Influence of the Local Structure on the Photocatalytic Properties of Zinc Spinel Ferrite Nanoparticles

The correlation between iron distribution in octahedral or tetrahedral sites in synthetic magnetite and zinc spinel ferrites and their catalytic properties were investigated in this work. The zinc-doped nanomagnetites were prepared by co-precipitation method and annealed at 420 °C. X-ray diffraction, UV-Vis, X-ray absorption and Fe Mössbauer spectroscopies were used to identify iron sites. The obtained results showed that Zn replaces the relatively smaller Fe cation in the tetrahedral sites, increasing the cubic lattice dimension, and this replacement remarkably enhances the catalytic effectiveness of the Zn-spinel ferrite towards the indigo carmine degradation compared to undoped magnetite. The indigo carmine degradation experiments were carried out using photo-Fenton method in which 50 mL of indigo carmine dye aqueous solutions (20 mg L, pH 3.0), 20 mg ZnxFe(3-x)O4, and 2.0 mL H2O2 (aqueous solution at 0.028 mol L) were placed under UV light at 25 °C. Under these experimental conditions the degradation rate of indigo carmine was superior to 98%.


Introduction
Ferrite spinels are magnetic compounds containing iron, for which the general chemical stoichiometry corresponds to , where M is a metal cation of valence k occupying tetrahedral sites; m and n are integer numbers. 1 For magnetite, M = Fe; k = 2; m = 1 and n = 1, and the ideal formula is [Fe III ]{Fe III |Fe II }O 4 , where Fe III and Fe II occupy tetrahedral and octahedral coordination sites, respectively. Ferrites are widely researched materials due to their broad range of technological applications in electronic devices, 2 such as high-speed digital tapes, recording discs, rod antennas, and humidity sensors, [3][4][5][6] gas sensor in environmental monitoring, 7,8 permanent magnets, 9 catalytic reactions, [10][11][12] and as absorbent materials for environmental applications. 13 Their properties are strongly influenced by particle size distributions, particle agglomeration degree and morphologies. 14 Those microstructural parameters can be well controlled during the synthesis. Doping processes of ferrites nanocrystals using different metals, such as zinc, 15,16 nickel, 17 copper, 11 manganese 18 and cobalt 19 are commonly used to improve their electric, catalytic, or magnetic properties. 17,18 Several methods using chemical synthesis of zinc ferrite nanoparticles have been reported such as coprecipitation, 11,17,18,20 hydrothermal, 14 sol-gel, 19 and polymeric precursors. 21 The properties of these materials are strongly influenced by the chemical composition and microstructure which are significantly dependent on the chemical route used for their synthesis. The most common method used to obtain these doped ferrites is co-precipitation which involves the use of metal salts at different molar ratio (Fe:Metal) in a basic environment (pH > 8). 11,17,18 In this work, zinc-doped ferrites were prepared and those compounds are spinel systems which present AB 2 O 4 -type structures, where 8 Zn II and 16 Fe III ions occupy tetrahedral and octahedral sites, respectively, in a crystallographic face-centered cubic unit cell. 16,22 The electric and magnetic properties of these materials are also highly influenced by their cation occupation in tetrahedral and octahedral sites. Due to the relatively small energy difference among Zn II ions in T and M sites, 9,11 cation redistribution is strongly influenced by the annealing temperature and cooling rate used during the synthesis. 22 Zn x Fe (3-x) O 4 is an important member of the spinel ferrite family and an efficient photocatalyst mainly due to its ability to absorb visible light. [23][24][25] The photochemical stability of Zn x Fe (3-x) O 4 remarkably promotes an efficient conversion of H 2 O 2 into highly reactive HO• radical, addressing its particular interest on the oxidative photocatalytic degradation of organic substrates. 11,12 Some reported data on the use of Zn x Fe (3-x) O 4 nanoparticles also showed their hydrophobic and hydrophilic anticancer behavior through local drug delivering systems, [26][27][28][29] another very interesting application of these materials.
