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. 2022 Dec 13;23(1):59–67. doi: 10.1021/acs.cgd.2c00694

Inductive Heating Enhances Ripening in the Aqueous Synthesis of Magnetic Nanoparticles

Jesús G Ovejero 1, María del Puerto Morales 1, Sabino Veintemillas-Verdaguer 1,*
PMCID: PMC9817478  PMID: 36624778

Abstract

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The search for competitive processes and products using environmentally friendly chemistry is, nowadays, one of the greatest challenges in materials science. In this work, we explore the influence of magnetic inductive heating on the synthesis of magnetic iron oxide nanoparticles in water, either by the coprecipitation of iron(II) and iron(III) salts or by the oxidative precipitation of an iron(II) salt. In the first case, the way the heat is transmitted to the system influences mainly the nanoparticle growth that is thermally activated reaching nanoparticles up to 16 nm. In the second case, it influences magnetic nanoparticle nucleation through the dissolution of the initial iron oxyhydroxide formed (the Green Rust) and the crystallization of magnetic iron oxide leading to nanoparticles up to 55–64 nm. This nonconventional heating method can produce monodisperse populations (size distribution <25%) of bigger magnetic iron oxide nanoparticles if the appropriate magnetic field conditions are used. The results were interpreted as an enhancement of the oriented attachment growth mechanism by the use of inductive heating, and suggest the possibility of increasing the size range of nanomaterials that can be obtained by sustainable aqueous routes using nonconventional heating, while maintaining low size dispersity.

Short abstract

Effect of alternate magnetic field heating on the nucleation and growth of magnetic nanoparticles prepared in aqueous suspensions by coprecipitation of iron(II) iron(III) salts or by oxidative precipitation from an iron(II) salt. The increase of temperature with time due to the magnetothermic effect provokes the enhancement of both Ostwald ripening and oriented attachment mechanisms. This phenomenon opens the possibility of preparing larger magnetic nanoparticles in aqueous solutions keeping narrow size distributions.

1. Introduction

One of the main objectives of materials science is to search for safe and environmentally friendly chemical routes that enable the massive production of nanomaterials with minimal impact on the environment.1 To achieve this goal, both reagents and nanoproducts must be nontoxic to both humans and ecosystems and also biodegradable or recyclable. This is the case for magnetic iron oxide nanoparticles (MNPs) synthesized from aqueous solutions. Water is undoubtedly the best “green” solvent and aqueous routes are generally considered the best choice for the production of nanomaterials. However, it is also important to develop synthesis methods that use safe and efficient environmentally friendly heating processes, according to the UN Agenda 2030 for Sustainable Development. Among the heating systems available for industrial proposals, conductive heating used by heating blankets, hot plates, etc. is highly inefficient. Alternatives that reduce thermal losses, such as microwave heating or magnetic induction heating using alternating magnetic fields (AMFs), can help in increasing the heating efficiency by several orders of magnitude.2 The potential use of the last heating technique is especially relevant for the synthesis of MNPs, because they are able to absorb the energy of AMFs and heat their local environment in a highly efficient manner. Since the growth of MNPs is thermally triggered, the way in which heat is transmitted to the system is expected to be relevant to the mechanism of MNP formation and deserves careful inspection.

The La Mer3 model describes the classical mechanism of homogeneous nucleation from solution, which in the first stage consists of the formation of small nuclei in a supersaturated precursor solution; in the second stage, the system evolves to reduce the total free energy by decreasing its surface component with the growth of the nuclei. These can grow by incorporating atoms from the solution or by ripening. In this theory, there are two possible mechanisms for ripening: Ostwald ripening (OR) or aggregation. The OR process is based on the solubility differences between nanoparticles caused by their curvatures;4 consequently, this mechanism proposes the selective dissolution of small particles (with higher curvature and more unstable), and the growth of larger ones (with lower curvature and more stable). If we consider that the transport of ions between particles is by diffusion, the theoretical dependence of the mean size with time at constant temperature is given by the Lifshitz–Slyozov–Wagner theory,5 predicting a continuous growth. This theory was successfully applied to a variety of systems, mainly semiconducting ZnO,6 GaAs,7 and CdTe,8 but also organic substances such as β-carotene.9 In the case of the aggregation mechanism, growth occurs as a consequence of collisions between nanoparticles. Due to the reactivity of the nanoparticles, these collisions cause stable aggregates.10 As for the case of nucleated MNPs at moderate temperatures, these aggregates are favored to be oriented and behave as a single crystal, so that in the following this mechanism will be referred to as “oriented aggregation” (OA). For this model, the growth of the average aggregate size can be determined by combining concepts of chemical kinetics with statistical mechanics.11 A time dependence of the average size is obtained, which presents a plateau at longer times where the size is constant, in contrast to OR ripening that predicts a continuous growth. This different evolution causes the progressive widening of the size distribution of the OR-matured nanocrystals with respect to those matured by OA. This has been experimentally verified in the case of Ag12 and in a wide variety of compounds, such as SnO2,13 ZnSe,11 α-Fe2O3,14 and ZnS.15 Recently, direct evidence of the OA process has been obtained by in situ electron microscopy.16

