Exploring wet chemistry approaches to ZnFe2O4 spinel ferrite nanoparticles with different inversion degrees: a comparative study

Paolo Dolcet*ab, Kristin Kirchbergc, Alice Antonellod, Christian Suchomskic, Roland Marschallce, Stefano Diodatia, Rafael Muñoz-Espíf, Katharina Landfesterd and Silvia Gross*a
aDipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo, 1, 35131, Padova, Italy. E-mail: Silvia.gross@unipd.it
bInstitute of Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Engesserstr. 20, 76131 Karlsruhe, Germany. E-mail: paolo.dolcet@kit.edu
cInstitute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany
dMax Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
eUniversity of Bayreuth, Universitaetsstrasse 30, 95447 Bayreuth, Germany
fInstitut de Ciència dels Materials (ICMUV), Universitat de València, Catedràtic José Beltrán 2, 46980 Paterna, Spain

Received 6th March 2019 , Accepted 2nd May 2019

First published on 6th May 2019

Zinc ferrite (ZnFe2O4) spinel nanocrystals were synthesised through three different low-temperature routes, namely hydrothermal, combined miniemulsion/hydrothermal, and microwave-assisted non-aqueous sol–gel synthesis. With the aim of studying the influence of the respective approach on the structural evolution of the compounds during the synthetic process, the samples were prepared with different processing times. The resulting nanoparticles were inspected with a wide range of analytical techniques, both with regards to the local (X-ray absorption spectroscopy – XAS) and the long-range (Raman spectroscopy, X-ray diffraction – XRD) order.


Spinel ferrites having the general formula MFe2O4 (where M is a bivalent metal)1 are a widely known class of oxides employed in a broad range of applications.2–4 They have been used as gas sensors,5 in diagnostic medicine6,7 as well as therapeutics,4 where their dielectric and magnetic properties2,8–10 make them very appealing functional compounds. Moreover, due to their small band gap, many are promising semiconductors for solar energy conversion.11 In particular, ZnFe2O4 is a very interesting spinel since its magnetic behaviour shows a dependence on particle size. Bulk zinc ferrite is a normal spinel with an antiferromagnetic behaviour and a Néel temperature of 10 K.12 In contrast, nanometre-sized ZnFe2O4 particles show superparamagnetic behaviour, featuring blocking temperature, coercivity, and saturation moment which are highly influenced by the synthetic history of the nanostructures.13

A tight control over the reaction pathways, particularly in terms of nucleation and growth, steering the structural evolution from precursors to nanoparticles and the related properties of the material, is of paramount importance to obtain crystalline materials with desired properties. As a matter of fact, different synthetic routes can lead to materials with the same elemental stoichiometry, but can display altered properties ascribed to fine (micro)structural differences. It is therefore of paramount importance to investigate whether and to what extent different synthetic procedures can steer the structural evolution of the target materials, especially with regards to the inversion degree of ZnFe2O4 spinel.

A number of comparative synthesis studies, as well as studies related to the effect of different experimental and synthesis parameters pertaining to the inversion degree have already been reported.14,15 For instance, in a work by Blanco-Gutiérrez et al.,12 ZnFe2O4 fine particles with sizes ranging from 3 to 19 nm were prepared by an ethylene glycol-assisted solvothermal method. Neutron powder diffraction measurements revealed a difference in inversion degree that was ascribed to different particle sizes. In general, a low inversion degree was ascribed to smaller particles. Furthermore, Corrias et al.16 drew a clear correlation between size and inversion degree through the analysis of both the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS). While, in a bulk ZnFe2O4 microcrystalline sample, Zn2+ ions are located in the tetrahedral sites and trivalent Fe3+ ions occupy octahedral sites (normal spinel), when particle size decreases, Zn2+ ions are transferred to octahedral sites and the degree of inversion is found to increase.

El-Sayed et al. investigated the effect of preparation methods and different annealing temperatures on the crystal size, crystal strain, bond lengths, bond angles, cation distribution, and degree of inversion by different analytical tools.17 It should furthermore be noted that a correlation between the degree of inversion and particle size generally exists and is dependent on the stoichiometry and the preparation method employed, as also suggested by other authors.18 Understanding this correlation could be useful for application purposes as it would allow an interested party to choose suitable synthetic protocols and parameters to obtain zinc ferrite nanoparticles with the desired characteristics for the chosen application. An extensive comparison as presented herein, encompassing three different syntheses approaches, has not however been described yet.