In this work the synthesis using coprecipitation method and the characterization of zinc-doped ferrite samples is described as well as photocatalysis tests under UV light. The crystallographic and hyperfine structures, along with the cation distribution of the undoped and Zn-doped samples, were assessed by UV-Vis, X-ray absorption (XAS) and 57 Fe Mössbauer spectroscopies and X-ray diffraction (XRD). XAS is an ideal technique to investigate the local structure around different cations due to its high chemical selectivity and sensitivity. The influence of the chemical structure on the catalytic efficiency of these zinc ferrite nanocrystals towards the degradation of the indigo carmine dye was also reported in this work which undoubtedly allows a better understanding of the main chemical mechanisms involved in dye degradation process.

Synthesis and characterization
Doped ferrites were synthetized by co-precipitation of Zn II and Fe III chlorides in aqueous solutions, at room temperature, through addition of ammonium hydroxide. All chemicals used, FeCl 3 .6H 2 O, ZnCl 2 , NH 4 OH, CH 3 COONH 4 , HCl and K 2 Cr 2 O 7 were purchased from Vetec (New Química, Vitória-ES, Brazil). The obtained solids were washed with ammonium acetate, dried, and decomposed under N 2 atmosphere at 420 °C for 2 h. 30 A sample of pure magnetite was prepared and used as reference. Total iron was then quantified by using the standard dichromatometric method; 31 in this analysis it was possible to quantify Fe II . In this method the samples were dissolved in HCl aqueous solution under CO 2 atmosphere and titrated with K 2 Cr 2 O 7 . Zn content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) on a PerkinElmer Optima 7000DV.
XRD data were acquired on a Rigaku Geigerflex diffractometer equipped with a Co tube (Kα radiation, λ = 0.17092 nm) and a graphite monochromator using a scanning rate of 1° 2θ min −1 and a dwell time of 5 s per step. Room-temperature 57 Fe Mössbauer spectra were recorded on a conventional transmission constant acceleration spectrometer using a 57 Co/Rh source. αFe foil was used to calibrate the Doppler velocity and isomer shift. All spectra were numerically fitted using the least-squares computer program NORMOS-90. 32 Samples were mixed with sugar in order to obtain an averaged Fe density of about 10 mg cm −2 . A 13 mm diameter acrylic disc was used as sample holder.
XAS measurements were performed using the DB04-XAFS1 beamline at the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. All spectra were collected at the Zn K-edge (9659 eV) for ZnFe 2 O 4 and ZnO (standard) in transmission mode using a Si(111) channelcut monochromator, previously calibrated using 7.5 µm Zn foil. X-ray absorption near edge structure (XANES) spectra were recorded from 9600 to 9700 eV at 0.5 eV steps with 5 s counting time. Extended X-ray absorption fine structure (EXAFS) data were collected from 9700 to 10700 eV at 3.0 eV steps and with 5 s counting time. EXAFS spectra were numerically treated using the WinXAS algorithm. 33 XANES data were background-corrected by using a polynomial fit in the pre-edge region and normalized in the 9600-9655 eV energy range for Zn samples. The edge position was determined as the zero of the second derivative of experimental spectra. Atomic absorption data were fitted by a cubic spline function in the k 3 space. EXAFS signals, k 3 ·χ(k), were Fourier-transformed using a Bessel window to obtain the radial distribution function (RDF). As we were interested only in the first coordination shell, which gives information about Zn II sites, we applied a Fourier back-transformation only to the first RDF peak and fitted the obtained χ(k) functions by the least-squares procedure, using experimental phases and amplitudes extracted from a standard compound, ZnO, which contains 4 Zn-O bonds at 1.98 Å.

Photocatalytic tests
The photocatalytic activity of Zn x Fe (3-x)  added to 50 mL of indigo carmine dye aqueous solution (20 mg L −1 , pH 3.0) at room temperature (25 °C). These reaction systems were kept at a distance of 15 cm from a UV lamp (Philips, 80 W). Aliquots were collected at equal time intervals and centrifuged and then their absorbance values were determined as the peak maximum at 610 nm using a Hach DR 5000 UV-Vis spectrophotometer. Dye removal efficiency was then calculated using the concentration values. Photolysis and adsorption tests of indigo carmine in the presence of Zn x Fe (3-x) O 4 were also carried out for comparison with the results of oxidation reactions.