The extension of the ripening process can be controlled by temperature and time17 in most nanocrystal syntheses, e.g., Ge nanostructures,18 or by using coating agents that hinder mass transport and avoid both ripening processes.19,20 However, other physical approaches such as sonication in the case of zeolitic imidazole framework nanoparticles,21 mechanical forces in the mechanochemical synthesis of PbTe,22 or light for the coarsening of Pt nanoparticles supported on TiO223 have also been exploited for the control of crystal ripening. In the synthesis of magnetic nanoparticles, the effect of static magnetic fields24,25 has been studied. Few studies analyze the effect of AMFs on the nucleation and maturation mechanism26−28 and, to our knowledge, none of them systematically analyze the effect of the AMF intensity and frequency. These can be considered as key parameters, since the absorption of magnetic energy by the MNPs and the thermal gradient induced on their surface depends critically on the matching between the size of the MNPs and the AMF conditions.29

In this work, we study the effect of magnetic inductive (AMF) heating on two commonly used aqueous syntheses of magnetic iron oxide nanoparticles. We compare the effects of the exceptional heat production capacity of MNPs in the presence of AMF with a conventional heating. For AMF heating, the complex time-temperature profiles reached during the process will be analyzed in relation to the size distribution and the frequency and intensity of the magnetic field used.

2. Experimental Section

2.1. Coprecipitation Synthesis with Conventional Heating

For the coprecipitation (CP) synthesis of MNPs, 40 mL of NH4OH 25 wt % (Solution A) was rapidly injected into a solution B containing 40 mL of 0.175 M FeCl2 and 0.334 M FeCl3 under vigorous stirring. The obtained suspension was immediately heated in an oven at 100 °C for 1 h and used as a reference (REFcp).

2.2. Oxidative Precipitation Synthesis with Conventional Heating

For the oxidative precipitation (OP) synthesis of MNPs, a solution C consisting of 270 mL of an hydroalcoholic solution 27.7% made 0.247 M in NaOH and 0.111 M in NaNO3 and a solution D consisting of 30 mL of H2SO4 0.01 M made 1 M in FeSO4 were prepared (C and D were deoxygenated by bubbling nitrogen). After mixing, a suspension of ferrous-ferric hydroxy salt (green rust, GR) was initially formed. This suspension was kept under strong stirring for 15 min in a glove box under nitrogen. For the reference sample with conventional heating (REFop), 300 mL of the suspension was heated to 90 °C and kept for 16 h by circulating ethylene glycol at 90 °C and 7 L/min through the thermostatic jacket of the reactor, as described in a previous work.30

2.3. Alternative Heating System

The experimental setup for the syntheses of MNPs in the presence of AMF is represented in Figure 1. The reactor was placed into the inductor in which the AMF is produced by a Fives Celes 12,118 M01 generator, purposely designed for magnetic hyperthermia using nanoparticles. This device is composed of the combination of a CELES MP 6 kW generator capable of generating resonant frequencies in the range 100–400 kHz (tunable with an ALU CU type capacitor box) and a 71 mm i.d. DT25901A chilled coil. The system is capable of producing AMFs up to 65 mT at 90 kHz. Higher frequencies could be obtained at the cost of decreasing the magnetic field intensity making the product H·f constant and equal to 6 × 109 Am–1 s–1. Temperature was measured with an OSENSA fiber optic probe Mod. PRB-G40-02 M-STM-MRI inserted into the solution through a glass thermowell.

Figure 1.