In the present work, we specifically focus our attention on low temperature synthetic protocols for the preparation of the zinc spinel ferrite ZnFe2O4, such as microwave,19 hydrothermal20,21 and miniemulsion combined with hydrothermal routes,22 these latter two both performed under subcritical conditions. The selected approaches are all able to yield nanocrystalline materials in short times and at low temperatures (<300 °C), thus complying with the principles of green chemistry23–25 as far as the mild synthesis conditions are concerned. The structural evolution over time, in relation to the different employed synthetic approaches, has been investigated in-depth with a variety of analytical methods, including X-ray diffraction (XRD), Raman and X-ray absorption spectroscopy (XAS), delivering information on different length scales, from atomic to long range order.

Experimental section


For the synthesis of spinel ferrites, sodium hydroxide (98.0%), tetraethylammonium hydroxide (20% w/w in water), zinc(II) nitrate hexahydrate (99.9%), iron(III) nitrate nonahydrate, iron(III) chloride hexahydrate (99.9%), n-pentane, cyclohexane and rac-1-phenylethanol (98%) were purchased from Sigma Aldrich (Milan, Italy). Acetone and ethanol were purchased from VWR (Milan, Italy). Oxalic acid dihydrate (99.8%) was purchased from Carlo Erba (Rodano, Milan, Italy). Polyglycerol polyricinoleate (PGPR) was kindly gifted by Palsgaard (Juelsminde, Denmark) All reagents were used without further purification.

Synthesis protocols

Hydrothermal synthesis. For the hydrothermal synthesis of ZnFe2O4, a suspension of metal oxalates, prepared from an aqueous mixture of metal salts and oxalic acid, was basified and transferred into an acid digestion reactor (hydrothermal bomb reactor), which was heated at 135 °C for a set time span (see Table 1). The reaction protocol has been described in greater detail in a previous work.21
Table 1 Prepared ZnFe2O4 samples and corresponding synthesis details (the number given to each sample refers to the treatment time)
Sample designation Approach Treatment time Crystallite size/nm Treatment temperature/°C
a Not calculated due to low crystallinity.
HY-1 Hydrothermal 1 h 3.6 135
HY-3 Hydrothermal 3 h 4.4 135
HY-6 Hydrothermal 6 h 4.7 135
HY-12 Hydrothermal 12 h 5.1 135
HY-24 Hydrothermal 24 h 6.8 135
ME + HY-0 Miniemulsion only No thermal step a RT
ME + HY-3 Miniemulsion + hydrothermal 3 h 8 RT + 80
ME + HY-6 Miniemulsion + hydrothermal 6 h 10 RT + 80
ME + HY-12 Miniemulsion + hydrothermal 12 h 13 RT + 80
ME + HY-24 Miniemulsion + hydrothermal 24 h 10 RT + 80
MW-5 MW-assisted sol–gel 5 min 8.8 275
MW-10 MW-assisted sol–gel 10 min 8.7 275
MW-15 MW-assisted sol–gel 15 min 9.1 275
MW-20 MW-assisted sol–gel 20 min 9.2 275
MW-30 MW-assisted sol–gel 30 min 9.7 275

Combination of miniemulsion and hydrothermal routes. In a typical synthesis, the inorganic precursors were dissolved in Milli-Q water to obtain a solution with a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 Fe[thin space (1/6-em)]:[thin space (1/6-em)]Zn molar ratio. This aqueous solution was mixed with the organic phase (PGPR 1 wt% solution in cyclohexane), in a 1[thin space (1/6-em)]:[thin space (1/6-em)]4 weight ratio. The aqueous and organic phases were mixed and pre-emulsified by stirring. The mixture was then ultrasonicated by a W-450D Branson digital sonifier (½′′ tip, 2 min sonication time, 70% amplitude, pulse 1 s, pause 0.1 s). Immediately after this ultrasonication, an excess (i.e., 12 times the moles of Fe) of NaOH (16.6 M aqueous solution) was added to the prepared stable miniemulsion, and this mixture was ultrasonicated again with the same parameters described above, to trigger the penetration of NaOH inside the droplets and to promote the precipitation of the inorganic precursors within the confined space of the droplets. The miniemulsion was then either (i) immediately heated to 80 °C and ambient pressure without stirring in a glass vial or (ii) poured into a Teflon-lined autoclave reactor (45 mL, mod. 4744 Parr Instrument), sealed and heated to 80 °C without stirring for a set different heating time (3, 6, 12 or 24 h).

