Citric acid concentration tune of structural and magnetic properties in hematite (α−Fe2O3) nanoparticles synthesized by sol−gel method

This study synthesized hematite nanoparticles using the sol-gel method. The physical properties are modified by the citric acid concentration used as fuel. The resulting sample’s rhombohedral (hexagonal) structure and space group R3c were revealed by the XRD data. The Scherer formula revealed that the crystallite size at the most substantial peak was 32.14 nm, 24.58 nm, and 23.21 nm with an increase in the citric acid concentration of 0.3 M, 0.4 M, and 0.5 M, respectively. The FTIR spectrum’s absorption band reveals the properties of hematite nanoparticles. Finally, the magnetic properties confirmed from the VSM data revealed a significant decrease in the coercive field at 935 Oe, 610 Oe, and 548 Oe as the effect of citric acid concentration increased.


Introduction
The most stable iron oxide under air conditions is hematite. It has many different applications, such as biomedicine [1], lithium-ion batteries [2,3], gas sensors [4,5], and catalysts [6,7]. Hematite is widely available, environmentally friendly, and has low production costs [8]. The hematite electrode has a theoretical capacity of 1007 mAh g −1 , which is much greater than that of regular graphite [9]. Hematite electrodes have high electrochemical performance and cycle stability at 200 mA g −1 and 1000 mA g −1 with respect to the capacity at various applied current densities (ranging from 100 mA g −1 to 4000 mA g −1 ) [10]. A different expression demonstrates that hematite shows steady performance for 40 cycles. Maintaining 330 mAh g −1 and 230 mAh g −1 of capacity, respectively, at discharge/charge rates of 25 mA g −1 and 200 mA g −1 [4].
The morphological shape [1,11], fabrication method [12], calcination temperature [6,13], and precursor concentration [14] also improve the performance of hematite. Different methods for synthesizing hematite nanoparticles to improve their structure have been described in the literature. These methods include hydrothermal [15,16], high-energy ball milling [17], rapid inductive heating [18], co-precipitation [19], and solgel methods [20,21]. Metal oxides have been created using the sol-gel method, which allows for high homogeneity control over size and shape [22,23]. In this method, the precursor solution is treated with the necessary reagents to prepare the polymerization and hydrolysis reactions that produce the sol. The process of gelation is carried out by adding polymers or condensing the sol into a gel [24]. In the gel cell method, citric acid is a reagent or chelating agent that is frequently utilized.
Three carboxylic and one hydroxyl group in citric acid can bind to metal cations or their hydroxyl species to create weakly soluble bond complexes, which can adsorb surface minerals and boost the negative particle charge [25][26][27]. Han et al [28] claim that citric acid can increase the efficiency of filling by reducing the surface charge and considerably increasing the energy for full inclusion between siderite and hematite particles after getting adsorbed onto their surfaces. Citric acid can alter the surface characteristics of the underlying minerals, which will change how they interact with other solutes and are soluble in solutions [29]. By changing the steric and electrostatic interactions that control particle aggregation and transport, citric acid can also have an impact on particles in the environment [30]. The concentration of citric acid affects the speed of response, which in turn affects how quickly the gel forms. As a result, research into citric acid content and how it affects structural and magnetic changes in hematite nanoparticles has become an intriguing area of study.

Experimental
Hematite nanoparticles were produced using the sol-gel method. The citric acid (C 6 H 8 O 7 .H 2 O) and nonahydrate form of iron (III) nitrate (Fe(NO 3 )3.9H 2 O) was used. Following stoichiometric weighing, in double-distilled water, the material is dissolved. The concentration of citric acid used is 0.3 M, 0.4 M, and 0.5 M. Initially, the precursor was heated under magnetic stirring until a dark red gel was formed. At 150°C the gel was dried for one hour. The dried samples were hand-mashed to obtain a powder sample. The final powder was annealed for 4 h at 600°C. Finally, the samples were mashed again to obtain a more homogeneous powder.
Several approaches, including x-ray diffraction, scanning electron microscopy, infrared spectroscopy employing Fourier transforms, and vibrating sample magnetometry, were employed to characterize the production of hematite nanoparticles. The XRD analysis was performed on the material using a PANalytical X'Pert Pro with a Cu-Kα radiation source (λ = 1.5405 Å). Measurements are recorded at an angle of 2θ in the range of 20°-80°. The scanning electron microscope image allowed for the identification of the sample morphology with the help of FEI Inspect-S50 at 100,000× magnification. A Shimadzu IR Prestige 21 equipment was used to perform FTIR spectroscopy in order to identify the functional groups of the sample. Wavenumbers were recorded from 4000-350 cm −1 . Magnetic behavior was observed using an vibrating sample magnetometer OXFORD VSM 1.2H with a 10 kOe maximum external magnetic field.

