with their sizes in the range 300-550 nm. The TEM image of several individual Zn-doped Fe 3 O 4 (Fig. 3b) confirms its octahedral structure. The small insert in Fig. 3b shows a normal structural model of an octahedral Zn-doped Fe 3 O 4 composed of eight [111] facets. Figure  The FTIR is the normal method to ascertain the chemical composition and structure of spinel Zn-ferrites. The FTIR spectrum of pure Fe 3 O 4 and Zn-doped Fe 3 O 4 was depicted in Fig. 4. The spectra data exhibited the distribution of cations in the crystal structure through their vibration modes. It is said that the metal cations are usually exposed at two different sub-lattices as the tetrahedral sites and octahedral sites in magnetite. The peak around 600 cm −1 is ascribed to the stretching vibration mode of the To investigate the valence states and chemical formation of Zn-doped Fe 3 O 4 octahedrons, the prepared product was checked using X-ray photoelectron spectroscopy (XPS) measurement. As shown in Fig. 5a, the survey spectrum of Zn-doped Fe 3 O 4 sample could be fitted with characteristic signals of elements as Zn, Fe, O, and adventitious C. The appearance of the peak at 284.8 eV in the spectrum can be attributed to carbon contamination and CO 2 penetrating in the surface of the sample after synthesis. The peak observed at 530.5 eV is O 1s, which can be ascribed to oxygen in metal oxides. In a spinel ferrite, the core level binding energies of Fe 2p electrons will different in the two Fe cations at octahedral sites and tetrahedral sites. In the prepared sample, the valence states of the Fe cations are mixed (Fe 2+ and Fe 3+ ), thus different from a normal spinel ferrite. It can cause the emitted photoelectrons from the 2p states with different energies. As shown in Fig. 5b, Fe 2p spectra for prepared Zn-doped Fe 3 O 4 are resolved into several subpeaks corresponding to 2p 3/2 and 2p 1/2 and their satellite peaks. All peaks are positioned at 709.11, 710.32, 712.77, 718.15, 725.01, 731.24 and 734.63 eV, respectively. In general, Fe 2p 3/2 peaks occur between 710 and 715 eV and Fe 2p 1/2 peak positions are at 724 and 731 eV, respectively. The peaks at 709.11 eV and 718.15 eV are satellite peaks associated with Fe 2p 3/2 and Fe 2p 1/2 , respectively. Similar results have been reported in the literature (Di et al. 2019). In the XPS spectrum of Zn 2p shown in Figure 5c, the presence of peaks at 1022 eV and 1045 eV are characteristic of to Zn 2+ 2p 3/2 and Zn 2+ 2p 1/2 . The O 1s XPS spectrum of the synthesized sample is shown in Fig. 5d. The binding energy peaks at 529.5 eV and 531.8 eV are attributed to the lattice oxygen binding with Fe and Zn (Fe-O and Zn-O), respectively (Kuang et al. 2019). In addition, no observed shoulder in the photoelectron spectrum suggests that zinc is The Brunauer-Emmett-Teller specific surface area, pore size, and pore size distribution of the materials are very important factors for the photocatalytic reaction. The BET surface of Zn-doped Fe 3 O 4 was checked by N 2 adsorption-desorption measurement. The specific surface, pore size and pore size distribution were obtained as shown in Fig. 6 (insert). It can be seen that the isotherm of the sample belongs to a type IV curve. The specific surface area of samples was 28.62 m 2 /g, which is higher than that of Zn-ferrites nanoparticles 2.66 m 2 /g and 16.2 m 2 /g reported by Dhiman et al. (2021) and Jadhav et al. (2020).