Figure 1

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Experimental scheme of the reactor employed in the preparation of magnetite nanoparticles under an AMF. T represents the optical thermometer. Note the position of the reactor in the center of the water refrigerated coil where the AMF is more homogeneous. Low filling degree of the reactor assures that most of the reactant is in the zone of the maximum magnetic field. A reflux column was used in the long experiments (not shown). W indicates the water flow inside the refrigerated coil. Solutions A and B used for CP are replaced with C and D in the OP experiments.

For the OP synthesis with AMF heating, magnetic fields with different frequencies and intensities (Table 1) were applied to 80 mL of the GR previously formed as described above but with both solutions C and D deoxygenated by bubbling nitrogen (1 h) outside the glove box. The heating by AMF started after 15 min of stirring of GR in the AMF reactor. The experiment was finished when 90 °C was reached or the suspension boiled. The amount of magnetite produced was 0.618 g (0.446 g Fe).

Table 1. Experimental Conditions for AMF Heating (F: Frequency and H: Field Intensity) of Initial MNPs Prepared by Coprecipitation and Green Rust Precursor Prepared by Oxidative Precipitation.

AMF heating
Coprecipitation Oxidative Precipitation
  F (kHz) H (mT)   F (kHz) H (mT)
AMF1cp 92 66      
      AMF2op 170 36
      AMF3op 205 30
AMF4cp 285 22 AMF4op 285 22
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For the CP synthesis with AMF heating, the addition of the ammonia solution A to the ferrous-ferric solution B and the application of the AMF were simultaneous and, once boiling was reached, the system was kept at 100 °C under reflux for 1 h. The amount of magnetite produced was 1.58 g (1.14 g Fe). No other energy source outside the magnetic field was used during both experiments.

In independent experiments, the temperature evolution was recorded for the mother solutions, (whose composition was obtained subtracting the stoichiometric amount of magnetite to the reactant solution), exposed to the same AMF.

The specific conditions of the heating by AMF are included in Table 1. The sample’s names are given by the heating source (AMF) and conditions (1–4) followed by the letter “cp” or “op” if made by CP or OP.

2.4. Characterization Techniques

Structural characterization of the MNPs was carried out by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The iron oxide phase was determined from XRD patterns recorded between 20° and 70° (2θ) in a diffractometer with a graphite monochromator (Bruker D8 Advance, Billerica, MA, USA) and Cu Kα radiation using a zero-background sample holder. The crystallite size was determined using peaks widths of the XRD patterns and the Scherrer equation using the facilities of the PC-APD computer program. A JEOL JEM 1011 transmission electron microscope with a Gatan ES1000W camera was used for TEM imaging. For the sample preparation, a drop of the aqueous suspension was deposited on a carbon-coated copper grid and allowed to evaporate at room temperature. To determine the particle size distributions (PSDs), direct measurements were done on TEM micrographs (counting more than 400 particles). All the PSDs were fitted to Log Normal functions.

For the AMF activated processes, the experimental time-temperature profile was used as an indicator of variations in the heating efficiency of the nanoparticles throughout the process. The specific adsorption rate (SAR) of the MNPs was evaluated in the final part of the process from the heating slope after subtracting the contribution of the mother solution and correcting for heat losses.

Magnetic characterization of the samples was carried out on a MagLabVSM vibrating sample magnetometer (Oxford Instruments) with a maximum field of 5 T. The hysteresis cycles of the powder samples were measured at room temperature at a rate of 0.3 T/min. Saturation magnetization was determined by extrapolating to infinite field the magnetization data above 2.5 T using the linear plot M(H) versus 1/H. Magnetic susceptibility was determined as the slope of M versus H below 0.02 T where the dependence of magnetization and magnetic field is linear. Mass magnetization was presented in emu/g.

3. Results

Regardless of the synthetic procedure, all materials produced in this study were identified as magnetite or maghemite nanoparticles by their XRD patterns and no secondary phases were identified in any case, as shown in Figure S1.

3.1. Coprecipitation

Figure 2 presents the time-temperature evolution of the medium during the CP synthesis under two extreme AMF conditions: low frequency-high field (AMF1) and high frequency-low field (AMF4). This figure also includes the heating curves of the mother solution (without iron salts) and distilled water under the same AMF conditions. These curves can be used as in situ indicators of the formation timing and the evolution of iron oxide nanocrystals. The heating curves are similar for both AMF conditions and comprise a sharp rise in temperature due to the mixing of the reagents and a temperature increase (step I) that reaches a maximum heat rate of 8.5 °C/min for AFM4 and 9.5 °C/min for AFM1 at the end of this step followed by a progressive heating (step II) that finally tends to stabilize (step III) due to boiling.

