1 Introduction

LaFeO3-type perovskite is one of the most important materials and has attracted attention because of their wide applications, such as electrodes materials for fuel cells, catalysts, chemical sensors, optoelectronic devices [1–5], etc. The substitution with various transition metal (TM) ions in the composition site is observed to improve the electrical/magnetic properties. In earlier report, (Pb0.8La0.2)(Ti0.8Fe0.2)O3 sample originates from combining A-site (lone pair) ferroelectricity with B-site magnetic order among substitution at A-site and B-site. Results of detailed dielectric, ferroelectric, and magnetic studies are reported to show that the broad transitions in dielectric spectra correspond to the magnetic ordering in the sample [6]. Therefore, the incorporation of divalent or trivalent cations into the La or Fe sub-lattices has been major investigated as alternatives [6–8].

Recently, the magnetic properties of LaFeO3 have been extensively studied but the magnetic study of LaFeO3 nanoparticles is rare [9]. Antiferromagnetic nanoparticles always show unusual magnetic properties due to the finite-size effects, surface anisotropy effects, interface effects, shape anisotropy effect and so on [10–12]. In addition, the introduction of Ba2+ ions into La3+ site causes a decrease in the number of covalence bonds leads to the limitation of displacement in octahedron [13]. Moreover, mixed valences of Fe3+/Fe3+/Fe4+ by oxygen non-stoichiometry of the materials were expected for improvement the magnetic property. Therefore, nano-size of LaFeO3 system has been major investigated as an alternative. Various types of LaFeO3 nanoparticles can be synthesized by many methods such as sol–gel [14–16], co-precipitation [17], bull milling [18], sonochemical [19], and hydrothermal [20].

Polymerized complex (PC) method has been synthesized on polyester network between citric acid (CA) and ethylene glycol (EG) solution. This solution plays an important role to balance the difference in individual behavior of metal ions in solution. It results in a better distribution of ions and prevents the separation of components at later process stages. Initially, the metal ions are introduced into the solution. During the heating reaction, the complexes and production of polymer gel were formed above 100 °C and then the oxidation and pyrolysis of the polymer matrix begin also above 400 °C. This step leads to forming amorphous oxide or carbonate precursor [21]. Therefore, the phase-pure with high-quality LaFeO3 nanocrystalline has been successfully prepared at lower temperature by PC method [22, 23]. This method is suitable in recent years for synthesizing magnetic nano-particles due to the magnetic properties are usually strongly dependent on the particle size.

In this study, the PC method is used to synthesize LaFeO3 nanoparticles and the effect of the particles size on the magnetic properties of LaFeO3 was investigated and discussed. The synthesized LaFeO3 nanoparticles were characterized by thermogravimetric-differential thermogravimetric(TG-DTG), X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Visible spectroscopy (UV–Vis), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The magnetic properties of LaFeO3 nanoparticles were investigated using a vibrating sample magnetometer (VSM) at room temperature (RT).

2 Experimental

In the preparation of LaFeO3 nanoparticles, LaN3O9·6H2O (Aldrich), Fe(NO3)3·9H2O (Kanto) and C6H8O7 (Analar Normapur) were dissolved in EG of 180 ml to promote the polyesterification. The lanthanum (III) and iron (III) ions are bound by the strong ionic bonds between the metallic ions and carboxylate ions in a polymeric chain or between the polymeric chains. The polymeric precursor compound is schematically represented in Fig. 1. The precursor solution was stirred at RT for 6 h and then heated at 180 °C until dried. Then the final product was pre-calcined in a furnace at 400 °C for 2 h, leading to the partial decomposition of the polymeric gel. The precursor was calcined at 750, 850, 950 and 1,050 °C for 2 h in air to obtain single phase.

