Introduction

Reduction of the particle size of Fe2O3 and Fe3O4 oxides to nanometric scale can be useful in some technological aspects. Nevertheless, for such specification level, standardizing the experimental conditions of sample preparation is essential [1–5], which e.g., prevents the formation of microstructured oxide particles with different crystallinities exhibiting similar sizes and shapes. These requirements are more distinguishable for some oxide properties such as superparamagnetism, quantum tunneling of magnetization, and large coercivities, which are revealed only when the particles attain sizes under a determined critical value [2, 4, 5]. For Fe3O4, the methods described are various, such as precipitation in the presence of polyvinyl alcohol, chemical reduction of α-Fe2O3, decomposition of oxalates, citrates and hydroxides [3–6]. Production of α-Fe2O3 nanoparticles involves from application of thermal transformations of metallic iron under oxidizing atmospheres, thermal oxidation of Fe3O4, iron salts hydrolysis until the use of surfactants containing large carboxylate alkyl chains [1, 2, 5, 7, 8]. Surfactants act in order to prevent the aggregation of α-Fe2O3 nanoparticles into clusters generated by the anisotropy of the dipolar attraction, which, by changing, causes loss of properties associated to the single domain. In the last years, successful protection of α-Fe2O3 nanoparticles have sometimes been achieved by using anionic alkyl-sulphonate surfactants such as dodecylbenzene sulphonate and 1-undecane sulphonic acid, besides 1-octanol (alcohol) [7]. It has been emphasized that surfactants may stabilize the morphology of Fe3O4, even though it depends on the initial amount of Fe(II) present in the process. However, particle sizes are between 2 and 20 nm in order to control these factors. Despite the advances, the points of view are not unanimous about the exact role of surfactants [1, 7]. Some inconveniences from the use of surfactants have been pointed out due to the difference on the interaction between the latter and the surface and inside of the particles [1]. According to this point of view, anionic surfactants interact less strongly with oxygen bonded to Fe on the surface of particles, modifying less the original bond length. Under these conditions, there aren’t variations between atomic vibrations between the center and the surface of particles, TEM and EXAFS data suggest that cationic surfactants, differently, attach to the oxygen atom and increase significantly the bond length (Fe–O) on the surface, ordering the nanoparticles and suppressing the atomic vibrations on the particle surface. The anharmonicity of the nanoparticles can be associated to this difference in the bond characteristics between center and surface atoms [1, 2, 4].

Here, the reactions in water—alcohol solution mixtures between diphenyl-4-amine barium sulphonate surfactant and iron (II) and iron (III) hydrate sulphates were studied aiming to obtain precursor complexes for the iron oxides. The interaction between anionic ligand and iron ions will be investigated here, as well as the coordination characteristics, thermal stability, binding site, conductivity in solution and type and characteristics of the produced complexes and oxides.

Nevertheless, the formation reaction of the complexes with the anionic surfactant DAS must include procedures that control the surfactant’s oxidizing photodegradation, catalyzed by Fe(III) ions. In this degradation process, the surfactant and Fe(III) ion complex absorbs light inside the visible spectral region. The excited ligand radical L• + in general reaches its most stable oxidation state through an intermolecular process that reduces the Fe(III) to Fe(II). Radiations with λ > 330 nm are enough for the ligand excitation [9–13].

If Fe(III) ion is not complexed to the ligand/surfactant, it reacts with the water molecules. Among the species hydrolyzed in aqueous solutions of Fe(III) (pH = 2.5–4.0) Fe(OH)2+ form dominates and is characterized by high photo activity at λ = 313 and 365 nm. If irradiated, the species Fe(OH)2+ acts as a photodegradation starter through the internal transfer of electrons, which photo reduces the Fe(OH)2+ species, producing the Fe(II) ions and the •OH radical [9, 11, 12].

In general, the oxidation reactions catalyzed by Fe(III) have always been limited to the photo-oxidizing effects of the hydrolyzed species, such as Fe(OH)2+ ion, discarding the role of organic ligand. However, organic ligands influence the chemistry of the iron redox processes [12, 13]. In the present work Fe(III) was complexed with a sulfate anion according to (1):

$$ 3 {\text{Ba}}\left( {\text{DAS}} \right)_{{ 2({\text{aq}})}} + {\text{Fe}}_{ 2} \left( {{\text{SO}}_{ 4} } \right)_{{ 3({\text{aq}})}} \to 3 {\text{BaSO}}_{{ 4({\text{S}})}} + {\text{Fe}}_{ 2} \left( {\text{DAS}} \right)_{{ 4({\text{aq}})}} + 2\left( {\text{DAS}} \right)_{\text{oxid}} $$
(1)