Results and Discussion
The chemical formulae obtained through chemical analysis, Table 1, showed that both samples present some degree of oxidation which was confirmed by the presence of cation vacancies. XRD patterns ( Figure 1) display reflections that are assigned to the cubic phase of ZnFe 2 O 4 (franklinite, JCPDS card number 22-1012). 34 Lattice parameters (Table  1) were calculated through the analysis of (220), (311) and (440) reflections using UnitCell software. 35 ZnFe 2 O 4 spinel cubic phase is reportedly stable at temperatures higher than 360 °C. 35 Mean particle size was estimated from the (311) reflection peak breadth, using the Scherrer's equation. 36,37 The lattice parameter value for this prepared magnetite (Fe 3 O 4 ) (a = 0.83889(5) nm) is well consistent with that of the standard magnetite (a = 0.8396 nm, JCPDS card No. . This result strongly suggests that if ferrous iron oxidation occurred, it did not lead to detectable changes in the crystalline structure of the obtained magnetite. For the zinc doped spinel, Zn x Fe (3-x) O 4 , the cubic lattice dimension (a = 0.84157(5) nm) was found to be higher than that of Fe 3 O 4 . 35,38 This cell expansion of Zn x Fe (3-x) O 4 was expected as the ionic radii of Zn II in tetrahedral and octahedral sites are 60 and 75 pm, respectively, which in both cases are higher than the ionic radii of Fe III (high spin), which is 49 or 65 pm, in tetrahedral and octahedral sites, respectively. 37 Table 2).
The room-temperature 57 Fe Mössbauer spectrum (Figure 2) of the Fe 3 O 4 sample showed two well-defined sextets assignable to iron in both tetrahedral and octahedral sites of the spinel structure. Experimental data were fitted by two Lorentzian-shaped sextets for each sub-spectrum, assignable to each Fe-O coordination sites, Table 3. These Mössbauer parameters indicate the coexistence of two populations of magnetite in the sample. 20 In a stoichiometric magnetite, the atomic ratio, , is equal to 2. To draw the corresponding relative subspectral Mössbauer area (RA) ratio, the    (Figure 3) of Zn x Fe (3-x) O 4 is much more complex compared to the one of stoichiometric frankilinite. The end-member franklinite ZnFe 2 O 4 itself has a normal spinel structure, with all Zn II located in T sites: its Mössbauer spectrum is composed by only one ferric doublet, with isomer shift relative to the αFe and quadrupole splitting values of δ = 0.348 mm s −1 and 2ε Q = 0.348 mm s −1 , respectively. 23 Franklinite is an anti-ferromagnetic oxide, 41 in which Zn occupies one tetrahedral site and Fe III the octahedral B-sites. 42 The antiferromagnetism of this compound at room temperature is due to the predominantly negative super-exchange interaction among Fe III ions at M sites. 23 On the other hand, if a solid solution composed of an intermediate phase between magnetite and franklinite is formed, at least two sextets corresponding to iron in T and M sites appear, despite the putative preference of Zn II for T sites. Regarding the Mössbauer results (Figure 3) of this Zn x Fe (3-x) O 4 sample, two broad and asymmetric sextets were observed, indicating partial replacement of Fe III by Zn II , mainly at tetrahedral sites of the spinel structure. The probability profiles of hyperfine field values for both coordination sites are, through this fitting model, linearly correlated with values of isomer shifts displayed in Figure 3.
For Zn x Fe (3-x) O 4 the probability profiles of the hyperfine fields, Figure 3, showed a maximum at 43.1 and 47.6 T for tetrahedral sites and 39.4; 42.9 and 45.7 T for octahedral sites.