Figure 2.

Figure 2

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Heating curves of: CP synthesis of MNPs in the presence of different AMFs (conditions in Table 1, brown and green continuous lines); corresponding mother solution (6 M NH3, 0.68 M ClNH4) (colored dash lines); and water (black line).

Interestingly, the mother solution produces a significant amount of heat under AMF. This heating capacity of dissolved ions has been reported previously31 and is responsible for much of the heating of the suspension. The formation of the MNPs is clearly reflected in Figure 2 as a nonlinear increment in the heating rate of the suspensions with respect to that of their corresponding mother solutions. As we expected, the water exhibited no temperature rise under AMF, ruling out any thermal artifacts in the reactor.

Figure 3 shows TEM images and PSDs of the CP samples obtained under the two AMF conditions together with a reference sample of the conventional heating (REFcp). The micrographs show the influence of the magnetic field on the microstructure of the MNPs prepared by this method. Both MNPs nucleated and aged under AMF showed similar TEM average sizes (16 and 17 nm), which are ∼80% larger than that of the reference REFcp (9 nm), also they present a more spheroidal shape. The polydispersity was also significantly reduced in both AMF conditions (σ/D = 0.6) with respect to the reference sample (σ/D = 0.8).

Figure 3.

Figure 3

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TEM micrographs, average size D ± σ and PSD of samples obtained by CP in the presence of AMFs (Table 1) and the reference (REFcp).

Figure 4 shows the evolution of the particle size determined by both TEM and XRD with time. It can be observed that regardless of the frequency and intensity of the magnetic field, the particles grow during the first 10 min of treatment, reaching a size plateau of 16 ± 1 nm after this time. In all cases, the differences between the mean TEM sizes and those obtained from XRD peak broadening (Figure S1) are small, indicating the single-crystalline nature of the samples.

Figure 4.

Figure 4

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Evolution of the TEM and Scherrer’s particle size with time for CP samples prepared under different AMF conditions (Table 1).

The data indicate that the average particle size of the crystals increases as the temperature increases during steps I and II (Figure 2), but after reaching a critical size around 15 nm at 12 min, the temperature increase moderates (step III), and the size of the MNPs stabilizes, as shown in Figure 4.

The evolution of the average sizes and PSD with time of sample AMF4cp presented in Figure 5 shows the bimodality that the system develops with time as well as the progressive rounding of the particles. In Figure S2, the XRD patterns of the transient samples are presented showing their monocrystalline nature.

Figure 5.

Figure 5

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Evolution of the particle average D ± σ and PSD with time (from 2 up to 30 min) during the synthesis of sample AMF4cp.

SAR values of the magnetic nanocrystals were calculated from the heating curves, after subtracting the contribution of the mother solution. The values, 51.8 and 27.6 W/g Fe for AMF4cp and AMF1cp, respectively (Figures S3 and S4, respectively), correlate with the particle size determined by TEM and XRD, both being well above those expected for MNPs obtained by CP using conventional heating32 and close to MNPs produced by organic decomposition.33

3.2. Oxidative precipitation

It has been well established that for OP synthesis, the initial iron oxyhydroxide GR plays a key role as iron reservoir.34,35 In this route, the heating process induces a progressive dissolution of GR by oxidation and hydrolysis and drives the crystallization of MNPs.36−38 Thus, the interaction of the previously formed GR and the AMF will determine the heating rate of the OP synthesis with important consequences on the final structure of the MNPs.

The temperature evolution of the reaction mixture under three different magnetic field conditions is presented in Figure 6, together with the reference sample (under conventional heating) and the mother solution. As in the coprecipitation case the temperature evolution during the AMF exposure can be also divided into three steps that now correspond to: (I) Heating of the initially formed GR, (II) sudden increase of the temperature due to the AMF absorption of the newly formed magnetic initial nuclei, and (III) net decrease of the heating rate at the end of the process.

Figure 6.

