The Effect of EDTA Functionalization on Fe₃O₄ Thermal Behavior

Abstract

The surface of Fe₃O₄ nanoparticles is very reactive and can oxidize to γ-Fe₂O₃ (maghemite) and α-Fe₂O₃ (hematite) structures. Based on this, the oxidation process of Fe₃O₄ nanoparticles must be prevented, and one of the strategies is surface functionalization with organic or inorganic molecules. Thus, this study analyzed the thermal behavior of Fe₃O₄ and Fe₃O₄-EDTA nanoparticles using X-ray diffraction (XRD), simultaneous thermogravimetry-differential thermal analysis (TG-DTA), differential scanning calorimetry (DSC). Results showed that γ- Fe₂O₃ was obtained as an intermediate in Fe₃O₄ and Fe₃O₄-EDTA decomposition, as confirmed by TG-DTA and DSC curves. Moreover, Fe₃O₄-EDTA exhibited a temperature peak (T_p = 573.5°C) of phase transformation (γ-Fe₂O₃ → α-Fe₂O₃) higher than that of Fe₃O₄ (T_p = 533.0°C), confirming that EDTA molecules stabilized the nanoparticles efficiently. The kinetic behavior of samples changed, and the activation energy for functionalized samples decreased.

Keywords:
Magnetite; Maghemite; Hematite; Non-isothermal kinetics

1. Introduction

Magnetite (Fe₃O₄) is an oxide formed by Fe³⁺ and Fe²⁺ ions in a 2:1 molar ratio, in the pH range between 9 and 14, and preferably in the absence of oxygen^1,2. Its unit cell is based on a cubic system (face-centered cubic), which provides different O^2- ion coordination according to the type of interstitial: in the octahedral, the anions to both iron ions (Fe²⁺ and Fe³⁺) are coordinated; while in the tetrahedral, only Fe³⁺ is coordinated³. The Fe³⁺ ions are equally distributed in octahedral and tetrahedral interstices, while all Fe²⁺ ions only occur in octahedral interstices. Hence, the magnetic behavior of Fe₃O₄ is associated with this ion^4-6. Considering their position in the crystalline structure, Fe²⁺ ions can easily oxidize to Fe^3+5,7. Consequently, the co-precipitation must be synthesized under an inert atmosphere (nitrogen or argon) to prevent the oxidation of Fe²⁺ to Fe³⁺. The main crystalline phases of iron oxide are magnetite (Fe₃O₄), maghemite (γ- Fe₂O₃), and hematite (α- Fe₂O₃). The first and second phases show similar properties, but α- Fe₂O₃ exhibits distinct characteristics from the others^3,7-10. Currently, Fe₃O₄ nanoparticles have received increasing attention due to their simple syntheses and stable storage in the colloidal form^7,11. Nevertheless, the agglomeration of nanoparticles is expected over time to reduce the surface energy provided by the large surface area of nanoparticles^1,12-14. Moreover, the oxidation process reduces magnetic properties. Based on this, strategies must be developed to provide colloidal protection and chemistry stabilization for Fe₃O₄ nanoparticles. For instance, there are already nanoparticles covered with organic (alkoxysilanes, proteins, etc)¹⁵ or inorganic (metal oxides, metals, etc) species¹⁶. The surface functionalization of nanoparticles can introduce new functions to nanoparticles, as it can change the chemical and physical properties. In the case of Fe₃O₄ nanoparticles, it appears that the functionalization can improve the colloidal stability^1,2,17-23, increase the chemical and thermal stability against the oxidation reaction²³, but depending on the functionalizing agent it can decrease the magnetic property^24-26 The oxidation reaction of the Fe₃O₄ nanoparticles is one of the factors that contributes to the decrease in the magnetic properties. In addition to these functions mentioned above, Marcos-Hernández et al.²⁷ studied nano-Fe₃O₄ electrodes with the aim of electrochemical reduction of nitrate. This expands the application possibilities of functionalized nanoparticles.