After the reaction time, the suspension was centrifuged (13[thin space (1/6-em)]000 rpm, 5 min) to collect the solid product. This product was then washed by centrifuging once with a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture of acetone and ethanol and three times with distilled water. The obtained solid was dried under vacuum at room temperature for 15 h and subsequently ground.

Microwave-assisted synthesis. In a typical synthesis,19 anhydrous zinc acetate (91.7 mg), was dissolved by sonication in dry rac-1-phenylethanol (15 mL). Subsequently, iron(III) acetylacetonate (353.2 mg) was added, followed by sonication for 5 min. The resulting dark red solution was transferred to a borosilicate vial (30 mL), sealed with a screw cap and heated at 275 °C under microwave irradiation for 5–30 min. The stirring rate was set to 300 rpm. After quenching with compressed cold air, the ZnFe2O4 nanoparticles were precipitated by adding n-pentane and collected by centrifugation, followed by washing twice with a solution of acetone and ethanol. Finally, the obtained brown powder was allowed to dry at room temperature. Microwave syntheses were performed using a Monowave 300 microwave (Anton Paar Germany GmbH). The temperature was monitored with a ruby thermometer (fibre-optic probe) placed inside the reaction vial. Pressure sensing was accomplished by a hydraulic sensor.

In Table 1, experimental details used for samples prepared by hydrothermal synthesis (HY), combination of hydrothermal and miniemulsion (ME + HY) and microwave-assisted sol–gel route (MW), together with selected crystallite sizes (calculated via Rietveld refinement – see Characterisation methods below), are reported.

Characterisation methods. X-ray powder data were collected with a Bruker D8 Advance diffractometer equipped with a Göbel mirror and employing the CuKα radiation. The angular accuracy was 0.001° and the angular resolution was better than 0.01°. All patterns were recorded in the range between 10–80° with a scan step of 0.03° (2θ) and a 7 s per step acquisition time. Crystallite sizes were determined by Rietveld refinement using the HighScore Plus program. The quality of the refinement was assessed by the values of the discrepancy factor (profile weighted residual error), Rwp, and the goodness-of-fit, χ2.

Non-polarised Raman spectra were collected, using a laser power of 0.2 mW, on a SENTERRA dispersive Raman microscope from Bruker Optics equipped with a Nd:YAG laser (excitation wavelength = 532 nm).

For X-ray absorption spectroscopy, Fe (7112 eV) and Zn (9659 eV) K-edge spectra were collected at the bending magnet of the XAFS beamline at the Elettra synchrotron facility (Trieste, Italy), under standard ring conditions (2.0 GeV, 300 mA). For energy selection, a Si(111) double-crystal monochromator was used. Spectra were collected in a detuned configuration of the monochromator. Data collection was performed in transmission mode using an ionisation chamber placed downstream with respect to the sample. Internal energy calibration was accomplished by simultaneous measurement of the absorption of a metal foil placed between two ionisation chambers located after the sample. Each sample was prepared as a pellet, using cellulose as a dispersant medium.

Data reduction and analysis was performed using the Demeter package;26 background removal, in particular, was carried out by the Autobk routine of the Athena software, which was also used for data alignment and calibration. The extracted EXAFS functions were fitted exploiting the software Artemis. Passive electron reduction factors (S02) were obtained from the fit to bulk metallic references (0.8 for Fe and 0.75 for Zn) and then kept constant in the fits for all the solid solutions.

