Influence of Synthetic Conditions on the Crystal Structure, Optical and Magnetic Properties of o-EuFeO₃ Nanoparticles

Abstract

In this paper, powders consisting of o-EuFeO₃ nanoparticles were obtained by the co-precipitation method using two different precipitating agents (NH₃ and (NH₄)₂CO₃ solutions; corresponding products were denoted as samples S.1 and S.2, respectively) and followed by annealing for 1 h at 750, 850, and 950 °C. The magnetic, optical, and physicochemical characteristics of S.1 and S.2 were evaluated by means of EDXS (energy dispersive X-ray spectroscopy), PXRD (powder X-ray diffraction), TG/DSC (thermogravimetry/differential scanning calorimetry), FTIR (Fourier-transform infrared spectroscopy), TEM (transmission electron microscopy), UV-Vis DRS (Ultraviolet–Visible diffuse reflectance spectroscopy), and VSM (vibrating-sample magnetometry). The o-EuFeO₃ structure was shown to be stable at annealed temperatures, and t = 850 °C is recommended for synthesizing such substances. The average size of o-EuFeO₃ crystals is around 27 (S.1) and 34 nm (S.2). The results revealed a homogeneous distribution of the main elements on the samples’ surfaces, with morphology consisting of isometric and highly agglomerated nanoparticles. The bandgap value (E_g) of the synthesized samples was 2.31 (S.1) and 2.39 eV (S.2). Besides, these nanoparticles appeared to possess paramagnetic behavior.

1. Introduction

To date, nanomaterials based on orthoferrites with the general formula RFeO₃ (R = Sc, Y, and Ln) have received increased attention due to their attractive perovskite-like structure, high values of the magnetooptical ratio of the specific Faraday rotation to absorption, and high domain wall mobility [1] as well as strong visible-light absorbance [2] and luminescent properties [3], etc. These properties make them fascinating in various innovative applications such as sensors and actuators [1], gas sensing devices [4], potential MRI contrast agents [5], multiferroic materials [6,7,8], catalysts for toluene oxidation [9,10], and photocatalysts in the degradation of various water-soluble dyes [11], Congo red molecules [12], and methyl orange [13,14].

Especially orthoferrites possess a magnetic structure typical of weak antiferromagnetic ordering; this interesting characteristic is being investigated by a variety of works regarding the magnetic material dependence of synthesis routes, as different synthesis methods can result in varying the product’s structure, crystallite/particle size, morphology, surface effect, etc. For example, Popkov et al. [15] discovered that the isometric morphology of o-YFeO₃ obtained via various synthesis methods increased remanent magnetization (M_r) and decreased coercivity (H_c) as crystallite size increased. In the paper [16], a relation between the zero-field exchange bias and o-LaFeO₃ particle size at room and low temperatures was demonstrated. Phokha et al. [17] established that weak ferromagnetic behavior on o-LaFeO₃ nanoparticles is caused by uncompensated spins at the surface and canted internal spins. Besides, for o-HoFeO₃ crystals, it was found that the weak ferromagnetic state is due to the Fe³⁺ ordering, and the antiferromagnetic transition is induced by the Ho³⁺ sublattice ordering at very low temperatures [18]. Moreover, our previous works showed that magnetic parameters (H_c, M_r, and M_s) varied with different annealed temperatures (650, 750, and 850 °C) of o-PrFeO₃ nanoparticles [19] and different synthesis procedures of single-phase o-HoFeO₃ nanocrystals [20].