Fig. 1
figure 1

Schematic representation of polymeric precursor in PC method of LaFeO3 nanoparticles

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The prepared samples were characterized using TG-DTG, XRD, TEM, UV–Vis, XPS, XANES, and VSM. Thermogravimetric-differential thermogravimetric analysis (Mettler Toledo Stare System TG-DSC), and X-ray diffractometer (XRD) using Cu Kα radiation with λ = 0.154184 nm (Bruker D2 Phaser, Germany) were used to study the phases of the LaFeO3 samples. The morphology and crystal structure of the samples were performed using TEM (FEI, TECNAI G2 20, Netherlands). UV–Vis was performed using a UV-3101PC UV–Vis-NIR scanning spectrometer (Shimadzu, Japan). XPS analysis was performed by AXIS Ultra DKD (Kotros analytical Ltd, Manchester, United Kingdom). The samples were excited with X-ray hybrid mode of monochromatic Al Kα1,2 radiation at 1.4 keV. XANES spectra of Fe K edge was studied using X-ray absorption near edge spectroscopy in transmission mode at the BL5.2 line at Siam Photon (Synchrotron Research Institute) in Nakhon Ratchasima, Thailand. The magnetic measurements were performed at RT using a vibrating sample magnetometer (VSM, Versa Lab, Quantum Design, USA).

3 Results and discussion

The thermal decomposition and crystallization temperature of the LaFeO3 precursor were obtained by TG-DTG analysis as shown in Fig. 2. The TG curve showed three steps of weight loss between 30 and 1,100 °C. The weight of the sample remained constant at around 700 °C until the temperature up to 1,100 °C, indicates that the reaction is complete and no evidence of a phase transition is present in the sample. On the DTG curve, three peaks were observed at ~238, 506 and 618 °C, suggesting that the thermal events related to the burn-out of moisture and trapped solvent (water and carbon dioxide), and nitrates corresponding reaction of LaCO3OH into La2O2CO3, and La2O2CO3 with Fe2O3 into LaFeO3 [24].

Fig. 2
figure 2

TG-DTG curve of LaFeO3 precursor

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Figure 3 shows the XRD patterns of the precursor and LaFeO3 samples calcined at various temperatures. The precursor showed amorphous with some weak peak of orthorhombic phase. With increasing the temperature to 750 °C, the diffraction peaks become stronger and sharper, indicating crystallinity of LaFeO3 becomes better during the calcination process. The calcined samples exhibited peaks consistent with the orthorhombic structure of LaFeO3 in the standard from JCPDS 88-0641 and no diffraction peaks corresponding to impurity phases were observed. The Rietveld analysis confirmed a single phase orthorhombic structure (space group Pnma (62)) without any impurity phase. Figure 4a, b show the Rietveld refined plot of the samples calcined at 750 and 850 °C, respectively. The final values for the quality factors and convergence are listed in Table 1. The average crystallite sizes of all samples were calculated from X-ray line broadening of the peaks at (101), (121), (220), (202), and (123) planes using Scherrer’s equation. The crystallite sizes were obtained to be 44.5 ± 2.4, 55.6 ± 2.9, 66.2 ± 4.0, and 74.1 ± 4.0 nm for the samples calcined at 750, 850, 950 and 1,050 °C, respectively, which increased with the increase in the calcination temperature. The values of the lattice parameter (a, b, c) were in good agreement with that of orthorhombic LaFeO3 (JCPDS 88-0641) with a = 0.5564 nm, b = 0.7855 nm, c = 0.5556 nm.

Fig. 3
figure 3

XRD patterns of precursor and LaFeO3 samples calcined at various temperatures for 2 h in air

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Fig. 4
figure 4

Refined XRD patterns of LaFeO3 samples calcined at a 750 °C and b 850 °C. The experimental data are indicated by the dots and the calculated data by the solid line overlaying them and the bottom curve shows the difference between the experimental and calculated data

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Table 1 Crystallite sizes from XRD line broadening, lattice parameter (a, b and c) calculated from Rietveld refined XRD patterns, and magnetization M of LaFeO3 nanoparticles calcined in air at 750, 850, 950 and 1,050 °C for 2 h
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The TEM images with selected area electron diffraction (SAED) patterns of the LaFeO3 samples are shown in Fig. 5. The samples showed small nanoparticles with particle size below 100 nm except for the sample calcined at 1,050 °C. Particle size of the sample 950 °C-calcined clearly shows that particles become larger as the calcination temperature increases, which agree with the XRD results. The SAED patterns of the samples showed spotty ring patterns indicative of a polycrystalline structure, which agree with the XRD results.