Results and discussion

The common denominator between the three employed approaches is the fact that they are all low temperature wet-chemistry routes. In the standard hydrothermal method (indicated as HY) the initial step involves the co-precipitation of zinc and iron oxalates, followed by hydrothermal decomposition (under low temperature, subcritical conditions) in a PTFE-lined closed vessel. The mild pressure conditions (about 3 bar) during the heating step at 135 °C are achieved autogenously.

The hydrothermal method is combined with miniemulsion synthesis (ME + HY) in the second approach, which was pioneered by some of us22 to pursue the low temperature crystallisation of different transition metal ferrites. The inverse miniemulsion procedure27–30 involves two phases: a dispersed (aqueous) phase and a continuous (organic) phase in which a lipophilic surfactant is dissolved. The dispersed phase forms nanosized droplets (50–300 nm), and due to the miniemulsion properties, any reaction occurring in the dispersed phase (i.e., the precipitation of inorganic oxides/hydroxides) is confined within the droplet volume. After the generation of the starting miniemulsion, containing the zinc and iron precursors, a NaOH solution was added to initiate the precipitation of the corresponding oxides/hydroxides. The obtained suspension was subjected to the same thermal treatment as in the HY approach.

The third method is based on a non-aqueous sol–gel route involving microwave (MW) heating to produce zinc ferrite particles with narrow size distribution on the time scale of minutes (i.e., 5–30 min).19 In this microwave-assisted approach, anhydrous zinc acetate, Zn(CH3CO2)2, and iron(III) acetylacetonate, Fe(C5H7O2)3, were exploited as raw materials along with rac-1-phenylethanol as high-boiling solvent. The latter compound facilitates dissolution of the precursors and uniform (dielectric) heating. In general, microwave reactor technology allows rapid heating and cooling and thus a fine-tuned synthesis protocol.

The evolution of the long-range order, as well as of phase purity and crystallite size, of the three different families of samples prepared with different reaction times was firstly monitored by X-ray diffraction, as shown in Fig. 1. The observed XRD patterns, corresponding to ZnFe2O4 (ICDD 00-022-1012) for all samples, immediately outline the effect of the different synthetic routes. The most interesting feature is that HY and MW already yield crystalline material after very short times (1 h and 5 min respectively), the MW approach being the fastest to produce a crystalline material. In the case of the ME + HY approach, the hydrothermal step appears to be essential to ensure crystallinity. Indeed, samples prepared without a heating step (ME + HY-0) show very poor crystallinity.

image file: c9qi00241c-f1.tif
Fig. 1 Diffraction patterns of ZnFe2O4 nanocrystals synthesised by (a) hydrothermal (HY), (b) miniemulsion + hydrothermal (ME + HY) and (c) microwave-assisted sol–gel methods (MW).

Although the involved reaction times and heating processes are largely different, particularly when comparing HY/ME + HY with MW samples, the different reflection broadenings of the patterns also point out how the different synthetic approaches influence the crystallite size of the particles. The reflections are broadest for the HY samples and become narrower for ME + HY and further to MW samples, thus suggesting a larger crystallite size. Related Rietveld refinement data are shown in the ESI. As suggested by the broader Bragg's peaks, sample HY-1 displays a crystallite size of 3.6 nm and increasing the heating step only has limited influence. After 24 h, the average crystallite size is indeed increased only up to 7 nm. In this regard, the MW series is even more striking, since the crystallite size is 9–10 nm regardless to the duration of the thermal treatment and apparent already after 5 min.

In the case of the ME + HY samples, on the other hand, a much broader size distribution is found by XRD since the peaks’ profile shape is super Lorentzian. For sake of comparison, this size distribution can be divided in two different superimposing populations (as determined by Rietveld refinement), having average sized of 3–5 nm and 12–15 nm respectively.