The typical UV-vis diffuse absorption spectra of the Zn-doped Fe 3 O 4 octahedrons are illustrated in Fig. 7. The result reveals that the synthesized Zn-doped Fe 3 O 4 exhibits wide absorption in the visible region. Zn-doped Fe 3 O 4 octahedrons display the absorption intensity in the wavelength range from 300 nm to 700 nm. Note that the good visible light absorption ability indicates that the sample is a potential catalyst for the photo-Fenton reaction and can be useful in wastewater treatment with visible light absorption. The bandgap energies of samples were calculated on the basis of the Kubelka-Munk function (Zhao et al. 2020). According to the equation αhʋ=A(hʋ-E g ) n/2 , where υ, h, α, E g and A are the photo frequency; the Planck's constant, the absorption coefficient, the band gap and the constant, respectively, the estimated bandgap for Zn-doped Fe 3 O 4 sample is 1.55 eV (see insert in Fig. 7). The result shows that the bandgap of the prepared Zn-doped Fe 3 O 4 is narrower than that of Fe 3 O 4 (2.25 eV) and Zn-doped Fe 3 O 4 nanoparticles (2.4 eV) (Anjana et al.

The Possible Formation Mechanism
To reveal the formation mechanism of octahedral Zn-doped Fe 3 O 4 , time-dependent experiments were conducted at 4, 8, 12, 16, 20 and 24 h. The time-dependent morphological evolution processes have been checked by SEM. Figures 8a-d depict the SEM images of products obtained at the fixed reaction time. It can be seen that when the reaction time was 4 h, bulk particles could be obtained. When the time reaction is longer than 8 h, the micro quasi-spherical morphology products can be observed (Fig. 8b), which is ascribed to the Ostwald ripening processes. As well known, the Ostwald ripening process is the growth of larger particles at the expense of smaller particles through a recrystallization process due to the energy difference among them. These processes have been widely used to explain successfully fabrication formation of many sphere nanomaterial systems ). Keep in mind and carefully observation of SEM images (Fig. 8c) reveals that the micro quasi-spheres changed into like-polyhedrons at 12 h reaction time. When the reaction time rises to 16 h, the polyhedrons can be clearly achieved as shown at Fig. 8d. This alteration could be ascribed to the crystallographic surfaces that enclose the particles. In the hydrothermal synthesis, ethylene glycol molecules play an important role in gradually reducing the difference among the surface energies, leading to the selective adsorption on the [111] facets and increase the growth of the [100] facets. As a result, the Zn-doped Fe 3 O 4 octahedrons are formed because the growth rate along the [100] direction is higher than that along the [111] direction (Yang et al. 2022). The morphology of the obtained products changed significantly with the presence of the octahedron rolls, when the reaction time was up to 20 h (as shown at Fig. 8e. Finally, the uniform Zn-doped Fe 3 O 4 octahedrons were prepared at the reaction time of 24 h (Fig. 8f). The arrangement of the energy surface process and the crystallographic growth process at high temperature may be a possible reason for this significant morphology evolution .
It is noted that some amount of N 2 H 4 .H 2 O is also critical to the formation of Zn-doped Fe 3 O 4 octahedrons. To uncover the role of hydrazine hydrate in the synthesis process, a series of parallel experiments were conducted by altering the amount of hydrazine hydrate. In our experiments, without N 2 H 4 .H 2 O being added into the reaction system, particles were not obtained. Like-sphere products prepared, when a small quantity (2 mmol) of N 2 H 4 . H 2 O was added in the system. Octahedron products in 300-500 nm size were formed when 5 mmol of N 2 H 4 .H 2 O were added. When increasing the amount of N 2 H 4 .H 2 O (10 mmol), the size of the prepared octahedrons were 400-700 nm. Figure 9 presents the SEM images of the synthesized samples derived at 0, 2, 5, 10 mmol of hydrazine hydrate solution (N 2 H 4 .H 2 O), respectively. Herein, N 2 H 4 .H 2 O could serve as a very necessary factor added for electrostatic stabilization to prevent agglomeration of particles, thus affecting the Then, the generated gaseous bubbles with high surface energy due to their small size diameter could serve as the heterogeneous nucleation center for aggregating newly formed nanoparticles around the gas-liquid interface. On the other hand, in solvothermal systems, these produced gaseous bubbles could provide a soft template to form quasisphere products. Without N 2 H 4 introduction, the decomposition reaction of NH 4 OH into gaseous bubbles might not occur completely. Without a heterogeneous nucleation center also as a soft template, particle products are dominant rather than like-sphere structure ( Fig. 9a-b). When more N 2 H 4 .H 2 O was added into the synthesis at 5 and 10 mmol, significant morphology change occurred leading to form single-octahedral Zn-doped Fe 3 O 4 (Fig. 9c-d) [111] facets with the lowest growth rate will be dominating leading to an octahedron shape product (Chakhoum et al. 2018). As a result, the simultaneous occupation of the processes as the Ostwald ripening, the crystallographic surfaces and the growth rate of the basal surfaces could play an important role for evolution morphology of the prepared products. The results suggest that the different sizes of the Zn-doped Fe 3 O 4 octahedrons could be synthesized controllably through modifying the amount of hydrazine hydrate reactant.