As is known, pure RFeO₃ materials are considered to be difficult to produce as the facile and preferred formation of the R₂O₃-Fe₂O₃ system [21]. Besides that, Ristić et al. [22] stated that heat treatment (to 900 °C) should be applied in order to obtain a well-crystallized sample and eliminate the amorphous fraction of the Fe₂O₃-Eu₂O₃ system. From those issues, many synthesis methods have been attempted to acquire specific orthoferrites with desired properties, including solid-state reaction (o-HoFeO₃ [18], o-EuFeO₃ [23], o-ScFeO3 [8]), microwave-assisted (o-EuFeO₃ [13]), sol-gel method (o-LaFeO₃ [2,16], o-YFeO₃ [21], o-LnFeO₃, Ln = Eu, Gd, Tb [5], ReFeO₃, Re = Sm, Eu, Gd [11]), solution combustion synthesis (o-EuFeO₃ [24,25,26], o-HoFeO₃ [14]), nebulized spray pyrolysis (o-LaFeO₃ [27]), hydrothermal method (o-LaFeO₃ [4], RFeO₃, R = Pr, Nd, Sm, Eu, etc. [7]), polymerized complex method (o-LaFeO₃ [17]), sucrose-assisted polymethyl methacrylate-templating method (o-EuFeO₃ [10]), and co-precipitation method (o-PrFeO₃ [19], o-HoFeO₃ [20], XFeO₃, X = Y, La, Gd [28]), etc.

Among the methods mentioned, the latter is commonly performed to synthesize nanoscale solid materials from solution, in which a liquid-phase reaction is used to produce insoluble compounds. This method has been widely applied due to its simplicity and convenience. The as-synthesized precipitates may consist of fine crystalline and/or amorphous particles [29]. Therefore, heat treatment is usually conducted to produce a single phase, which results in the product having stable properties. Materials obtained through such routes provide a pure and homogeneous phase. The size, shape, and morphology of the prepared particles depend much on precipitation conditions, precursor mixtures, precipitating agents, and processing procedures [30].

In the present work, europium orthoferrite was chosen as the model of the research object. The compound has been shown to be active in photocatalytic activities [11,12,13], magnetic behaviors [7,26], multiferroic properties [24], biomedical applications [5], and others [3,9]. As described above, choosing the right method of orthoferrite preparation is important, but more important is finding the most optimal synthesis conditions for that method. Thus, in terms of the co-precipitation method, we aim to study the influence of different precipitating agents in the synthesis process on the properties of the resulting product, such as crystalline structure, morphology, particle size, optical characteristics, and magnetic behaviors.

2. Materials and Methods

2.1. Sample Preparation

The chemicals for the synthesis of EuFeO₃ nanoparticles include europium (III) nitrate hexahydrate (Eu(NO₃)₃·6H₂O, 99.9% purity, Acros Organic, Geel, Belgium), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.9% purity, Sigma-Aldrich, St. Louis, MO, USA), NH₃ solution (85% purity, density of 0.901 g/mL), ammonium carbonate ((NH₄)₂CO₃, 99.9% purity, Sigma-Aldrich, St. Louis, MO, USA), and double distilled water. The chemicals are of high purity and were used without further purification.

Experimental synthesis of EuFeO₃ nanoparticles was conducted according to the co-precipitation method mentioned in [20]. In general, the procedures are as follows:

(1)

Slowly add 50 mL of a solution containing a stoichiometric molar mixture of the salts Eu(NO₃)₃ and Fe(NO₃)₃ to 400 mL of boiling water on a heated magnetic stirrer (t ≥ 95 °C). The system was then boiled for another 15 min and then cooled to room temperature. After that, a 5% NH₃ solution was added dropwise to the reaction mixture. The amount of 5% NH₃ solution was taken in excess to precipitate all the Eu³⁺ and Fe³⁺ cations present in the solution (pH = 9–10). The resulting sample is assigned S.1;

(2)

The second procedure is carried out in the same way as the first procedure, except that the 5% NH₃ solution is replaced with a 5% (NH₄)₂CO₃ solution. The obtained sample was denoted as S.2.

In both cases, the precipitate formed was stirred for 60 min, then allowed to settle for 30 min before being vacuum-filtered and washed several times with distilled water to a pH of about 7. The as-synthesized products were annealed at 750, 850, and 950 °C for 1 h, followed by cooling to room temperature, and then finely ground and analyzed.

2.2. Sample Characterization

Energy dispersive X-ray spectroscopy (EDXS) was used to determine the quantitative and qualitative composition of the synthesized samples using an EDX H-7593 HORIBA (Horiba, UK). The quantitative elemental composition was calculated as the average of three different values obtained in each sample.

