Low-Temperature Co-Precipitation Synthesis of HoFeO 3 Nanoparticles

: In this research, we investigate and discuss the characteristics of HoFeO 3 nanoparticles synthesized by the co-precipitation method at low temperature (t ◦ ≤ 4 ◦ C). The single-phase HoFeO 3 samples with the orthorhombic structure formed after annealing of the precipitates at different temperatures up to 950 ◦ C. The annealed HoFeO 3 nanoparticles have an average crystal size of 10–20 nm (SEM, TEM). UV-Vis spectrum of HoFeO 3 sample annealed at 750 ◦ C showed strong UV and Vis absorption with small optical energy gap (E g = 1.56 eV). In the range temperature of 100–300 K, the HoFeO 3 samples showed superparamagnetic behaviour at 5 kOe with high magnetization (M s = 1.3–2.4 emu/g) and very low susceptibility ( χ << 1).


Introduction
The features of rare-earth orthoferrites RFeO 3 (R = rare-earth elements) such as thermal, electrical, magnetic and optical properties depend on chemical component, particle size and synthetic methods [1][2][3][4][5][6]. Based on specific use purposes, there are different requirements about performance of orthoferrites. This can be carried out by adjusting morphology and particle size via preparation method or doping other elements in the RFeO 3 [7][8][9][10]. Noteworthy, one of the rare-earth orthoferrites receives much attention as HoFeO 3 , which is synthesized by different methods such as solid-state reaction technique [11], hydrothermal [12] and sol-gel technique [13][14][15][16]. Especially, HoFeO 3 particles with a micrometer size synthesized by solid-state reaction technique possess an optical energy gap (E g ) of 3.39 eV; saturation magnetization (M s ) of 25.5 emu/g, coercivity (H c ) of 2659 Oe and remanent magnetization (M r ) of 4.08 emu/g at 10 K [11]. Meanwhile, with a size of 10-20 µm, HoFeO 3 particles prepared by hydrothermal method acquire a M s < 0.1 emu/g at 300 K [12].
In the previous reports [13,14], HoFeO 3 nanoparticles (40-100 nm) with E g value of 2.12-2.14 eV have been employed as photo-catalysts in the visible light. The M s value of HoFeO 3 nanoparticles (100-300 nm) gradually decrease with increase of temperature [15], specifically, which reach the amount of 5.14, 0.6 and 0.2, respectively, at 50, 100 and 300 K. HoFeO 3 particles in size 25-30 nm was also prepared by co-precipitation, using ethanol solvent [17]. However, ethanol is volatile, pollutant, and flammable solvent compared to water. Furthermore, the water solvent is cheaper. The orthoferrite HoFeO 3 nanoparticles with a size of 10-20 nm and without conglomeration, which are prepared by simple co-precipitation method at low temperature (t • ≤ 4 • C), have not been reported, so far.
In this work, HoFeO 3 nanoparticles (with a size of 10-20 nm and without conglomeration) were successfully synthesized and studied on structure, thermal, optical and magnetic properties.