The competition established is limited to the reducing species SO3R− and to the highly coordinating SO4 2−. Only with the reaction progress and the consumption of sulphate ion producing BaSO4, the SO3R− anion starts to reduce Fe(III) to Fe(II) [13]. For comparison, product of reaction 2 will receive the same care.

$$ 3 {\text{Ba}}\left( {\text{DAS}} \right)_{{ 2 ({\text{aq}})}} + 3 {\text{ Fe}}\left( {{\text{SO}}_{ 4} } \right) \, _{{({\text{aq}})}} \to 3 {\text{BaSO}}_{{ 4 ({\text{S}})}} + {\text{Fe}}_{ 3} \left( {\text{DAS}} \right)_{{ 6 ({\text{aq}})}} $$
(2)

Experimental

Synthesis of FexDAS2x complexes

The synthesis of Fe x DAS2x complexes (x = 2 or 3) involves the slow addition of aqueous solutions of the hydrated sulphates of Fe(II) and Fe(III) to water/ethanol 7:1 solutions containing Ba-DAS at 60 °C under inert atmosphere, the exclusion of light and with constant agitation until total precipitation of BaSO4. The final precipitate was filtered and washed several times with purified hexane.

Obtainment of iron oxides from the FexDAS2x complexes

The FexDAS2x complexes are thermal treated in a muffle type furnace up to 1000 °C, at a heating rate of 10 °C min−1 under air.

Characterization of the FexDAS2x complexes and the iron oxides

Carbon, hydrogen and nitrogen contents of the complexes were determined using PerkinElmer 2400(CHN) equipment. Spectra of solutions at 10−4 mol L−1 concentratios of the iron complexes were obtained in the UV-VIS range using a Varian spectrophotometer model Cary 1E. IR spectra were recorded in a FTIR Bomem MB 100 in the 350–4000 cm−1 operation range, in KBr pellets.

TG/DTG curves of Fe complexes were obtained in a Shimadzu TGA-50H equipment under dynamic O2 and N2 atmospheres (60 mL min−1) and at a heating rate of 10ºC min−1 from room temperature up to 1000 °C. X-ray diffractograms (XRD) of the iron oxides were obtained in a HZG diffractometer with quartz monochromator for Kα of Co (λ = 1.79020 Å) radiation line.

The 57Fe Mössbauer spectra of the FexDAS2x complexes were obtained in an energy modulation system through Doppler Effect of the 57Fe δ radiation (57Co isotope source). The δ energy modulation system comprises a Mössbauer velocity transducer, a driving system and a function generator.

Results and discussion

Synthesis and characterisation of FexDAS2x complexes

Ba-DAS solubilization occurred at 60 °C after ethanol addition, in a ratio of 1:7 in relation to the aqueous solution volume. Besides improving solubility of DAS-Ba in water, the addition of purified ethanol to the solution makes the photo degradation of the surfactant by the Fe(III) ion more difficult. The Fe(III) ions, even when complexed by the sulphate ion, are still susceptible to a slow hydrolysis that generates the photo active species Fe(OH)2+ or to the photodegradation by the anion SO3R¯. The addition of alcohol inhibits these processes. This care, the sulphate ion, the excess of surfactant, the inert atmosphere and the temperature and pH (<5) control minimize the action of the oxidizing species [5, 9, 11–13].

The blue solid produced by reaction 1 (complex A, Fe2DAS4) was characterized as [Fe2(C12H10NO3S)4]·9H2O (1265.6 g/mol) and yield over 64%. The greenish product obtained in reaction 2 (complex B, Fe3DAS6) with yield over 70% calculated from [Fe3(C12H10NO3S)6]·12H2O (1871.4 g/mol), Table 1.

Table 1 Analytical results for elemental composition and Mössbauer spectroscopy parameters of the complexes A [Fe2(C12H10NO3S)4]·9H2O(Fe2DAS4) and B [Fe3(C12H10NO3S2)6]·12H2O(Fe3DAS6)
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Figure 1 shows UV-Vis spectra of BA-DAS and of both Fe(II) complexes. The maximum of the spectra is close to λ = 246 nm, normally attributed to the transition of π electrons and associated to the displacement of double bond inside the aromatic rings. The small difference found between the λmax of the Ba-DAS and the Fe(II) complexes and the band intensities indicate that the ions are slight affected by the DAS ligand [9, 11]. The reactions destined to the ligand photo degradation catalyzed by Fe(III) use λ = 365 nm for the excitation of the photo active species FeOH2+. In this study, however, it was impossible to attribute a band to this species in the UV-Vis spectra [9, 11–13].