Figure 6

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Heating curves of the OP synthesis of magnetite MNPs in the presence of different AMFs (conditions in Table 1); corresponding mother solution (0.11 M Na2SO4, 0.1 M NaNO3, 0.0014 M NaOH) (dash); and reference sample (REFop) prepared by conventional heating.

A clear effect of frequency and field intensity on the temperature evolution was observed. At low frequencies high fields (AMF2 and AMF3), the GR heats up much faster than the mother solution and nucleation and boiling temperatures are reached quickly after 50–60 min. On the contrary, at AMF4, the GR contributes less to the initial heating, nucleation is consequently delayed, and the temperature increases progressively reaching stability at much longer times without reaching boiling.

TEM images of the MNPs produced by OP are presented in Figure 7. Under conventional heating conditions, OP produces cubic MNPs of 37 ± 9 nm (REFop). The presence of the AMF during GR aging and magnetite nucleation leads to an increase of the average particle size up to 55–64 nm as a function of the applied magnetic field. These values correlate approximately with their crystal size determined by XRD in Table S1, indicating, together with the polyhedral morphology, that the MNPs are single-crystalline.39

Figure 7.

Figure 7

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Average particle sizes D ± σ, PSDs, and TEM micrographs of selected OP samples obtained under different AMF conditions (Table 1) and the corresponding reference sample obtained by conventional heating.

Interestingly, the PSD shows a clear dependence on frequency and field, with AMF3 (205 kHz and 30 mT) being the condition that minimizes polydispersity (23%). Lower frequencies lead to incomplete removal of the smaller crystalline fraction around 35 nm (the usual result when conventional heating is employed), and higher frequencies promote the formation of large MNPs that increase their average size, but also lead to an increase in polydispersity (above 30%). These observations must be the result of differences in the AMF absorptivity of the MNPs and not to the total energy input delivered to the system, since in all experiments the amplitude-frequency product was maintained constant.

The evolution of the PSD with time of the sample AMF3op presented in Figure 8 shows the initial bimodality of the system that disappears with time as well as the change in morphology form round to polyhedral coincident with the pass form zone I to zone II (Figure 6). Rests of the GR in the process of dissolution in zone I could be appreciated after 32 and 38 min of reaction. Interestingly, both the spherical and the polyhedral intermediate samples are monocrystalline as deduced form the coincidence between TEM and Scherrer sizes (Figure S5).

Figure 8.

Figure 8

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Evolution of the particle size average D ± σ and PSD with time during the synthesis of the sample AMF3op. Progressive dissolution of GR could be observed between 32 and 38 min and the inset at 32 min show the aggregates of MNPs.

The specific absorption rate (SAR) evaluated from the slope of region III of the heating curves gives increasing values of 82.9, 133, and 181 W/g Fe as the field conditions change from low to high field intensity (AMF4, AMF3, and AMF2, respectively) (see Figures S6–S8 for the details of the calculations). The maximum SAR value was recorded for the smallest average size and most polydisperse sample obtained under low-frequency and high-field conditions (AMF2).

Figure 9 shows the hysteresis cycles of the samples measured at room temperature. The cycles correlate with what it is expected for the average size of the MNP, presenting higher saturation magnetization (∼75 Am2/kg for CP and 85 Am2/kg for OP) and low coercivity values (<4 mT for CP and <10 mT for OP), being in all cases greater than those for the reference sample REFcp. Table S1 shows all the values of magnetic properties and particle sizes.

Figure 9.

Figure 9

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Hysteresis loops of some representative samples prepared by CP, OP using conventional and AMF heating. Field conditions are described in Table 1.

4. Discussion

There are two main factors that differentiate AMF heating from conventional heating. On the one hand, the energy of the AMF can be transformed into heat by the aqueous solutions, through the induction of ionic currents, and by the magnetic cores through their hysteresis losses. In the first case, the solution is heated homogeneously. In the second case, the locally dissipated heat is able to create a thermal gradient from inside, i.e., at the nanoparticle surface, that alters the transport and perhaps the atomic order at the surface of the cores themselves. The second and more important difference between AMF heating and that produced by conventional sources is that the energy supplied to the system depends on the size of the particles themselves. At low field strengths, such as those used in the present work, the magnetization of the MNPs follows a linear response with the magnetic field,40 and the amount of heat generated increases with frequency until it reaches a maximum (at resonance frequencies well above those explored in the present study). At a fixed frequency, the generated heat increases dramatically when its intensity exceeds the anisotropy barrier of the MNPs. The larger the size of the MNPs, the larger this barrier is and the greater the heat produced when it is exceeded. Consequently, for a given frequency and field, there is an optimal particle size for heat production.41 In OP and CP, the evolution of particle size over time feeds back into the inductive heating capacity of the system and justifies the complex temperature profiles observed, as well as the dependence of the particle size obtained on frequency and magnetic field strength. This feedback loop does not exist in conventional heating.