Result and discussion
Hematite nanoparticles' XRD pattern is seen in figure 1 (annealed at 600°C for four hours) prepared via the solgel method. The peaks that appear around 24 214), and (300), respectively, which correspond to the International Centered Diffraction Data (ICDD) 89-0598 [31]. The presence of these peaks supports the sample's rhombohedral (hexagonal) structure and the presence of the space group R3c [14,32]. An additional peak near an angle of 30.28°is determined to be an unidentified contaminant. The difference in intensity in the resulting crystal structure is possible because of the differences in the shape of hematite [24,33]. The Debye-Scherrer formula may be used to determine the crystallite size for all samples, as follows [34]: where λ is the wavelength of the Cu-Kα radiation source (0.15405 nm), θ is the Bragg diffraction angle, and β is the full width at half maximum (FWHM) of the largest peak in radians. The density and lattice parameters (a = b, and c) were derived using the following equation [35]: where Z is for atomic number per unit cell, M for molecular weight, N A for Avogadro's number, V c for unit cell volume, d is the distance between the planes (d = λ/2sinθ), hkl is the Miller index, and d x is the density of the sample.   Table 1 lists the estimated XRD parameters, including crystal size, lattice parameters, and sample density. At a precursor concentration of 0.3 M, the crystallite size was discovered to be 32.14 nm, while at 0.4 M and 0.5 M, it was determined to be 24.58 nm and 23.21 nm, respectively. The observation by Cornell and Schwertmann [36] that Fe (III) solutions to which citric acid was added during hematite synthesis caused hematite to expand intensely along the surface is evidence for the presence of citric acid at certain points on the hematite surface (110) and (104). This decrease was due to the peak broadening as the citric acid concentration increased. In contrast, the lattice parameters of hematite nanoparticles were found to be approximately a = 5.036 Å and c = 13.764 Å. These results are consistent with those reported previously [14,37]. Meanwhile, the sample density increased from 5.256 g cm −3 to 5.274 g cm −3 with an increase in citric acid concentration. Here, the consequence of the smaller the crystallite size is the greater the mass density. Figures 2 (a)-(c) shows the SEM images and grain size particles of hematite nanoparticles with three citric acid molarities at 100,000× magnification. Here, the grain size particle distribution was calculated for a typical 80 selected grains using the ImageJ software. The morphology of the sample is seen to be influenced by citric acid concentration. The nanoparticles of hematite synthesized using citric acid have characteristics that match the size of the nano and have a spherical morphology. Changes in concentration cause the morphology to become clearer so that the agglomeration boundaries can be seen more clearly. Agglomeration appears larger as concentration decreases. The relationship between grain size and acid citric molarities is depicted in inset figures 2(a)-(c). The average particle size was 29.48 nm with a citric acid concentration of 0.3 M. The grain size  Fe-O octahedral decreased to 21.53 nm and 18.31 nm for a concentration of citric acid 0.4 M and 0.5 M, respectively. The particle size distribution changes as citric acid concentration increases, which is consistent with a reduction in crystal size in the XRD data. This suggests that the saturation state of product formation in the sol-gel process is determined by increasing the citric acid concentration. Low concentrations of the citric acid present larger grain sizes and crystal sizes compared to higher concentrations. According to Davar et al [13], nanoparticles made with citric acid exhibited lower and narrower size values than nanoparticles made with tartaric acid. Compared to tartaric acid, the citric acid molecule is bigger and contains more chelating agent groups. The three acid groups and one alcohol group in citric acid make it more potent than tartaric acid with two acid groups and two alcohol groups. Therefore, the cictric acid molecule is adsorbed onto the surface more strongly than tartaric acid, through covalent bonds between the -COO− and Fe (III) groups [38]. Figure 3 and table 2 show the FTIR curves and typical oxide bond absorption location of hematite nanoparticles produced by sol-gel for different citric acid concentrations. At wavenumbers between 400 and 350 cm −1 , the FTIR spectra of nanoparticles with different citric acid molarities were examined. The FTIR spectrum revealed well-defined vibrational peaks at around 1573 cm −1 , 883 cm −1 , 551 cm −1 , and 441 cm −1 . Furthermore, the absorption band at approximately 883 cm −1 is associated with the bending of C-H bonds. Prominent absorption peaks appeared at approximately 551 cm −1 and 441 cm −1 . The bond stretching vibration metal-oxygen (Fe-O) at the tetrahedral site was linked to the high-frequency absorption band (∼551 cm −1 ). In the meantime, the metal-oxygen (Fe-O) bonds bending vibrations at the octahedral site are responsible for the low-frequency absorption band (∼441 cm −1 ). The range of absorption bands is based on previously reported characteristics of hematite nanoparticles [17,31,39,40]. Figure 4 shows the room-temperature hematite nanoparticles' hysteresis loop. According to table 3, the various citric acid molarities may have had an effect on the magnetic behavior of the nanoparticles hematite based on the changes in the hysteresis curve that represented the coercive field's intensity, saturation magnetization, and remanent magnetization. The coercive field was 935 Oe at 0.3 M citric acid concentration, but it fell to 610 Oe and 548 Oe at 0.4 M and 0.5 M citric acid molarities, respectively. Table 3 shows the hematite nanoparticles with different citric acid concentrations. The saturation magnetization of the hematite   [42] also produced a low magnetic saturation value of hematite with a value of 1.24 emu g −1 which indicates paramagnetic behavior. As a result of variations in citric acid concentration, the magnetization (Ms and Mr) in the study's results did not show any particular tendency The highest saturation magnetization was obtained at 44.88 emu g −1 when the precursor concentration was 0.3 M. This is related to the larger magneto-crystal anisotropy when the concentration of citric acid was 0.3 M.

Conclusion
Using the sol-gel process, hematite nanoparticles were successfully fabricated. The XRD, SEM, FTIR, and VSM were used to characterize the generated samples. As the quantity of citric acid increased, the size of the crystallites in hematite nanoparticles reduced from 34.14 nm to 23.21 nm. FTIR spectroscopy reveals the identification of functional groups involved in the formation of metal oxides. In addition, strong absorption bands at approximately 551 cm −1 and 441 cm −1 were confirmed according to the characteristics of hematite nanoparticles. The magnetic properties confirmed by VSM showed changes with increasing citric acid concentration. As the critic acid concentration increased, the coercive field decreased from 935 Oe to 548 Oe. Citric acid's concentration, which serves as a chelating agent, has an impact on the material's ability to control its size and magnetic properties.