Fig. 1
figure 1

UV-Vis spectra of the ligand Ba(C12H10NO3S)2 and Iron (II) complexes (A and B)

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Figure 2 shows the infrared spectra of Ba-DAS, and Fe2DAS4 and Fe3DAS6 complexes. Table 2 presents the attempts to attribute the bands of the IR spectra of the complexes [14–18]. The analysis of the IR spectra, before and after complexation allows identifying the complexation process of the Fe(II) ions by DAS. The bands attributed to the stretching of the aromatic C=C bond appear in ligand spectrum at 1597, 1580, 1516 and 1420 cm−1 decrease significantly after complexation. However, this effect is much stronger for the band at 1580 cm−1, typical of the ring conjugation, but that practically disappears in the spectra of both complexes. The bands associated to torsion in and outside the C–H aromatic bond plans appear at 1138, 1008 and 843 cm−1 in the ligand, remaining with weak intensities in the complexes spectra [14, 17, 18]. The most important change in the IR spectra and associated to the complexation process refers to the bands attributed to the sulphonate group (R–SO3)¯of the DAS ligand. The 1229, 1184 and 1051 cm−1 bands appearing in the ligand spectrum are usually associated to (doubly degenerate) asymmetric and symmetric stretching of the sulphonate group that shows symmetry C3V. With the complex formation, the 1229 cm−1 band totally disappears in the spectra. This effect is due to decrease in the symmetry C3V in the complexes, which removes the double degeneration of the asymmetric stretching of the group and lowers the symmetry of the group. The other bands are slightly displaced to frequencies of the symmetric stretching (1036 cm−1) and asymmetric (1188 cm−1) of the group S=O [14–18].

Fig. 2
figure 2

IR spectra of the ligand Ba(C12H10NO3S)2 and complexes A and B

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Table 2 Infrared data attributions for the ligand (DAS-Ba) and complexes A (DAS-Fe)red and B (DAS-Fe)
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Figure 3 correspond to the Mössbauer spectra of the Fe(II) ion in the two complexes suitable to investigate the coordination sites [3, 15, 19].

Fig. 3
figure 3

57Fe-Mössbauer spectrum of complexes A and B

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The quadrupole splitting parameters (∆) and isomeric displacement (δ) were obtained from the computer-adjusted spectra of Fig. 3. In Table 1 the ∆ and δ values of the Fe(II) ion in both complexes and at room temperature are compared for FeSO4·7H2O [19]. For both complexes, it was verified a single doublet with the δ values for Fe2DAS4 and Fe3DAS6 equal 1.282 mm/s and 1.261 mm/s, respectively; and ∆ values of 2.937 and 3.053 mm/s, respectively. Comparison with accepted values for FeSO4·7H2O (δA = 1.261 mm/s; ∆ = 3.217 mm/s) indicates total reduction of Fe(III) initial (reaction 1). Quantitatively, these values imply that in the complexes the Fe(II) ion occupies the distorted octahedral sites with high spin values [3, 15, 19].

Thermogravimetric Characterization of the FexDAS2x Complexes

The complexes were characterized by TG/DTG under dynamic O2 and N2 atmospheres [20–22]. Figure 4 illustrates the evolution of the mass losses occurred in both complexes as temperature increased. The mass loss and the respective residues under different atmospheres are presented in Table 3 (complex A, Fe2DAS4) and Table 4 (complex B, Fe3DAS6). Figure 5 contains the scheme illustrating the event temperature intervals and the type of mass losses simulated corresponding to Tables 3(a) and 4(b).

Fig. 4
figure 4

TG/DTG curves of the complexes A and B, under dynamic O2 and N2 atmospheres

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Table 3 Summarized TG/DTG data and mass-loss tentative assignments of complex Fe2DAS4 under flow N2 (a) and O2 (b)
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Table 4 Summarized TG/DTG data and mass-loss tentative assignments of Fe3 DAS6 under flow N2 (a) and O2 (b)
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Fig. 5
figure 5

