Synthesis routes for multi-shape Fe3O4 nanoparticles through oxidation-precipitation of hematite and modified co-precipitation method without surfactant

Abstract Uniform fine particles of iron oxide were precipitated in different morphologies through forced hydrolysis and co-precipitation. The effect of reaction parameters on particle uniformity and morphology was studied, and SEM analysis explored that particle morphological features were strongly affected by the applied reaction conditions. Therefore, monodispersed particles of magnetite precursors with narrow size distribution were prepared under the extensively optimized experimental parameters. The growth mechanism of the precipitated solids was observed to be sensitive to the medium temperature and pH, as high temperature and pH resulted in large particles. In contrast, small particles were precipitated at low temperatures and pH. Magnetite nanoparticles were precipitated directly through co-precipitation under controlled reaction conditions and from the calcination of the as-synthesized hematite at high temperatures. Their SEM demonstrated that calcination at high temperatures affected the particle morphology to a greater extent along with the phase transformation. Similarly, XRD and FTIR analysis of the as-prepared magnetite particles from two routes confirmed the single-phase magnetite. The response of the as-prepared particle systems to the higher temperature and composition of the surface groups was analyzed by TG/DTA and FT-IR techniques. Graphical Abstract


Introduction
Nanoparticles are considered significant materials in nanotechnology; recently, nanoparticles have gained great interest due to their exclusive and new properties and recently received considerable interest due to their unique and novel properties and performance in many innovative processes. [1] Nowadays, excessive consideration has been given to iron oxide particles, promoting a new era of photocatalysis, magnetic applications, sensors, electrocatalysis, optics, anticorrosion, and biomedical electronic, etc. [2,3] Iron oxide occurs in several phases, such as maghemite (c-Fe 2 O 3 ), Akageneite (b-Fe 2 O 3 ), hematite (a-Fe 2 O 3 ), and magnetite (Fe 3 O 4 ). Among all these phases, magnetite (Fe 3 O 4 ) is the utmost attractive and significant metal oxide. It has been synthesized by various methods like co-precipitation, sol-gel, forced hydrolysis, etc., in a diverse configuration of nanostructures, i.e., wires, spindles, rods, cubes, tubes, flowers, spherical, microspheres, rhombohedra, and ellipsoidal, etc. [4][5][6][7][8][9] Literature regarding the synthesis and application of iron oxide showed that researchers successfully synthesized hematite nanoparticles by a surfactant-assisted hydrothermal method and produced nanoparticles with spherical shape. [10] Similarly, magnetite nanoparticles with various shapes and sizes have been prepared through the forced hydrolysis method of FeOOH, c-FeOOH, b-FeOOH, as well as from reduction of a-Fe 2 O 3 in multiple gas atmospheres and the presence of different surfactants. [11,12] Likewise, Cursaro et al. [13] precipitated magnetite nanoparticles in a single step under the inert gas atmosphere using double iron salts.
However, the current work pointed to the direct synthesis of magnetite nanoparticles by co-precipitation method via using the single salt solution and indirectly from the calcination of hematite at elevated temperatures, which were synthesized through the forced hydrolysis method without surfactants. The synthesis strategies utilized were simple, economically practical, and environmentally friendly compared with the work of the research groups of Kongsat and Cursaro. [10,13] Furthermore, the working methods are free from any type of modifier, reducing agent, or surfactant.

Materials
Ferrous chloride (FeCl 2 ), Ferric chloride (FeCl 3 ), ammonium hydroxide (NH 4 OH) (25%), sodium dihydrogen phosphate (NaH 2 PO 4 ), ethanol (C 2 H 5 OH), and hydrochloric acid (HCl) were acquired from Scharlau of A. R. grade and used without additional purification. Throughout the experimental work, deionized water was used to prepare both working and stock solutions in Pyrex glass vessels. Stock solutions were filtered to eliminate the insoluble impurities using a vacuum filtration device and membrane filter paper with 0.2 mm pore size.

Synthesis of precursors for monodispersed Fe 3 O 4 particles
The controlled precipitation method was used to prepare the precursor particles. For this method, aqueous solutions of ferrous chloride (1 Â 10 À1 -1 Â 10 À3 mol/L) and ammonium hydroxide (10-25%) were taken and mixed in appropriate ratios under precise magnetic stirring at different temperatures (30 C-90 C). The resulting reactant mixtures were aged for various intervals of time. The air was bubbled for a known duration through the resultant dispersions at the end of predetermined time. The resultant dispersions were cooled to room temperature, then filtered the precipitated solids through a membrane filter and isolated from their mother liquors. The filtered powder was thoroughly washed with ethanol and deionized water, dried in open air then stored in a desiccator.
Besides, an alternative method was also used to prepare magnetite nanoparticles. For this purpose, an aqueous solution of sodium di-hydrogen phosphate (2 Â 10 À3 -1 Â 10 À3 mol/L) and ferric chloride (1 Â 10 À1 -1 Â 10 À3 mol/L) were slowly mixed in a proper ratio; then the resulting mixture was heated for a specific time interval (30-96 hours) at a temperature between 70 C and 98 C in firmly closed 100 mL Pyrex glass vessel. The reaction mixture was removed from glass vessels at the end of heating and cooled to room temperature. The subsequent precipitates were filtered off from the mixture solution, washed, dried, and stored for further characterizations.