Photo-Fenton Activities
The photocatalytic reaction of the samples was evaluated through the degradation of rhodamine B aqueous solution in the presence of H 2 O 2 under visible irradiation. To study the photocatalytic activity of the prepared Zn-doped Fe 3 O 4 octahedrons, the results were compared with the efficiency of Zn-doped Fe 3 O 4 nanoparticles, where Zn-doped Fe 3 O 4 was fabricated as in Liu et al. (2016). The results of photocatalytic activities of the samples prepared at different conditions are shown in Fig. 10  can be associated with their crystalline structure and octahedral morphology. The adsorption of RhB on the prepared octahedrons in the dark was also examined. The RhB adsorption ability was negligible, indicating that the degradation of RhB was due to photodegradation only with no adsorption. Figure 10c displays the absorption spectra of the RhB solution which show a characteristic peak at 554 nm. From Fig. 10c, it is seen that the intensity of the absorption peak at 554 nm decreased gradually following the time reaction. At a reaction time of 60 min, the absorption peak intensity is minimal, demonstrating that the RhB was removed. The mineralization of RhB was also investigated as depicted in Fig. 10b. The results reveal that the TOC removal of RhB using Zn-doped Fe 3 O 4 octahedrons as catalysts gained about 72%. The results suggest that Zn-doped Fe 3 O 4 synthesized exposes high capacity for the mineralization of contaminants. The efficiency of Zn-doped Fe 3 O 4 photocatalysts for dye degradation is compared to similar materials reported by other groups in Table 1. Effect of catalyst amount and pH (in the range of 2 to 8) on RhB degradation efficiency was also studied (Fig. 11). The results show that the degradation rate of RhB was enhanced with an increase in catalyst amount as depicted in Fig. 11a. However, at higher catalyst dosage, the percent dye removal was slightly decreased. Based on the experiment, 1.5 mg/L of Zn-doped Fe 3 O 4 octahedrons is consistent for the RhB photo-Fenton removal. The experimental results on the effect of pH reveal that the optimum pH was 5.5 (Fig. 11b). With pH below 5.5, at high H + concentration, the formation of stable oxonium ion H 3 O 2 + makes hydrogen peroxide more stable and then decreases its activity with ferrous ions. Moreover, the formation of Fe(II) complexes and the precipitation of ferric oxyhydroxides at pH values above 5.5 are probably reasons for decreased efficiency in the photo-Fenton RhB removal processes (Domingues et al. 2019).
To investigate the possible mechanism of the photocatalytic process, a series of quenchers were added to the system for catching the major active species. In these experiments, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), isopropyl alcohol (IPA) and benzoquinone (BQ) were added to the reactant solutions as the active scavengers of H + , • OH and • O 2 − , respectively (Cui et al. 2019;Han et al. 2019). The effectiveness of the RhB photo-degradation process by Zn-doped Fe 3 O 4 octahedrons with different scavengers is depicted in Fig. 12, where it can be clearly seen that the reaction rate was decreased using EDTA-2Na, IPA, BQ as trapping agents. The photocatalytic degradation of RhB efficiency was changed slightly using the addition of IPA, BQ scavenger, while the RhB photo-degradation rate dropped insignificantly with added EDTA-2Na scavenger. The results indicate that • OH and • O 2were free radical species in the degradation reaction in the photo-Fenton degradation of RhB by Zn-doped Fe 3 O 4 octahedrons. The results are similar to others reported (Haounati et al. 2021).