Powder X-ray diffraction (PXRD) was applied in determining the phase composition and crystal structure of the obtained powders using an EMPYREAN X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) using a step of 0.019° and a Bragg angle (2θ) of 10–80°. The average size of crystals (D) was determined according to the Scherrer equation (Equation (1)). The unit cell volume (V) of crystals was the product of lattice parameters (a, b, and c) obtained from the Profex XRD application.

$$D=\frac{k·\lambda }{\beta ·\mathrm{c}\mathrm{o}\mathrm{s}\left(\theta \right)},$$

(1)

where λ is an X-ray wavelength, nm; β is the full width at half maximum (FWHM), in radians; θ is the Bragg angle, in radians. The Scherrer constant (k) depends on how the width is determined, the crystal shape, and the size distribution. The k can vary from 0.62 to 2.08. Generally, the k is taken as 0.94 (for present paper) and 0.89 for the FWHM and integral breadth of spherical crystals with cubic symmetry, respectively. The k can also be rounded to 1.0 or 0.9 without greatly affecting the resulting calculations. A more detailed discussion of k refers to [31].

The samples were thermally analyzed using differential scanning calorimetry (DSC) and thermogravimetry (TG) on a SETARAM-Labsys Evo 1600 °C (Caluire, France) with a heating rate of 10 °C/min.

Fourier-transform infrared (FTIR) spectra were recorded with a Nicolet Model NEXUS 470 FTIR spectrometer using KBr pellets.

Transmission electron microscopy (TEM) was used to examine the morphology of the nanoparticles using a JEOL-1400 (JEOL Ltd., Tokyo, Japan).

The UV-Vis diffuse reflectance spectra (DRS) were measured with a UV-Visible Spectrophotometer UV-2600 (Shimadzu, Kyoto, Japan) equipped with an integrating sphere (specialized for measuring the optical absorption of a solid powder). The equipment used D2 (deuterium for UV light source) and WI (halogen for visible light source) lamps, typically in the wavelength range of 190–900 nm. Barium sulfate (BaSO₄) powder was used as a baseline material. The result of measuring provided the wavelength dependence of absorbance, which was then used to determine the bandgap (E_g, eV) value employing the Tauc plot method. The optical absorption coefficient (α, cm⁻¹) was calculated via Equation (2).

$$\alpha =\frac{A}{l·\log \left(e\right)},$$

(2)

where A is the absorbance, a.u.; l is the sample thickness, cm.

The hysteresis loop and magnetic parameters such as coercivity (H_c, Oe), remanent magnetization (M_r, emu·g⁻¹), and saturation magnetization (M_s, emu·g⁻¹) were derived from a vibrating sample magnetometer (VSM, MICROSENE EV11, Japan) at 300 K in a magnetic field (H) ranging from −16,000 Oe to +16,000 Oe.

3. Results and Discussion

3.1. Physicochemical Properties of the Synthesized Samples

All obtained PXRD patterns of the samples S.1 and S.2 show that the synthesized EuFeO₃ compound has an orthorhombic structure with the space group Pbnm (o-EuFeO₃, ICDD PDF #47-0066) of the typical perovskite type [5,10,24]. By varying annealed temperatures in the range of 750–950 °C, the phase composition and structure form do not seem to be different among the samples (see Figure 1). Further, Figure 2a shows the average size of o-EuFeO₃ crystals (D) calculated for the highest intensity peak (hkl = 112). As can be seen, the D decreases gradually (from 23 to 31 nm (S.1) and 30–43 nm (S.2)) as the annealed temperature decreases; this is clearly understandable as, at a higher temperature, the components in the reaction mixtures gain more motion energy, resulting in larger crystals [26]. It is demonstrated that the D obtained from the first synthesis procedure is smaller than that obtained from the second one.

Moreover, Figure 2b shows the corresponding unit cell volume (V)—the product of 3 lattice parameters a, b, and c (see Table S1)—of o-EuFeO₃ crystals derived from the Profex XRD application using the obtained PXRD data. The results reveal that as the annealed temperatures are changed, the lattice parameters and thus the V values change slightly, which can be explained by a minor distortion of the crystalline structures. However, there is almost no significant peak shift between the samples, indicating the crystalline structure’s stability [25,26].