4.1. Temperature Profiles and Nucleation Aging for AMF versus Conductive Heating

4.1.1. Nucleation Period (Step 1)

The temperature profile obtained by conventional heating OP (and CP) is the inverse of that obtained by AMF heating. The former, originated after dropping the GR suspension into a vessel maintained at 90 °C (or introducing the initial coprecipitated MNPs into the oven at 100 °C), supplies energy to the solution by heat conduction from outside through the walls. Consequently, faster heating rates are produced at the beginning with a slow asymptotic temperature rise at the end (see the OP case in Figure 6). This initial rapid heating leads to an initial massive nucleation ending in smaller particles with a narrow size distribution (36 nm, σ = 24% for OP). On the contrary, AMF heating in OP and CP is slower at the beginning and tends to accelerate during the process as the particles grow and produce heat in a more efficient way.

The differences in the shape of the heating curves in zone (I) for OP and CP reflects the different nature of the two processes. In CP, the massively generated iron oxide clusters/nanoparticles immediately after mixing the iron salts and the base are probably too small to produce heat from AMF absorption, so the important initial heating is generated by the ionic currents induced in the solution, as in the case of the mother solution. However, thanks to the small particle size of the MNPs obtained by CP and the constancy of the H·f, both AMF1 and AMF4 are able to overcome easily the anisotropy barrier along step I. As a result, the heating must be directly controlled by the frequency in CP as observed. In addition, the broad size distribution of the MNPs obtained by CP mitigates the dependence of AMF absorption with the size, because there is always a minority fraction capable of absorbing the AMF. As a result, the change to phase II happens similarly for both AMF1 and AMF4 (Figure 2) and the crystallization is also similar in both cases. On the contrary, for OP, the diluted mother solution and reduced ion mobility of the GR are responsible for the delay in the nucleation and the prolongation of phase(I) with respect to conventional heating. The bigger sizes of the MNPs generated by OP makes the transition to phase II (the one in which the inductive heating produced by the MNPs overcomes that produced by the mother solution) steeper than in the CP case and the narrower PSD of OP MNPs justify the dependence of the time-temperature profiles with the frequency and field intensity.

4.1.2. Aging Period Steps II and III

During the aging period (step II) both the time-temperature profile and the final product depend on the features of the magnetic crystals generated in the previous step and their suitability for the heat production in the AMF. For the OP synthesis, the intermediate frequency and field AMF3 which produced the earliest onset of step II were also the best for the preparation of uniform MNPs (Figure 7). The lower frequency even with the most intense field AMF2 induced less heating when the MNPs were smaller and delayed the passage to phase II. During this phase, both AMF3 and AMF2 produced similar heating rates due to the compensating effect of frequency and fields when MNP’s become larger. As the MNPs grow, the MNP anisotropy barrier rises and hysteretic dissipation is boosted so that the temperature slope suddenly increases (step II). Thus, the heat dissipation in step II is mainly controlled by the magnetic field intensity, and the highest slope is observed for the highest field intensity (AMF2). Unfortunately, the PSD of the AMF2 sample is rather polydisperse (Figure 7) due to the large amount of energy delivered to the particles. In contrast, under AMF4 conditions, the smaller field strength is less able to overcome the anisotropy barrier of the MNPs so the heating slope is reduced in step II, and a longer ripening period results in larger MNPs with a broad size distribution (Figure 8). Finally, in step III, as the crystal size of the MNPs grows, the anisotropy field of the anisotropy barriers continues to increase and eventually becomes larger than the field strength applied decreasing the heat production.39 When this occurs, only the smaller fraction of MNPs is able to maintain the self-heating effect, while the larger fraction of MNPs is inactivated and reduces their growth rate. Such size dependent heating improves the homogeneity of the sample. Therefore, the manipulation of the intensity of the AMF field seems an interesting strategy to filter the final achievable crystal size and reduce the size dispersion. This explains why the well-tuned AMF2 conditions produce an earlier transition to step III than AMF3, despite having a similar average heating slope in the previous step and a lower heating power in this final period.