Temperature changes can promote several chemical and physical processes in nanoparticles. Under heating, Fe₃O₄ converts to other oxides, following the reaction sequence showed in reaction (1) ^3,9-11:

$2F{e}_3{O}_4+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.O_2\to 3\gamma -F{e}_2{O}_3\to 3\alpha -F{e}_2{O}_3$ Reaction(1)

Hence, a thermal study with simultaneous thermogravimetry-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) associated with X-ray powder diffraction (XRPD) can provide information about the influence of functionalization on the oxidation of Fe₃O₄. Functionalization is not expected to change the reaction sequence but only modify the mechanism and stabilize the crystalline phase. Non-isothermal kinetics could investigate the modification in the reaction mechanism, as data are obtained in situ, which is considered an advantage and, therefore, has been applied to several materials^28-42. Thus, this study aims to investigate the influence of EDTA functionalization on thermal behavior and the sequence of reactions observed when heating Fe₃O₄ nanoparticles.

2. Materials and Methods

2.1. Chemicals

The reagents used for the synthesis were iron (II) chloride tetrahydrate (Merck), iron (III) chloride hexahydrate (Sigma Aldrich), ammonium hydroxide ~ 28% (Synth), acetone P.A. (Synth), and Ethylenediaminetetraacetic acid- EDTA (99%) (Synth).

2.1. Synthesis and modification of Fe₃O₄

The nanoparticle syntheses were reported in a previous study²³. Quantities of 0.03 mol of iron (III) chloride hexahydrate and 0.015 mol of iron (II) chloride tetrahydrate were mixed in 100 mL of distilled water. Then, 10 mL of ammonium hydroxide was added to the system, followed by 50 mL of EDTA solution (0.002 mol L^-1). The mixture was stirred for 1 hour at room temperature in an inert atmosphere (nitrogen). The resulting precipitate was separated with a magnet and washed several times with distilled water. Then, the samples were dried at 60.0°C in a laboratory oven. This methodology leads to the formation of Fe₃O₄-EDTA nanoparticles. The Fe₃O₄ nanoparticles were obtained in the same way but without adding the EDTA solution.

2.2. Analytical instruments

2.2.1. Simultaneous thermogravimetry-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC)

The TG-DTA curves were obtained with a thermal analysis system from Netzsch, model STA 449 F3. Approximately 5 mg of the sample was measured and placed in an open crucible of 200.0 µL of α-alumina. A dry air atmosphere was used, with a flow rate of 70.0 mL min^-1, and the temperature ranged between 30-800°C. The heating rate was 10.0°C min^-1.

The DSC was analyzed on a Mettler-Toledo equipment, model DSC 1 Star^e System. The sample mass was approximately 4 mg, placed in an open crucible of 40.0 µL of α-alumina, with flow and heating rates of 50.0 mL min^-1 and 10.0°C min^-1, respectively. The experiment was performed under air and nitrogen atmospheres. Under air, the temperature ranged from 25°C to 300°C, while under nitrogen, the sample was heated from 25.0-150.0°C for 5 minutes (to eliminate the residual water in the sample), then cooled to 0°C, and heated to 400.0°C. It is worth noting that before this experiment, Fe₃O₄ was heated to 140.0°C to eliminate the water.

2.2.2. X-ray powder diffraction (XRPD)

The X-ray powder diffraction was measured in a Rikagu, model MiniFlex 600, using Cu Kα radiation (λ = 1.54056Å) and settings of 40 kV and 20 mA in the 2θ range of 10 to 80°, 10° min^-1.

2.2.3. Kinetic parameters

Non-isothermal kinetics was analyzed following the ICTAC recommendations⁴³. The curves with different heating rates (5.0, 10.0, 15.0, and 20.0°C min^-1) were obtained in the same equipment and experimental conditions as the previous topic in a dry air atmosphere. The data were processed with the NETZSCH kinetics Neo Trial software by Netzsch⁴⁴. The Friedman model-free method was used^{45, a} derivative method that is more sensitive than integrative ones³⁵. Its expression is shown in Equation 1:

$ln[{\beta }_i{(\frac{d\alpha }{dT})}_{\alpha ,i}]=ln\left[f\left(\alpha \right)A_{\alpha }\right]-\frac{E_{\alpha }}{R{T}_{\alpha ,i}}$Equation(1)

where β_ι is the heating rate (°C min^-1), R is the gas constant (8.3145 J K^-1 mol^-1), and A_α (the unit of the lnA_α is cm³ mol^-1 s^-1) and Ε_α (kJ mol^-1) parameters are the pre-exponential factor and activation energy, respectively.