Further structural and complementary details can be obtained by the analysis of the Raman spectra, shown in Fig. 2. The three T2g modes give rise to signals at 220, 350 and 460 cm−1, with the first partially superimposed on the Eg mode at 245 cm−1. The peak at 460 cm−1 originates from the local symmetry (T2g) vibration of metal cations in Td sites. The last peak at about 670 cm−1 corresponds to a phonon with A1g symmetry and originates from a vibration involving the oxygen atoms in MO4 environments.31–34 The peak presents a splitting, connected with the inversion of the bivalent and trivalent cations in the lattice, and is thus indeed a convolution of two components, the first one at about 641 cm−1 [(Zn–O)tet] and a second one at approx. 685 cm−1 [(Fe–O)tet].35 The ratio between these two components (I685/(I641 + I685), see Table 2) serves as a rough estimation of the inversion degree. The ME + HY method leads to nanoparticles with the highest inversion degree, and the parameter is roughly constant throughout the series. For both the remaining methods, the I685/(I641 + I685) ratio tends to decrease with heating time (more clearly for the HY series, see also Fig. S2), signifying that the samples evolve towards a normal spinel. Bahnemann et al.31 also evidenced an effect of the inversion degree on the other modes (in particular on the T2g vibration at about 460 cm−1), but due to the low experimental resolution, the effect was not as noticeable as in our case. Additionally, no secondary phases were identified in the spectra, especially no hematite (α-Fe2O3) impurities (these could also be generated by laser-induced oxidation), as witnessed by the absence of two diagnostic sharp bands at about 227 and 292 cm−1.

image file: c9qi00241c-f2.tif
Fig. 2 Non-polarised Raman spectra of zinc ferrites nanoparticles prepared by hydrothermal (left), miniemulsion + hydrothermal (middle) and microwave-assisted sol–gel methods (right) after different preparation times. Spectral intensities have been normalised to the most intense Raman band centred at roughly 680 cm−1, so that changes in intensity and peak width are visible by the naked eye.
Table 2 Ratios of the (Fe–O)tet/(Zn–O)tet components of the Raman active A1g mode
Hydrothermal Miniemulsion + hydrothermal Microwave
Time I685/(I641 + I685) ratio Time I685/(I641 + I685) ratio Time I685/(I641 + I685) ratio
1 h 0.499 0 h 0.566 10 min 0.495
3 h 0.488 3 h 0.561 15 min 0.467
6 h 0.478 6 h 0.573 20 min 0.458
9 h 0.474 12 h 0.563 30 min 0.472
12 h 0.474 18 h 0.572    
18 h 0.474 24 h 0.558    
24 h 0.469        

In parallel with the long-range order determined by XRD, the effects of the synthetic route and the heating time on short range order (in the Å scale) were investigated by means of X-ray absorption spectroscopy (XAS), yielding a more reliable description of the degree of inversion. Measurements at the Fe and Zn K-edges were performed at the XAFS beamline at Elettra Synchrotron Facility (Trieste, Italy). Fig. 3 depicts the Fe K-edge XANES spectra collected for the zinc ferrites prepared with different heating times. For better clarity, only the curves corresponding to the shortest, the middle and the longest heating times are reported for each route. The complete set of curves is shown in the ESI. The XANES region however already reveals interesting differences. The positions of the absorption edges are typical for Fe in a +3 oxidation state, in agreement with the compound stoichiometry. This is further confirmed by the detailed analysis of the pre-edge peak, by deconvolution of the signals using pseudo-Voigt peaks, after subtraction of the edge jump contribution using an arctangent function (see variogram reported in Fig. 4 and Table S5). For all samples the centroid is found at 7114.3–7114.4 eV, a value typical for trivalent iron.36 In addition, the pre-edge peak is symptomatic of the symmetry around the absorber. This peak arises from a 1s → 3d transition, which is forbidden by dipole selection rules and thus weak, but its intensity increases for non-centrosymmetric environments, i.e. tetrahedral ones.36 The pre-edge area is highly affected by the applied synthetic route: in particular, it is the most intense for the ME + HY approach, indicating how, for these samples, the symmetry around the Fe absorber is, on average, lower that for the other samples. This result indicates that, for these samples, the inversion degree is higher, i.e. there is a larger number of FeIII ions occupying tetrahedral sites, as already hinted by Raman spectroscopy. Most interestingly, the influence of the synthetic method is stronger than that of size since, on average, the HY and MW samples have similar pre-edge intensities despite significantly different crystallite sizes. On the other hand, the influence of heating time is not as relevant, since a clear trend in decreasing pre-edge area is present only for the HY series.

image file: c9qi00241c-f3.tif
Fig. 3 Fe K-edge XANES spectra of ferrites prepared with different hydrothermal approaches; in the inset, zoom-in in the pre-edge region (spectra are shifted for better clarity).

image file: c9qi00241c-f4.tif
Fig. 4 Fe K-edge pre-edge variogram of the integrated intensity and centroid position for the different ferrites series. Black dots represent positions of reference compounds,36 red crosses are associated with the ME + HY, squares with HY and triangles with MW samples.