Scheme of TG/DTG mass losses corresponding to Tables 3 and 4

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The Fe2DAS4 complex presents expressive differences in the mass loss as well as in its respective development for both atmospheres. The first mass loss of 12.30% at t = 120 °C (calcd. 12.80%) in N2 agree with the evaporation of crystallization water. In despite this, in TG/DTG curve under O2 the first loss was distributed in a larger temperature range (120–480 °C), indicating probably thermal decomposition of compound A as suggested by attribution (15.00%) to the experimental loss (14.71%). For other temperatures the mass losses show differently, i.e., in (O2), there is higher loss (72.65%) within a shorter temperature range (505–515 °C), whereas under N2, the mass losses are distributed within the temperature range 400–1000 °C. Although the difference, the final results of the processes result in the residues of 12.62% under O2 and 12.61% under N2 corresponding to α-Fe2O3. This simulation allow to the calculation of molecular mass of the Fe2DAS4 complex as 1264.7 g mol−1(O2) and 1265.7 g mol−1 (N2) (elemental analysis 1265.6 g mol−1).

For Fe3DAS6 complex, under O2 and N2 atmospheres, the TG/DTG curves keep the same previous characteristics. The first mass losses in both experimental curves have a different percentage of 15.43% (O2) and 15.35% (N2). However, under N2 atmosphere, there is a shorter temperature range centered on 118 °C, differently from the mass loss under O2 (40–100) °C. The both simulations refer to the release of hydration water molecules and the apparent beginning of thermal decomposing of the compound (under N2 and O2). For other temperatures, it is seen that up to 500 °C all the complex is decomposed under O2, while under N2 atmosphere; the mass losses are extended up to 1000 °C. The both residues of mass losses simulation in O2 (12.37%) and N2 (12.37%) resulted in the Fe3O4. The percentage values led to the calculation of molecular mass of the Fe3DAS6 complex as 1870.7 g/mol (O2) and (N2) (elemental analysis 1871.4 g/mol−1).

Characterization of oxides from the thermal treatment of the FexDAS2x complexes

The product of thermal decomposition of the Fe2DAS4 complex presents an XRD with band lines located at 2 θ: 24.2, 33.2, 35.8, 40.9, 49.5, 54.1, 62.6, 64.1 (Fig. 6) and attributed to the phase α-Fe2O3 (Fig. 6) [7, 8, 23–25]. The same way, the residue of the complex Fe3DAS6 sample showed band lines at 2 θ: 37.9, 44.0, 64.3, 77.4 attributed to phase Fe3O4 (Fig. 6) in accordance with literature data [3, 4, 6, 23–25]. The XRD attributions confirmed the results found in the TG/DTG analyses.

Fig. 6
figure 6

XRD patterns of the residues of complexes A and B

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Figure 7 shows the 57Fe Mössbauer spectra (room temperature) of α-Fe2O3 and Fe3O4 obtained in the thermal decomposition of the Fe2DAS4 and Fe3DAS6 complexes, respectively [3, 19].

Fig. 7
figure 7

57Fe-Mössbauer spectrum of α-Fe2O3 and Fe3O4

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α-Fe2O3 is constituted of dense packing oxygen cells where Fe3+ ions occupy Oh sites manifesting paramagnetic characteristics when at room temperature and antiferromagnetic, at low temperatures. The room temperature spectrum of α-Fe2O3 is represented by six lines from the hyperfine field (δ = 0.38 mms−1 and ∆ = +0.12 mms−1) resulting from the alignment of spins, which does not occur at low temperatures. Under these temperatures, the change in the spins magnetic alignment will invert the ∆ interaction value, making it negative. The temperature at which the ∆ value will invert itself is called Morin [19] temperature. The α-Fe2O3 particle size determines the fraction of material with superparamagnetic properties originated due to the decrease in spin relaxation time. Therefore, the decrease in particle size also lowers Morin temperature when inducing the increase in surface effect and decreasing ∆ interactions [3, 19].