Fig. 5
figure 5

TEM images with SAED patterns of the LaFeO3 samples calcined at a 750 °C, b 850 °C, c 950 °C and d 1,050 °C

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Figure 6a shows the UV–Vis absorption spectra of the LaFeO3 nanoparticles. All samples showed a strong absorption in the ultraviolet (~200–400 nm) and visible light region (~400–800 nm). This absorption is interesting because LaFeO3 could be developed a new visible light photocatalyst. The direct band gap energy (E g) was determined by fitting the absorption data to the direct transition as equation αhν = A(hν−E g )1/2, where α is the optical absorption coefficient, hν is the photon energy, E g is the direct band gap, and A is a constant [25]. The extrapolation of the linear portions of the curves toward absorption equal to zero (y = 0) gives E g for direct transitions (see Fig. 6b). The estimated direct band gaps of all samples were in the range of 2.15–2.23 eV. These band gaps are closed to the values reported in the literature for LaFeO3. For examples, Parida et al. [15] and Yang et al. [26] have reported direct band gap value of 2.1 eV for LaFeO3 nanoparticles synthesized by sol–gel auto-combustion method. Saad et al. [16] have reported a direct band gap value of 2.15 eV for LaFeO3 nanoparticles prepared by sol–gel combustion method. These small band gaps of LaFeO3 are interesting for application in photocatalytic, sensor materials and electrode material in solid oxide fuel cells (SOFCs) [15, 27–30].

Fig. 6
figure 6

a Room-temperature optical absorbance spectra of the LaFeO3 samples b plot of (αhv)2 as a function of photon energy for LaFeO3 nanoparticles

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The valence states of La, and Fe in the prepared samples were investigated by XPS, which is more sensitive to surface. The XPS spectra of La3d, Fe2p, and O1s of LaFeO3 nanoparticles for samples calcined at 850 and 1,050 °C were measured at RT as shown in Fig. 7a–c for the sample calcined at 850 °C, and Fig. 7d–f for the sample calcined at 1,050 °C. Figure 7a, d show that the peak position at approximately 833.5–833.6 and 850.3–850.6 eV are assigned to La 3d5/2 and La 3d3/2 [15, 31], respectively, indicating that the La ions are in the La3+ ions. The Fe peak of the samples included two components of Fe3+ (709.9–710.0 and 723.2–723.5 eV) and Fe4+ (711.6–711.7 and 724.9–725.2 eV) [15, 32] as shown in Fig. 7b, e. This result indicates that the Fe ions in our LaFeO3 samples are in a mixed of the Fe3+ and Fe4+ valence state. For O ions in LaFeO3, the samples showed similar peak position of crystal lattice oxygen (OL) at approximately 529.2–529.3 eV and hydroxyl oxygen (OH) at approximately 531.3–531.6 eV, indicating that it is attributed to the contribution of La–O and Fe–O in LaFeO3 crystal lattice for the OL signal [31, 32] as shown in Fig. 7c, f.

Fig. 7
figure 7

XPS spectra of La3d, Fe2p and O1s for the LaFeO3 samples calcined at 850 and 1,050 °C

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To confirm the valence states of La and Fe in the prepared LaFeO3 samples, we performed experiments by measuring the XANES. The XANES spectra at Fe K-edge were measured in transmission mode at RT. Figure 8 shows edge energies of the Fe foil, Fe3O4 (Fe2+, Fe3+) standard, Fe2O3 (Fe3+) standard, and LaFeO3 samples calcined at different temperature for comparison. The shift of the edge position can be used to determine the valence state. From Fig. 8, edge position of Fe3O4 (Fe2+, Fe3+) standard is approximately 7,124 eV, while Fe2O3 (Fe3+) standard is approximately 7,125 eV. In addition, edge position of FeO (Fe2+) standard (as not shown) is approximately 7,120 eV. These can be used simply as a fingerprint of phases and valence state. It is seen that the edge positions of all the samples were upper than those of the Fe3+ at approximately 7,126 eV. Thus, this result indicated that most of the Fe ions in our samples are in a mixed valence state of Fe3+ and Fe4+, which agree with the XPS results.