3. Results and Discussion

3.1. Thermogravimetry-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC)

Figure 1 shows TG-DTA curves. The TG curve of Fe₃O₄ (Figure 1a) shows a mass loss between 30.0 and 120.4°C (Δm = 1.86%), which refers to the water evaporation of the sample. After this thermal event, there was a small mass increase (Δm = 0.54%) in the TG curve between 160.0 and 288.9°C, and this value did not change up to 800°C.

Figure 1
TG/DTA curves of Fe₃O₄ (a) and Fe₃O₄-EDTA (b).

Some studies in the literature have reported that, under air atmosphere, γ- Fe₂O₃ is obtained as an intermediate in Fe₃O₄ thermal decomposition^32,36,46, which causes a mass gain in the TG curve. This change in sample mass is resulted from oxygen (O₂) incorporation and has been well described by Sanders and Gallager³², in which they have suggested a complex process for Fe₃O₄ thermal decomposition into a-Fe₂O₃ based in the global reaction (2):

$$2F{e}_3{O}_4+O_2\to X\to Y\to 3a-F{e}_2{O}_3$$ Reaction(2)

In the present reaction, X and Y species can be considered as γ- Fe₂O₃ and are intermediates of the reaction, since its consumed immediately after is formation³². In addition, the experimental value obtained by the authors (∆m_exp = + 3.47%) was very close to the theoretical value (∆m_exp = + 3.46%), corroborating the suggested mechanism. Nie et al.³⁴ monitoring the surface structure of magnetite (100) during O₂ exposure at a temperature of 650°C and the authors found no evidence of maghemite formation. The oxidation of Fe₃O₄ depends on its origin and nature⁴⁶, besides other factors such as temperature and sample size^3,47. Considering that the mass gain observed did not result in any signal in the DTA curve, and the Δm observed is lower than the equipment error (1%), it was impossible to confirm the formation of γ- Fe₂O₃ as an intermediate of Fe₃O₄ synthesized in this study. The DSC was analyzed to confirm this event and will be discussed later. Despite the absence of any other event in the TG curve until the end of the analysis, the DTA curve showed an exothermic peak at 533.0°C, which could be associated with a formation in the α- Fe₂O₃ phase.

Figure 1b shows the TG-DTA curves of Fe₃O₄-EDTA nanoparticles. Between 30.0 and 351.2°C, there was a subtle and continuous mass loss (Δm = 5.20%) associated with water evaporation and degradation of organic matter from EDTA molecules. One of our previous studies²³ discussed the interaction between Fe₃O₄ nanoparticles and EDTA molecules, and the results showed that the Fe₃O₄ surface is binding to the carboxylates of the EDTA molecules. Based on this, it is suggested that EDTA molecules stabilize Fe₃O₄ nanoparticles²³. As a result, the exothermic event associated with a formation in the α- Fe₂O₃ phase was dislocated in the DTA curve to a higher temperature (573.5°C) than that of non-functionalized Fe₃O₄ (533.0°C). This result corroborates the stabilization of Fe₃O₄ nanoparticles, which are less susceptible to oxidation when functionalized with EDTA and corroborate the previous result obtained by other techniques for this system²³. In addition, it also supports the stabilization of Fe₃O₄ nanoparticles by the coprecipitation method with organic molecules, such as also observed when used chitosan¹⁷.

The temperature overlaps of organic matter decomposition of Fe₃O₄-EDTA did not allow verifying whether there was any mass gain in the TG curve between 160.0 and 288.9°C. It is possible to affirm that the events in DTA curves are endothermic or exothermic considering the technique principle, which in a very simplified description indicates the temperature difference between the sample and a reference, which are exposed to the same controlled experimental conditions. In all DTA curves, the axis direction must be represented; as a result, in the present paper, they are exhibited as an up arrow in all DTA curves. The complete technique details can be found in specialized reference⁴⁸.