It is worthy noting that the metal-doped can play the role as electron-trapping centers, and thus, it prevents the combination process of photogenerated electron/hole pairs (Khalid et al. 2022;Song et al. 2018). In addition, many previous reports revealed that the octahedron materials exhibited excellent photocatalytic activity due to its good charge separation.   . With the available free radical species, RhB molecules can be oxidized to CO 2 , H 2 O and mineralization products (Liu et al. 2015). In addition, the active hydroxyl radicals can be generated by reaction between Fe 2+ and H 2 O 2 according to the following reactions (Liang et al. 2019): (1) Catalysts + hυ → h + + e − Moreover, the enhancement of the photo-Fenton reaction of Zn-doped Fe 3 O 4 could be ascribed to the Zn 2+ substitutes in tetrahedral sites and octahedral sites. It is worth noting that the crystalline structure and octahedral-shaped morphology of the fabricated photocatalyst can accelerate the electron transfer process, leading to the recombination of holes and electrons diminished. As a result, more active free radicals could be generated for the degradation of RhB molecules.
The reuse and the recycling availability of the photocatalyst play an important role in practical applications. Reusability studies of octahedral Zn-doped Fe 3 O 4 were conducted to survey the stability of the obtained photocatalysts in the RhB photo-Fenton degradation process under visible light irradiation. Magnetite activity is a special property of ferrite. Many research have reported about the excellent magneticity of Zn-ferrite (Alani et al. 2022;Wang et al. 2018). In this experiment, an internal magnet was used to collect the prepared photocatalysts after the photocatalytic processes. The results show that the catalyst was easily separated by an internal magnet and the RhB degradation effectively has no significant change during the four successive cycles, indicating high reliability and sustainability of the photocatalyst (Fig. 10d).
To confirm further the stability of the photocatalyst, XRD spectra of Zn-doped Fe 3 O 4 after 4 runs was characteristic (Fig. 10e). The results reveal that XRD spectra of Zn-doped Fe 3 O 4 before and after four runs changed insignificantly, suggesting the high stability of materials (Hu et al. 2019;Huang et al. 2018). It is worth noting that Zn 2+ ion can diffuse into Fe 3 O 4 lattice and subtiture Fe 3+ ion, which do not lead to any destroyance in the crystal structure, so the Zn-doped Fe 3 O 4 products remain stable (Ismael 2021). With interesting properties such as high photocatalytic activity, stability and easy separation, the Zn-doped Fe 3 O 4 octahedrons can be promising candidates for photo-Fenton application in dye degradation in textile industry.

Conclusions
Uniform octahedral Zn-doped Fe 3 O 4 was successfully synthesized by simple solvothermal route in ethylene glycol solvent with absence of any surfactants. The single crystalline Zn-doped Fe 3 O 4 octahedrons can be synthesized easily by adjusting the amount of hydrazine hydrate solution. The experimental results suggest that the simultaneous occupation of the processes as the Ostwald ripening, the crystallographic surfaces and the growth rate of the basal surfaces could play an important role in manufacturing the prepared products. Owing to the octahedral structure and high crystallinity, the Zn-doped Fe 3 O 4 octahedrons exhibited high photo-Fenton properties and excellent stability for the degradation of rhodamine B compared to Zn-doped Fe 3 O 4 nanoparticles. Specially, because of the local magnetic properties, the synthesized catalysts can be easily recovered to recycle by using a magnet. The results suggest that the synthesized products can be efficient photocatalysts for the degradation of rhodamine B.