The Fourier transforms (FTs) of the EXAFS functions are given in Fig. 5 (data not corrected for phase shift). The first shell, centred at about 1.9 Å, represents the Fe–O distance. The position and intensity of the ME + HY samples are lower than for the other samples. This observation is in agreement with a higher tetrahedral contribution, affecting the number of O scatterers (and thus the intensity) and the Fe–O distance (shorter in Td environments).

image file: c9qi00241c-f5.tif
Fig. 5 FTs of spectra recorded at Fe K-edge for the different transition metal ferrites.

The second shell, i.e. the region between 2.5 and 4 Å, is particularly meaningful, since it mainly consists of the cation–cation contributions. It can thus provide a qualitative indication of the inversion degree. The distance between two cations in octahedral sites is about 3 Å, whereas for two cations in tetrahedral sites this distance increases to about 3.65 Å. The interaction between cations in different symmetry sites (MTd − MOh) is 3.5 Å. With respect to the HY samples, the curves corresponding to the ME + HY show a broader second shell. This indicates that the tetrahedral contribution is higher, in accordance to the results gained from the XANES region, and might also reflect the poorer crystallinity of theses samples. On the other hand, the MW samples show a more distinct contribution at 3.6 Å. Since both Raman and XANES indicate that the inversion degree for this series is similar to the hydrothermal case, the presence of this second shell can be related to a bigger crystallite size and increased crystallinity.

A complete picture of the short-range order and useful insights into the different growth phenomena in the three classes of samples were afforded by acquiring X-ray absorption spectra at Zn K-edge. The collected XANES spectra are reported in Fig. 6 and consist of three main peaks, at 9665 (A), 9668 (B) and 9672 eV (C), with a shoulder (D) at about 9679 eV. Features B and D are particularly interesting, since they are correlated to the degree of inversion of the spinels. For higher inversion degrees, peak B was found to increase, while shoulder D decreases.15 In accordance with Fe K-edge measurements, the ME + HY samples show a greater intensity in the B feature, coupled with a decreased D feature. Some influence from the heating time is also found: for example, HY samples prepared with short heating times (t ≤ 3 h) exhibit a greater inversion degree than samples with a longer hydrothermal step, confirming the trend evidenced by Raman. Once again, the heating time does not show the same influence when a hydrothermal treatment is coupled with miniemulsion preparation (ME + HY samples). This effect is not size-related; the nanoparticles prepared by HY treatment show a lower, monodisperse crystallite size dispersion, so the possibility of partially inverting the spinel structure would be greater for these samples. Experimental outputs, on the other hand, evidence the opposite behaviour. This could however simply be due to the formation of a Fe-rich franklinite in the ME + HY samples, which has already been evidenced in a previous work.22

image file: c9qi00241c-f6.tif
Fig. 6 Zn K-edge XANES spectra of ferrites prepared with different hydrothermal approaches (spectra are shifted for better clarity).

The corresponding Zn K-edge Fourier transforms (Fig. 7) further evidence the trend outlined by the XANES region. In the ME + HY series, the contributions of Zn ions in octahedral (3 Å) or tetrahedral sites (3.6 Å) are almost equivalent, compatible with a higher inversion degree. These spectra show a greater dependence on the thermal treatment duration with respect with analogous Fe K-edge spectra. For example, the HY-3 sample shows a significant octahedral site contribution, but this becomes less important when lengthening the hydrothermal treatment. A similar trend is seen in the MW series, for which the octahedral symmetry contribution is less relevant, pointing to a lower degree of inversion.

image file: c9qi00241c-f7.tif
Fig. 7 FTs of spectra recorded at Zn K-edge for the different transition metal ferrites.