Figure 2 shows the DSC curve under the air atmosphere of Fe₃O₄. There was an endothermic event (T_p = 51.1°C and ΔΗ = 39.2 J g^-1) associated with water evaporation in magnetite. From 187.8°C, the baseline increases proportionally with the temperature, indicating a potential material change. However, there is an evident exothermic event between 247.0 and 280.1°C (T_p = 263.6°C and ΔΗ = 2.1 J g^-1). These events agree with those of the TG curve of Fe₃O₄ (Figure 1a).

Figure 2
DSC curve of Fe₃O₄ under air atmosphere_.

Figure 3 shows the cycle of DSC curves of Fe₃O₄ under a nitrogen atmosphere to investigate this event better. The first heating procedure (Figure 3a) showed a small endothermic event between 75.7°C and 132.0°C associated with residual water evaporation. The first cooling procedure showed many thermal events. However, the second heating procedure showed an exothermic peak (T_p= 339.7°C) with ΔΗ= 79.4 J g^-1. Under a nitrogen atmosphere, the temperature peak (T_p) was dislocated to a higher value, which could be explained by the occurring oxidation reaction: converting Fe²⁺ to Fe³⁺ in Fe₃O₄ resulting in γ- Fe₂O₃. Although the atmosphere used is considered inert, the nitrogen cylinder has a minimum amount of oxygen enough to cause the reaction under higher temperatures.

Figure 3
Cycle of DSC curves of Fe₃O₄ under nitrogen atmosphere: 0-150°C (a), 150-0°C (b), and 0-400°C (c).

Both experiments (air and nitrogen) showed a thermal event, confirming that the Δm observed in the TG curve of Fe₃O₄ (0.54%) was a mass gain instead of a baseline deviation. Moreover, this event might have resulted from the formation of γ- Fe₂O₃. The DSC of Fe₃O₄-EDTA was not analyzed because the organic thermal degradation of the sample starts at low temperatures, which could mask other signals in the DSC curve.

3.2. X-ray powder diffraction (XRPD)

The samples of Fe₃O₄ and Fe₃O₄-EDTA were heated from room temperature to 350°C and 600°C to compare the phases obtained at each temperature. Then, the samples were analyzed with X-ray diffraction. Comparing the diffractograms of Fe₃O₄ at room temperature (Figure 4a) with Fe₃O₄ at 350°C allowed verifying a small peak around 2θ = 25° associated with the α-Fe₂O₃ structure. The α-Fe₂O₃ has a hexagonal structure seen in the curve of Figure 4c. However, the diffractogram at this temperature is similar to the structures of Fe₃O₄ and γ-Fe₂O₃ with cubic systems³. Thus, the DSC data suggests that Fe₃O₄ nanoparticles were oxidized to γ-Fe₂O₃, and the transformation to α-Fe₂O₃ was initiated at a peak temperature of around 533°C (Figure 1). Above this peak temperature, the α-Fe₂O₃ formation reaction was completed, as shown in the diffractogram of Figure 4c. Although the literature reports that γ- Fe₂O₃ is completely converted to α- Fe₂O₃ above 325°C, this study only showed signs of the beginning of α-Fe₂O₃ formation reaction at a temperature of 350°C.

Figure 4
X-ray diffractograms of nanoparticles at different temperatures: Fe₃O₄ at room temperature (a), Fe₃O₄ at 350°C (b), Fe₃O₄ at 600°C (c), Fe₃O₄-EDTA at room temperature (d), Fe₃O₄-EDTA at 400°C (e), and Fe₃O₄-EDTA at 650°C (f). It agrees with the peak pattern from Crysmet 867299 (Fe₃O₄), Crysmet 967300 (γ- Fe₂O₃), and Crysmet 854228 (α- Fe₂O₃).

The surface functionalization of Fe₃O₄ nanoparticles was performed in a single step and this changed the mechanism of formation of the functionalized nanoparticles, decreasing the size of the nanoparticles²³ and consequently changing the properties of their colloidal²³ and thermal properties of the nanoparticles. The diffractogram results of EDTA-functionalized nanoparticles showed the formation of α-Fe₂O₃ only at 600°C. This indicates that functionalization has protected from the oxidation reaction compared to non-functionalized nanoparticles.