The 57Fe Mössbauer spectra of α-Fe3O4 at room temperature obtained from a sample of Fe3DAS4 thermally treated in air contains six resulting lines of a hyperfine field of 51.17 T with (δ = 0.34 mms−1 and ∆ = 0.0) (Fig. 7). Comparing this α-Fe2O3 spectrum to the ones that show the effect of variation of the sizes of the particles, it is noticed a close similarity to the one that refers to samples of α-Fe2O3 with the sizes up to 15 nm and that have a high content of super paramagnetic fraction [3, 19]. On the contrary, the spectrum of 57Fe Mössbauer of Fe3O4 at room temperature obtained from Fe3(DAS)6 thermally treated in air contains many singularities [3, 19]. Magnetite has been described as a spinoidal structure Fe2+ (A) [Fe3+] (B) O4, in which Fe2+ ions occupy the tetrahedral (Td) site (A) and the octahedral (Oh) site (B) contain the Fe2+ Fe3+ ions. However, the temperature above 77 K (Verwey temperature) starts in the Td site A, the process of oxidation and electronic transference: Fe2+ → Fe3+ + e−. Those electrons move towards the interstices B in a way that Fe3+ + e− → Fe2+. The vacancies in the Oh sites (B) will be occupied by an equal number of Fe2+ and Fe3+ ions. Site A starts to be mainly occupied by Fe3+ ions. At 300 K, for example, the exchange of electrons is very fast and produces a balanced spectrum with a sextet without quadrupole [3, 19]. In samples that were submitted to heating above that temperature and/or to oxidation processes, the Oh site A is occupied by Fe3+ ions and site B can be occupied by Fe2+ (inverted spinel) or by a mixture of Fe2+ and Fe3+, depending on the conditions of treatment of the sample [3, 19, 25].

The Fe3O4 resulting from heating the precursor A to 1000 °C, under atmospheric air, presented the Mössbauer spectrum with two sextets and one doublet. The spectrum analysis reveals that the first sextet (broken spectrum) can be associated to the occupation of the Td site A by the Fe3+ ions (δ = 0.225 mms−1 and ∆ = 0.112 mms−1). A second sextet (continuous) refers to the Oh site B, occupied by Fe2+ (δ = 0.564 mms−1 and ∆ = −0.120 mms−1). The doublet corresponds to the occupation of the site A by the Fe3+ in a cubic tetrahedrically distorted system (δ = 0.227 mms−1 and ∆ = 0.779 mms−1). These data show that, possibly, the Fe3O4 spinel produced in this work is inverted [3, 19]. However, there is an intriguing point in this occupation. The total percentage of the Fe3+ in the various sites is 88.81%, distributed in such a way that 26.43% of the ions are responsible for a hyperfine Td field of 48.83 T in A, while 62.38% are responsible for a null hyperfine field resulting from Td distortion in site B. This unbalanced occupation of the ions contained in the site could have been caused by difficulties in the process of electronic transference from site A to site B. The latter contains only 11.19% of Fe2+. The values of δ that were lower than the prevision indicating that an incomplete electronic transference occurs and stresses the unbalance of the inverted magnetite [3, 19]. However, the little participation of the Fe2+ ions in site B can be connected to a possible process of stabilization of the Fe3+ ion. The values for the hyperfine fields of the samples that were studied showed to be higher than the ones of the common magnetite as well as the line widths for the Td field from 0.50 to 0.83 mm/s for the Oh field are much higher than the traditional (0.19 mm/s). As mentioned, it seems reasonable that a distribution of hyperfine fields on the surface of the ultra fine magnetic particles would be able to induce an increase in the contribution of the demagnetization for the total field in the micro crystals. This evidence seems to be a reasonable explanation and it is supported in the data in Table 5, where the larger fraction of those nanoparticles (62.38%) occupies a tetrahedrically distorted place where they present superparamagnetic characteristics [3, 19].

Table 5 Mössbauer spectroscopy parameters for the Fe3O4 and Fe2O3
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Conclusions

In this study, the anionic surfactant was put to interact with the Fe atom allowing to DAS ligand two possible alternatives: the complexation or reduction. In reaction 1, the Fe(III) is reduced to Fe(II) due to the excess of DAS. In the oxidizing degradation, the excess of surfactant was consumed. The conclusion is that the complexation would be sufficiently strong with Fe(II) ion, however, if there are ions Fe(III) in aqueous solution, the reduction would overcome.

The TG/DTG data contributed to investigate the stability of the two Fe(II) complexes relatively to the oxidation in O2 atmosphere. As is seen, the total thermal decomposition of the two complexes is completed before 500 °C. However, the starting temperature of the thermal decomposition (120 °C) is much lower for the Fe2DAS4 compound, than it is for the Fe3DAS6, where only water loss is observed to 200 °C. This question seems to have a more affirmative answer if the products of TG/DTG and the thermal treatment are investigated. The thermal decomposition product of the Fe2 DAS4 is α-Fe2O3 while the one obtained from Fe3 (DAS)6 refers to magnetite, Fe3O4. The obvious conclusion is that the Fe2 DAS4 compound containing Fe(II), reduced by the DAS surfactant, is in a reasonable level of instability and when treated under an oxidizing atmosphere, it produces an oxide that contains only Fe(III) ion.