EXAFS experiments were carried out in Zn-doped magnetite sample in order to determine the Zn local   structure. For the raw EXAFS spectrum of the Zn sample (Figure 4a) a good signal-to-noise ratio was achieved. χ(k) functions weighted by k 3 were obtained from the Fourier transform of these spectra (Figure 4b) and adjusted using experimental phase shift and amplitude values extracted from ZnO spectra (standard material). The k 3 ·χ(k) functions were Fourier transformed over the 2.3-9.2 Å range using the Kaiser-Bessel window to obtain the partial RDF functions (Figure 5a) in which the peaks represent shells of atoms surrounding Zn atoms. As depicted in Figure 5a, the RDF of ZnO and Zn x Fe (3-x) O 4 are relatively similar with the first peak located at 1.51 Å. This first peak is associated to Zn-O interactions (first coordination shell). The second peak is associated to Zn-Zn and Zn-Fe interactions. EXAFS parameters of Zn x Fe (3-x) O 4 (Table 4) were obtained from the fitting of the Fourier back-transformed RDF functions of the first coordination shell using experimental phases and amplitudes extracted from the experimental data obtained from standard ZnO, Figure 5b. These parameters are in excellent agreement with those found for Zn x Fe (3-x) O 4 . 34 The coordination number obtained for Zn-O shell, 3.58, showed that Zn ions mainly occupy the T site, in agreement with experimental results of co-precipitation followed by annealing at 420 °C. 16,22 Figure 6 shows the XANES spectra obtained for Zn x Fe (3-x) O 4 sample and ZnO. XANES spectra of compounds containing Zn in tetrahedral sites present more features than those of Zn in tetrahedral sites. 33 The energy of the first edge feature in the XANES spectrum is indicative of Zn II coordination; generally, tetrahedral edges are shifted to lower energies by 2 eV or more compared to octahedral Zn edges. 33 In Zn x Fe (3- Fenton (iv) and H 2 O 2 + UV light (v) experiments resulted in low degradation efficiencies (40%) after 100 min of reaction. Compared with Fenton reaction, photo-Fenton significantly increased the degradation rate, affording a discoloration efficiency of more than 98% after 100 min (Figure 7b). In photo-Fenton reactions, active oxidizing species such as HO • are produced in a very short time interval compared with Fenton reactions, resulting in rapid oxidation of contaminants, similar behavior was previously observed in the literature. 11,12 The mechanisms of photo-Fenton oxidation reactions are well explained in the literature. 43    showed that the energies of the direct band-gaps when z = 0.0, 0.05, 0.1 and 0.15, vary from 2.02, 1.98, 1.92 and 1.90 eV, respectively. Figure 8 shows UV-Vis absorption bands of indigo carmine solutions before and after discoloration as a function of time. In these spectra the absorption bands below 340 nm are associated to the absorption of degradation products, and the bands at 280 nm are related to the absorption of the aromatic rings of the indigo carmine structure. According to Terres et al. 43 and Giri et al. 44 decolorization of indigo carmine is likely to produce intermediates such as 2-amino-5(sodium benzenesulfonate)-benzoic acid, anthranilic acid, benzoic acid, and aniline. These fragments present absorption bands below 340 nm due to the presence of an aromatic benzene ring. The spectra showed in Figure 8, display the absorption bands at 246 nm, suggesting the formation of those afore mentioned reaction intermediates. Throughout the reaction, the intensity of these absorption bands, in the range of 300-400 nm, decreases until completely disappears after 300 min indicating the degradation of the intermediate species. Absorption bands below 230 nm are associated to the residual hydrogen peroxide which absorbs in this spectral range. 43,44

Conclusions
The effect of Zn-doping in Fe 3 O 4 nanoparticles through the chemical coprecipitation route was investigated. Structural information of this Zn-ferrite was assessed by XAS, XRD and 57 Fe Mössbauer spectroscopy. XRD measurements indeed confirmed the formation of a cubic spinel phase, of the Fd3m space group; and it was observed that the cell dimension increase and the average crystal size decrease with Zn-doping. 57 Fe Mössbauer data revealed that Zn II cations replace Fe III ions in tetrahedral sites. EXAFS results confirm that dopant Zn ions mainly occupy tetrahedral sites. Photo-Fenton tests showed that the use of this spinel ferrite powders showed a significant catalytic efficiency, on degrading 98% of aqueous indigo carmine after 100 min reaction. This zinc-doped Zn x Fe (3-x) O 4 ferrite was revealed to be highly efficient as catalyst, and it opens many more perspectives on the development of new related Zn ferrites prepared through different chemical routes with high photocatalytic performance in order to promote environmental remediation of wastewater of natural water bodies.