According to the TG results of the samples S.1 (Figure 3a) and S.2 (Figure 3b), three temperature intervals can be distinguished on the curves, which are accompanied by mass losses and corresponding thermal effects: t^I = 50–200 °C is associated with the removal of water; t^II = 200–500 °C refers to the decomposition of iron(III) and europium(III) hydroxides; t^III = 500–750 °C is related to the decomposition of europium hydroxycarbonate Eu₂(OH)₄CO₃. There is a slight difference for sample S.2, that is, there is an additional mass loss at the temperature of 750–900 °C, which can be attributed to the further decomposition of europium oxycarbonate Eu₂O₂CO₃ [26,32]. It can be seen that the mass of the sample ended (Δm = 0) at a heated temperature of t > 800 °C (for sample S.1) and 900 °C (for sample S.2). The trend of the TG curves obtained in this work accords with our previous work regarding Pr-Fe-O and Ho-Fe-O systems [19,20].

Further, the DSC results have two similar facts: a broad endothermic peak observed with a maximum at 123 °C (for sample S.1) and 105 °C (for sample S.2), which is associated with the dehydration of the solvent; a significant exothermic peak observed with a maximum at 698 °C (for sample S.1) and 690 °C (for sample S.2), which refers to the crystallization of o-EuFeO₃ particles. The DSC curves also show that, in the temperature range of 300–400 °C, a small exothermic effect appears at the peak of 345 °C (for sample S.1), which may be attributed to the phase transformation of Fe₂O₃ and Eu₂O₃ particles from less to more stable ones; however, for sample S.2, an endothermic peak occurred, which can be explained by the decomposition of the hydroxides. Through the TG/DSC data, it is recommended that the heat treatment for producing stable o-EuFeO₃ should be at least 850 °C. As a result, all the following properties of the synthesized cases will apply to the samples annealed at 850 °C.

The EDXS results of the samples annealed at 850 °C for 1 h show spectra of the three expected elements (Eu, Fe, and O) with an Eu:Fe ratio that is close to the intended 1:1 (see Figure 4). Therefore, the samples synthesized in this work can be considered pure, and the synthesis method is reliable. Besides, EDX-mapping (Figure S1) reveals a homogeneous distribution of the main elements (Eu Lα1, Fe Kα1, and O Kα1), and a similar appearance between the two samples. Moreover, Figure 5a,b exhibit TEM images of the o-EuFeO₃ nanoparticles. The sample morphology consists of isometric and highly agglomerated nanoparticles, which are characteristic of nanomaterials obtained by heat treatment of various chemical compositions in air.

Furthermore, FTIR spectra in the range of 400–4000 cm⁻¹ for samples S.1 and S.2 are shown in Figure 6. The presented results indicate the presence of certain fragments in the molecules of the sample, including Eu-O and Fe-O, as well as species for H₂O and CO₂ adsorption.

Regions 416–418 and 553–555 cm⁻¹ refer to the stretching vibrations of ν(Eu-O) and ν(Fe-O), respectively [26,33];

Characteristic peaks of adsorbed moisture located at 1633–1638 cm⁻¹ and 3420–3447 cm⁻¹ for vibrations of bending δ(H-O-H) and stretching ν(H-O-H), respectively.

• Peaks in the range of 2880–2971 cm⁻¹ correspond to ν(C-H in the -CH₂, -CH₃), indicating traces of methanol used for the purpose of cleaning equipment;

Stretching modes of symmetric ν_s(CO₃²⁻), synchronous asymmetric ν′_s(CO₃²⁻), and asynchronous asymmetric ν″_s(CO₃²⁻) centered at 1046–1049, 1382–1384, and 1433–1460 cm⁻¹, respectively [34].

Sample S.1 has an additional peak of bending vibration δ(CO₃²⁻) and sample S.2 has an additional peak of stretching mode ν(C=O), which the other sample does not seem to own. When taken together, these facts only show that the samples are capable of adsorbing CO₂ and water from the air but do not represent a large difference between the two samples being compared.