Fig. 8
figure 8

XANES spectra at the Fe K absorption edge of the Fe foil, Fe3O4 (Fe2+, Fe3+) standard, Fe2O3 (Fe3+) standard and LaFeO3 samples calcined at different temperatures for comparison

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The magnetization curves obtained from VSM measurements for all LaFeO3 samples measured at RT with magnetic field (H) in the range of ±10 kOe are shown in Fig. 9. All the LaFeO3 samples exhibit weak ferromagnetic behaviour with the highest magnetization (M) at 10 kOe of ~1.0 emu/g, HC of ~25–125 Oe. The magnetizations of the LaFeO3 samples in this work are summarized in Table 1. It can be seen that nanoparticles of LaFeO3 show an increase of M with decreasing nanoparticle size as 44.5 ± 2.4 nm in size. This result is of great interest because bulk LaFeO3 is antiferromagnetic behavior due to the superexchange interactions between these neighboring Fe3+ ions of Fe 3+–O 2−–Fe 3+. The origin of ferromagnetism in our LaFeO3 nanoparticles could be a result of several reasons. The first possibility is because of the large fraction of uncompensated spins from the surfaces of the nanocrystals and the canted internal spin. This behavior has been also observed in other perovskites LaFeO3 samples as well as in BiFeO3 [33, 34] and YFeO3 [35]. Therefore, for ferromagnetism in perovskites oxides, the small crystallite size is also important consideration because of the increase in surface area. The second is the double exchange (DE) interaction, which is often reported for several oxides. In this work, the XPS and XANES results show the evidence of Fe3+ and Fe4+ ions in LaFeO3, which can be attributed to the mechanism of Fe 3+–O 2−–Fe 4+ ions for DE interaction. Therefore the RT-FM in these samples is suggested as being a result of the FM according to the DE. Finally, ferromagnetism may be originated by TM clusters such as Fe, FeO or Fe2O3. Fe is a well-known ferromagnetic material, as FeO (TN ∼ 200 K) and Fe2O3 (TC ∼ 240 K) show antiferromagnetic or weak ferromagnetic behavior at low temperature [36]. Our FM results are not from the presence of TM metal clusters because if TM clusters exist the magnetic moment would be proportional to the amount of TM concentration. In comparison to other works, the M value of 0.1 emu/g in this work is lower than the value of 0.38 emu/g for LaFeO3 nanoparticles (~21.9 nm) synthesized by sol–gel reported by Saad et al. [16], the value of 0.44 emu/g for LaFeO3 nanoparticles (~50 nm) synthesized by milling method reported by Thuy et al. [18], and the value of 0.9 emu/g for LaFeO3 nanofibers (~20 nm) synthesized by electrospining reported by Lee et al. [37]. The difference in M of LaFeO3 materials depends on the preparation conditions or size of nanoparticles or surface of the nanoparticles. However, further work is needed to study on the bond length and bond angle between Fe and O, and this will be of great interest to understand the ferromagnetic coupling on the DE mechanism.

Fig. 9
figure 9

M–H curves at ±10 kOe measured at RT of the LaFeO3 samples

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4 Conclusions

LaFeO3 nanoparticles have been successfully prepared by PC method. Structural characterization showed that the structure had a single phase of orthorhombic LaFeO3. The nanoparticles consisted of the particles of approximately 44.5 ± 2.4–74.1 ± 4.9 nm. UV–Vis spectra showed the prepared LaFeO3 nanoparticles having a strong UV-light absorption. XPS and XANES spectra further confirmed the main composition of the Fe ions were in mixed valence states of Fe3+ and Fe4+. Study of magnetic properties at RT showed that LaFeO3 samples with the smallest particle of 44.5 ± 2.4 nm exhibited soft ferromagnetic behavior with magnetization at 10 kOe of ~0.1 emu/g. This behavior indicated that ferromagnetism is due to the uncompensated spins at the surface and the canted internal spin, which is the nature of size-induced magnetism on nanoparticles.