4.2. Ripening Mechanism

Apart from the direct observation of the aggregates, information on the mechanism of ripening arises from the evolution of the PSD during the aging process. We expect that OR will proceed continuously maintaining the monodispersity of the initial distribution, but attachment of particles (oriented or not) will lead to the appearance of a second distribution of bigger sizes.

For CP, the presence of a plateau at 10 nm (Figure 4) during the temporal size evolution along the step II and the monocrystalline nature of all the samples suggest the predominance of the OA mechanism previously postulated for MNPs prepared by conventional heating.10,42 This is confirmed by the observation of the formation of a second distribution of bigger particles during steps II and III (Figure 5). OR could cause the shift of the PSD to bigger sizes observed during step I of the process. With the OA being limited by the surface reaction,43 it is not surprising that the enhancement of OA appears at the step III, when the temperature of the particles is higher by the magnetothermal effect. The unusually rounded appearance of the particles of this size in the absence of surfactants also points in this direction.44 As a result of the OA, the average particle size goes from 9 nm for REFcp without the AMF to approximately 16 nm for both the AMF1cp and AMF4cp samples, which is closer to the sizes obtained by high-temperature organic decomposition45 with a significant reduction of the polydispersity. However, due to the broad size distribution of CP samples, we conclude that specific AMF conditions do not have a relevant effect on heating rate or size differences.

In OP, the magnetite nucleation is controlled by the dissolution of GR at much lower supersaturation that in CP. Additionally, the slow increase of temperature during the step I enables the formation of much bigger particles, the smallest being approximately the same size as the final CP particles. During strep I, the OA mechanism is evident not only by the presence of a bimodal PSD (Figure 8) but for the round shape of the particles that are agglomerated as shown at higher magnifications (Figure 8 inset). In the absence of surfactants, the transformation of such agglomerates to single particles by internal OR is observed, in agreement with previous results14 but much faster due to the exceptional thermal activation generated by the AMF. This is also responsible for the fast attainment of the equilibrium polyhedral form of the particles, as soon as the process attains the fast heating of step II. The final shift of the PSD to bigger sizes observed at the end of the process (Figure 8) could be explained by standard OR as previously mentioned at the beginning of the CP process.

5. Conclusions

The syntheses of magnetite nanoparticles from aqueous solutions by CP and OP are profoundly influenced by the way in which energy is supplied to the system. Inductive heating is an alternative method to produce an inverse temperature gradient affecting nucleation and ripening processes. Replacing conventional heating by inductive heating with AMFs enhances nanoparticle ripening, which is a complex process that consists in a combination of oriented attachment and the Oswald ripening, ending in nanoparticles sizes above 15 and 50 nm with a narrow PSD for CP and OP, respectively. AMF heating enable the preparation of iron oxide nanoparticles similar to those obtained by organic decomposition at high temperatures, by a much greener method, in aqueous solutions and also the increase of size of the nanocrystals obtained by OP. The differences in the ripening observed among the samples depend on their sizes and polydispersity that control their efficiency as nanoheaters. The coupling of the heating power supplied and the particle size obtained by the inductive heating is necessary for the efficiency of the process.

Glossary

Abbreviations

OP

oxidative precipitation

CP

coprecipitation

GR

green rust

AMF1

alternating magnetic field 1 (92 kHz–66 mT)

AMF2

alternating magnetic field 1 (170 kHz–36 mT)

AMF3

alternating magnetic field 1 (205 kHz–30 mT)

AMF4

Alternating Magnetic Field 1 (285 kHz–22 mT)

SAR

specific absorption rate

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.cgd.2c00694.

  • Heating curves of temperature versus time showing the procedure for the estimation of SAR values, XRD patterns of the final and transient products, OP and CP particle sizes, and magnetic parameters (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

This study was funded by the Spanish Ministry of Economy and Competitiveness, Mag4Spinal project, No. PID2020-113480RB-100 and the EU project H2020-FETOPEN- RIA 829162, HOTZYMES.

The authors declare no competing financial interest.

Supplementary Material

cg2c00694_si_001.pdf (704KB, pdf)

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