3.3. Kinetic results of the formation of α-Fe₂O₃

Some studies have reported kinetic analyses on the solid-solid transition of Fe₃O₄ to γ- Fe₂O₃^32,36. A non-isothermal kinetic study was performed to investigate better the second phenomenon (α-Fe₂O₃ formation) and the functionalization effect on phase transition. Figure 5 presents the DTA curves obtained at different heating rates for each sample. A kinetic model should provide pertinent information about the dependence between activation energy and extent of reaction, as well as determine the kinetic triplet (E_α, A_α, and reaction type).

Figure 5
DTA curves at different heating rates for the formation of α- Fe₂O₃ from Fe₃O₄ (a) and Fe₃O₄-EDTA (b).

The software allowed fitting the extent of reactions with the Friedman model-free method⁴². Figure 6 shows the graphic dependence of E_α vs. α from the formation of α-Fe₂O₃ obtained for Fe₃O₄ and Fe₃O₄-EDTA samples. For Fe₃O₄, the activation energy values were almost constant at around 149.0 kJ mol^-1 up to α = 41.0%. Then, there was a constant decrease in activation energy values up to 82.0 kJ mol^-1. This profile (without shoulders or high variations in activation energy values with the increase in the extent of reaction) suggests an independent reaction (A → B)⁴². The graphic dependence profile of E_α vs. α of the Fe₃O₄-EDTA sample showed different behavior. The activation energy value begins at 234.0 kJ mol^-1 and decreases slowly up to 224.0 kJ mol^-1 (α = 67.0%). Hence, the value keeps decreasing up to 211.0 kJ mol^-1. Moreover, these values were higher than that of the non-functionalized material. Although the value changes were small, the profile did not suggest the occurrence of independent reactions.

Figure 6
$E_{\alpha }$ dependence on α by non-isothermal analyses of DTA data for Fe₃O₄ and Fe₃O₄-EDTA samples using the Friedman method.

The transformation of γ- Fe₂O₃ into α- Fe₂O₃ requires the atomic rearrangement from a cubic to a hexagonal system. This process occurs with the diffusion of iron and oxygen atoms. However, there was a different diffusion of these atoms (Fe₃O₄ to γ- Fe₂O₃), consequently creating vacancies in the crystalline structure. Then, these vacancies were consumed in the transition of γ-Fe₂O₃ to α-Fe₂O₃³⁴. The hematite nucleus previously formed catalyzed the reaction (autocatalytic), justifying the decrease in activation energy values observed in Figure 6 for the Fe₃O₄ nanoparticles.

Besides modifying the Fe-O bond by nanoparticle functionalization, the sample surface also changed²³. Additionally, the literature shows that γ- Fe₂O₃ grows from the core to the surface of magnetite³⁴. These facts corroborate the kinetic data obtained in this study, which shows a decrease in activation energy for EDTA-functionalized nanoparticles.

4. Conclusions

The γ- Fe₂O₃ was obtained as an intermediate in Fe₃O₄ and Fe₃O₄-EDTA decomposition, as confirmed by TG-DTA and DSC curves. Moreover, the Fe₃O₄-EDTA exhibited a higher temperature peak (T_p = 573.5°C) of transition (γ- Fe₂O₃ → α-Fe₂O₃) than Fe₃O₄ (T_p = 533.0°C), observed in DTA curves, confirming that EDTA molecules stabilize the nanoparticles efficiently. Then, the transformation reaction also changed, decreasing the activation energy of EDTA-functionalized nanoparticles.

5. Acknowledgements

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES) - Finance Code 001. The authors would like to thank CAPES (proc. 024/2012 and 011/2009 Pro-equipment), POSMAT/UNESP, FAPESP (processes: 2013/09022-7, 2015/00615-0, and 2016/01599-1), and CNPq (Processes 302267/2015-8 and 302753/2015-0) for the financial support.

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Publication Dates

• Publication in this collection
  28 Oct 2022
• Date of issue
  2022

History

• Received
  11 July 2022
• Reviewed
  21 Sept 2022
• Accepted
  06 Oct 2022