Experimental
A mixture containing 25 mL of Fe(NO 3 ) 3 solution (0.2 M) and 25 mL of Ho(NO 3 ) 3 solution (0.2 M) was mixed at room temperature (t •~3 00 K) and then gradually added to 400 mL of cool water (t • ≤ 4 • C). This strategy is to stabilize the obtained precipitate; consequently, it results in the controllable growth of crystals better than the co-precipitation at room temperature [4,10]. Subsequently, a 5% NH 3 solution was slowly introduced until pH = 9.2-9.4 to entirely precipitate the cations of Fe (III) and Ho (III) [18][19][20]. The precipitate was filtered, washed by deionized water until pH~7 and dried in air. Finally, the products were then grinded and carried out thermogravimetric analysis to determine appropriate temperature for obtaining the single-phase of HoFeO 3 orthoferrite.
The crystal structure of the products has been determined via X-ray diffraction (XRD) on a D8-ADVANCE (Brucker, Bremen, Germany) with Cu Kα radiation (λ = 1.540 Å) using a step size of 0.019 • in the range of 10-80 • . The materials' element component and surface morphology have been investigated via energy dispersive analysis of X-ray (EDAX) and scanning electron microscopy (SEM) on S-4800 FE-SEM (Hitachi, Tokyo, Japan), Raman spectrometer on Horiba XploRa ONE (American), and transmission electron microscopy (TEM) on JEM-1400 instrument (Jeol Ltd., Tokyo, Japan). The thermal property has been determined via thermal gravimetric analysis and differential scanning calorimetry (TGA-DSC) on a LABSYS Evo 1600 • (SETERAM Instrumentation, Caluire, France). While the optical and magnetic properties have been studied via UV-Visible spectrophotometer (UV-Vis) on UV-2600 (Shimadzu, Tokyo, Japan) and vibrating sample magnetometer (VSM) on a EV11 (MicroSense, Tokyo, Japan) with a maximum applied field of 16,000 Oe.