Overall, these results evidence how the crystallinity of the samples and their degrees of inversion may vary significantly depending on the employed synthesis protocol. This suggests that a closer examination of the mechanisms involved may yield further insights on the reasons why these differences manifest.

In the case of the microwave-assisted sol–gel, the reaction proceeds via a first hydroxylation step involving the phenylethanol and the zinc acetate or iron acetylacetonate precursors, generating zinc hydroxyacetate and iron hydroxydiacetylacetonate. In both reactions, 1-phenylethyl acetate is formed through an ester elimination.19 The so-formed hydroxylated species further react through condensation reactions, generating Fe–O–Zn bonds and leading to the formation of the target material. The homogeneous heating ensured by microwave likely leads to a fast growth step which is completed in the first few minutes, as witnessed by the only minor increase in crystallite size with time.

In the case of the hydrothermal step, two mechanisms are most commonly proposed. The in situ mechanism assumes the formation of initial [Fe(OH)n]−(n−3) cores which react with soluble Zn species to form a superficial ZnFe2O4 layer. This layer further grows inward as the reaction proceeds, finally consuming all the core and leaving only ZnFe2O4. In the second possible mechanism, known as “dissolution–precipitation”, the hydrothermal conditions favour the re-dissolution of the oxohydroxide or oxalate species that originally form upon basification, which are then able to react via condensation.21

The hydrothermal step is essential to achieve sufficient crystallinity, as witnessed by the ME + HY sample, which was not subjected to these conditions. On the other hand, the confinement induced by the miniemulsion system also has an effect on the crystallinity, as previously demonstrated.22 Whereas this confinement likely does not impact significantly on the mechanism, it strongly influences the local concentrations and kinetics, thus reflecting on the final crystallinity of the sample.


In conclusion, the differences in inversion degree observed among the samples can be ascribed to both difference either in the heating deposition mechanisms (microwave vs. conventional heating) as well as to the differences in the synthesis conditions, or to combination thereof. The rationalisation of the single effects is hardly accomplished. In the literature, correlations between size and inversion degree have been reported, but no clear dependence on heating protocols or synthetic conditions in general has yet been reported.

Indeed, whereas many previous reports evidenced the influence of the crystallite size on the inversion degree of nanoparticular ferrites,16,17 little speculation was available on the effect of synthesis route. By means of XRD, Raman and XAS, we thoroughly investigated the distribution of Fe3+ and Zn2+ ions in the Oh and Td sites of the spinel structure depending on the approach used for the preparation of these materials.

Comparing particles with similar sizes (around 8 nm), it was shown that a combined miniemulsion and hydrothermal approach is likely to favour a transfer of the divalent Zn ions to octahedral sites, while in the case of the microwave-assisted decomposition, following a different crystallisation mechanism, the inversion degree was rather low.

In addition, the inversion degree can be modulated, to a certain extent, by varying the duration of the thermal step, given a certain crystallinity, allowing for a targeted synthesis with the desired nanoparticles characteristics. These results disclose exciting perspectives in controlling the inversion degree, and thus the functional properties, of ferrites by tuning the synthetic experimental conditions. This will be particularly useful in several applicative avenues for ZnFe2O4 where the size, shape and inversion degree of the employed nanoparticles have been shown to affect the performance of the system, such as magnetic devices,31,37,38 drug delivery and medical applications,39 anti-fungal formulations,40 photocatalysis,41 and in particular photocatalytic water oxidation.31,42

Conflicts of interest

There are no conflicts to declare.


Financial support was provided by the DFG via the GrK (Research training group) 2204 “Substitute Materials for sustainable Energy Technologies”. S. G. gratefully acknowledges DFG and the Justus-Liebig Universität Gießen for the provision of a Mercator Fellowship (2016-2020). RME acknowledges the Max Planck Society for financial support through the funding of the Max Planck Partner Group on Colloidal Methods for Multifunctional Materials (CM3) at the University of Valencia. We would like to thank the Centre of Materials Research (LaMa) at Justus-Liebig Universität Gießen for the support of this project. KK and RM gratefully acknowledge by the German Research Foundation DFG under the priority program SPP 1613, project MA 5392/5-1.


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qi00241c

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