3.2. Diffuse Reflectance Spectroscopy

The optical properties of o-EuFeO₃ samples annealed at 850 °C were assessed by DRS, the results of which are shown in Figure 7. The high ability of the samples’ light absorbance in the visible radiation zone, especially in the wavelength range of 400–600 nm, is presented in Figure 7a; these results indicate that the o-EuFeO₃ compound can be classified as a photoactive material that can be used, for example, in photocatalysis [12,13]. It is shown that sample S.1 exhibits a higher absorbance than sample S.2, which can be attributed to its smaller grain size. In addition, the direct bandgap values of samples S.1 and S.2 were determined to be 2.31 and 2.39 eV, respectively, indicating that they are low-bandgap materials (Figure 7b).

We have estimated the microstrain value (ε) of samples S1 and S2 (annealed at 850 °C for 1 h) using the Williamson–Hall plot according to [35]. A brief description of the method is shown in Figure S2. The calculation results are shown in

Table 1. A comparison of the optical results and other characteristics with those found in the literature.

Sample	Bandgap (Eg), eV	Microstrain (ε)	Unit Cell Volume, Å3	Average Crystallite Size (D), nm	Ref.
S1-850	2.31	1.83·10−3	230.83	26.5	This work
S2-850	2.39	1.99·10−3	231.20	34.1
YFeO3	2.11	0.183 (lattice strain)	224.15	176.4	[37]
Y0.85Sm0.15FeO3	2.16	0.488 (lattice strain)	225.81	106.3
SmFeO3-800	2.72	1.3·10−3	231.89	78.7	[38]
SmFeO3-850	2.79	1.4·10−3	232.09	105.4
BiFeO3-500	2.30	–	–	38.8	[36]
BiFeO3-550	2.32	–	–	65.0
PrFeO3-750	1.66	–	238.11	24.4	[19]
HoFeO3-850	2.62	–	225.35	26.3	[20]
LaFeO3-800	2.10	–	241.33	47.3	[39]
YbFeO3-800	2.16	–	–	65.5	[40]

. The variation of E_g can be attributed to more than one reason; a detailed discussion refers to [36,37,38]. In this work, the increase in energy gap may be related to the increase in lattice spacing as unit cell volume increases as well as crystallite size. The relationship obtained between E_g and microstrain is that they are directly proportional; the trend is similar to that of another work [38].

3.3. Magnetic Performance

Figure 8 depicts the magnetic performance of o-EuFeO₃ samples (annealed at 850 °C for 1 h) as a magnetization dependence of the magnetic field (M-H) at 300 K. The obtained M-H curves are typical of highly positive linearity in the applied field from −15 to +15 kOe, indicating the magnetic behavior is of the paramagnetic type [19,26,41]. The magnetic properties of such materials can be enhanced by oxygen defect engineering, as shown in [42]. Apparently, the S-shape of M-H curves is not observed in this work, which is commonly related to the superparamagnetism of materials [43]; there is no M_s as the M-H response monotonically increases with the field’s growth.

4. Conclusions

5% NH₃ (1st procedure) and (NH₄)₂CO₃ (2nd procedure) solutions were used as precipitating agents in the co-precipitation method for synthesizing materials based on orthorhombic EuFeO₃ particles (o-EuFeO₃). For the convenience of comparison between these two procedures, their products were designated as samples S.1 and S.2. As was shown in PXRD results, by varying annealed temperatures in the range of 750–950 °C, the phase composition and structure form do not seem to be different among the samples; the average size (D) of o-EuFeO₃ crystals decreases gradually as the annealed temperature decreases; in this case, the D obtained from the first procedure is smaller than that obtained from the second one. According to the TG/DSC results, the heat treatment for completing o-EuFeO₃ synthesis was 800 and 900 °C for samples S.1 and S.2, respectively. It was observed from TEM graphs that the sample morphology consists of isometric and highly agglomerated nanoparticles. Further, FTIR results revealed that the samples are capable of adsorbing CO₂ and water from the air. Moreover, sample S.1 exhibited a higher light absorbance in the visible radiation region than sample S.2. It was found that the samples appear to behave as paramagnetic materials. The findings established a suitable synthesis condition in the co-precipitation method for obtaining material with desired properties, specifically in photocatalysis and/or high-magnetic-field devices.