Results and Discussion
In order to determine the optimal temperature range for the structural investigation, thermal gravimetric analysis (TGA) was performed with the results revealed in Figure 1. As a consequence, the initial weight loss (~35%) starts from 50 to 650 • C, meanwhile, which has not considerably changed at t • > 650 • C (<1%). The endothermic and exothermic peaks specifically appear on the DSC curve at 125.49 and 749.37 °C, respectively. Herein, the weight loss after annealing is attributed to the water removal from the surface, resulting from the crystallization and dehydration of Fe2O3.xH2O (x = 1-5) [21] and HoO(OH).yH2O [22]. Hence, the temperatures of 650, 750, 850 and 950 °C, were chosen for investigating structure and morphology of HoFeO3 na- The endothermic and exothermic peaks specifically appear on the DSC curve at 125.49 and 749.37 • C, respectively. Herein, the weight loss after annealing is attributed to the water removal from the surface, resulting from the crystallization and dehydration of Fe 2 O 3 .xH 2 O (x = 1-5) [21] and HoO(OH).yH 2 O [22]. Hence, the temperatures of 650, 750, 850 and 950 • C, were chosen for investigating structure and morphology of HoFeO 3 nanoparticles. Figure 2a shows the XRD pattern of HoFeO 3 after annealing at 650, 750, 850 và 950 • C for 1 h. As a result, HoFeO 3 reveals an amorphous state at 650 • C. Whereas, at the different annealing temperatures, the samples are in good accordance with the standard JCPDS: 46-0115 of the orthorhombic HoFeO 3 single-phase, which are correspondent to the exothermic peak on the DSC curve (749.37 • C). In the Raman spectrum of the HoFeO3 sample at room temperature, three modes are observed at 129.1, 514.1 and 594.2 cm −1 , which possess high peak intensities (Figure 3b). The Raman active modes of the HoFeO3 were assigned based on the method recently proposed by Gupta et al. [23] for RFeO3 (R = Tb, Dy, Ho, Er, Tm) compounds. Noteworthy, the wavenumber, appearing at 129.2 cm −1 , was attributed Ho-O vibration modes. The Raman bands above 200 cm −1 correspond to oxygen ions. The high frequency mode in the RFeO3 crystal may be assigned to the internal vibration related to the mutual Fe-O motion within the oxygen octahedron [24], in this work, which presents at wavenumbers of 514.1 and 594.2 cm −1 . The UV-Vis absorption spectra of the HoFeO3 nanoparticles revealed a strong absorption in the visible light region (~400-600 nm) (Figure 3c). This is interesting Furthermore, the SEM and TEM images of HoFeO 3 nanoparticles annealed at 750 • C are shown in Figure 2b,c. Herein, the orthoferrite HoFeO 3 particles are discrete with an average size of 10-20 nm, which are smaller and without conglomeration of particles comparing to HoFeO 3 nanoparticles prepared by hydrothermal [12], sol-gel [13][14][15], and co-precipitation using ethanol [17]. Figure 3a shows the EDAX spectra of HoFeO 3 , in which presence of Ho and Fe ions are clearly seen. An analysis of the weight and atomic percent of HoFeO 3 nanoparticles is in agreement with the theoretical value calculated from the chemical formula.
Crystals 2021, 11, x FOR PEER REVIEW 5 of 8 4a,b) and magnetic susceptibility is very low (χ << 1) (Figure 4e,f). HoFeO3 nanoparticles are super-paramagnetic material at temperatures greater than 100 K [25]. When the particles do not aggregate and the particle size is very small, the thermal energy is more dominant than directional energy, resulting in the superparamagnetic property of nano Ho-FeO3 (See Figure 2b,c).   [9] 149.3 3.39 -2.55 -- [10] 10-20 μm --<0.1 -- [11,12] 40-100 2.12-2.14 ---- [13] 100-300 -0.6 0.2 -- In the Raman spectrum of the HoFeO 3 sample at room temperature, three modes are observed at 129.1, 514.1 and 594.2 cm −1 , which possess high peak intensities (Figure 3b). The Raman active modes of the HoFeO 3 were assigned based on the method recently proposed by Gupta et al. [23] for RFeO 3 (R = Tb, Dy, Ho, Er, Tm) compounds. Noteworthy, the wavenumber, appearing at 129.2 cm −1 , was attributed Ho-O vibration modes. The Raman bands above 200 cm −1 correspond to oxygen ions. The high frequency mode in the RFeO 3 crystal may be assigned to the internal vibration related to the mutual Fe-O motion within the oxygen octahedron [24], in this work, which presents at wavenumbers of 514.1 and 594.2 cm −1 . The UV-Vis absorption spectra of the HoFeO 3 nanoparticles revealed a strong absorption in the visible light region (~400-600 nm) (Figure 3c). This is interesting because HoFeO 3 could be developed as a new visible light photocatalyst. The direct band gap energy (E g , eV) was determined similarly to the work [11] and is shown in Figure 3d. As a consequence, the band gap value of HoFeO 3 nanoparticles is~1.56 eV which is lower than in the previous works ( Table 1).
The obtained results on magnetic properties of nanoparticles HoFeO 3 indicated that the magnetization increases when the temperature decreases (Figure 4c,d). Additionally, the magnetization is much greater than reported papers (Table 1). Interestingly, the magnetization curve continuously goes up when the external magnetic field increases (Figure 4a,b) and magnetic susceptibility is very low (χ << 1) (Figure 4e,f). HoFeO 3 nanoparticles are super-paramagnetic material at temperatures greater than 100 K [25]. When the particles do not aggregate and the particle size is very small, the thermal energy is more dominant than directional energy, resulting in the superparamagnetic property of nano HoFeO 3 (See Figure 2b,c).

Conclusions
In conclusion, syntheses and full characterizations of HoFeO3 nanoparticles are performed. With property analysis, HoFeO3 revealed a single-phase state after annealing at 750, 850 and 950 °C for 1h adopted the grain sizes in the range of 10-20 nm. HoFeO3 na-

Conclusions
In conclusion, syntheses and full characterizations of HoFeO 3 nanoparticles are performed. With property analysis, HoFeO 3 revealed a single-phase state after annealing at 750, 850 and 950 • C for 1h adopted the grain sizes in the range of 10-20 nm. HoFeO 3 nanopowders have low band gap energy (E g = 1.56 eV), which is beneficial for application in optical catalysis. The HoFeO 3 nanoparticles are antiferromagnetic with a superparamagnetic behaviour (low susceptibility χ << 1 and high magnetization M s = 3.2-7.5 emu/g), which makes them the potential candidates for making the devices operating at a high magnetic field.