Heat treatment
The as-prepared precipitates were heated in a Nabertherm, M7/11 furnace up to different temperatures at the measured rate of 5 C/min with a stay time of 1 hour in a programmable furnace. The furnace was then turned off, and the solid sample in the furnace was cooled down to room temperature, and the calcined powder was stored for further use in a desiccator.

Scanning electron microscopy (SEM)
The morphological features and microstructure of the synthesized samples and calcined particles were investigated by Scanning Electron Microscope (SEM; JSM-6490, JEOL). For SEM imaging analysis, the desired dry synthesized sample powder was applied on aluminum stubs. These stubs were spluttered for 30 sec with a well uniform film of platinum by using a fine auto coater (JEOL, JFC-1600). The size and shape structure of these samples were examined by keeping the distance of 10 mm among the sample holder and tip of the electron gun at an accelerating rate of 15 KV.

Fourier transform infrared spectrometry (FT-IR)
Fourier Transform Infrared (Shimadzu, IR Prestige-21 FTIR-8400) Spectrometer was used to explore the presence of numerous species/functional groups on the surface of the synthesized precipitated particles. The range of spectral scanning was about 4000-400 cm À1 .

X-ray diffractometry (XRD)
The crystallinity of the synthesized nanopowders and calcined samples of iron oxide compounds were examined with the X-ray diffractometer, "JEOL JDX-3532" through Cu Ka radiations. Samples were scanned in a 2h range of 10-80 at the speed of 0.05 and a step angle of 0.1 /sec. The JDX-3500 and CMPR software were used for the examination of crystalline phases and peaks of the samples. Thermal gravimetric/differential thermal analysis (TGA/DTA) Thermal analysis of the as-prepared iron compound solid samples was done by a simultaneous TGA/DTA analyzer (Perkin Elmer, Diamond TGA/DTA). In this method, a known quantity of the sample was taken and heated at about 30 C-800 C in a crucible on electromagnetic balance at the rate of 5 C/min in the air. Weight loss of the sample was recorded as a function of temperature.

Synthesis of hematite (Fe 2 O 3 ) nanoparticles
The forced hydrolysis method was used to prepare uniform fine nanoparticles of iron oxide (Fe 2 O 3 ). [14] For this method, an appropriate amount of mixture solution containing ferric chloride and sodium dihydrogen phosphate was taken and heated for various time periods at high temperatures (40 C-98 C) in tightly closed reaction vessels, resulting in the precipitation of Fe 2 O 3 particles. Hydrogen was extracted in this process from the hydrous soluble metal ions, which were initiated thermally, leaving behind insoluble particles of Fe 2 O 3 as a result of olation and deprotonation . The mechanism of precipitation is given below: [15,16] Mechanism; Initial experiments showed that the reactant mixture composition greatly influenced the shape and size of the synthesized particles. Most of the experimental trails also generated gels and irregular particles. To obtain and synthesize reasonable uniform nanoparticles of iron oxide, numerous experimental parameters, such as aging time, temperature, the composition of the reaction mixture, and stirring rate, were improved extensively, mainly affecting the morphological features of the as-prepared particles. Therefore, it is considered essential to control the morphology of the particles during the precipitation process. In the present study, the effect of numerous parameters such as the pH of the medium, aging time, the temperature of the system, and reactant concentration of the solution mixture, on dispersity and uniformity of particle was thoroughly investigated.

Optimization of experimental parameters
To examine the effect of concentration of metal salt on the morphology of particles, several reactant mixtures containing (0.05-1.0 mol/L) of iron (III) chloride were prepared while other reaction parameters, i.e., the concentration of sodium dihydrogen phosphate (0.001 mol/L), temperature (98 C), aging (96 hours), etc. were kept constant. The subsequent precipitates obtained from various batches (Supporting Information, Figure S1) showed that the particle size increases when the concentration of iron (III) chloride rises up to a specific limit. [14] A high concentration of iron chloride (1.0 À 0.5 mol/L) eased the growth of initially formed particles, which led to the agglomeration of these particles [17] and formed the gelatinous material due to increased particle diameter. Therefore, it could be inferred that the morphology of the particles was greatly influenced by the concentration of FeCl 3 salt in the reaction mixture. While for the production of uniform fine Fe 2 O 3 colloidal particles, the appropriate concentration of iron(III) chloride was nearly 0.1-0.05 mol/L in the present study as well as reported before. [17,18] Furthermore, the influence of NaH 2 PO 4 concentration on particle morphology was examined in 0.001 to 0.2 mol/L under the constant experimental state. Results (Supporting Information, Figure S2) showed that NaH 2 PO 4 represented as the shape directing species [14] and remained active up to 0.01 mol/L.
The thermal effect on the hydrolysis of the same system has been studied at various aging temperatures (40 C-98 C) (Supporting Information, Figure S3). It was found that uniform fine particles of hematite were produced at high temperatures (90 C-98 C) because the hydrolysis of the nucleation rate increases due to the kinetic reaction mechanism, which is followed by the rise in growth of crystal. [19] Similarly, the effect of aging time (48 hours to 100 hours) was also investigated on the same particle's morphology at 98 C. It was noted that the particles were agglomerated with each other at the initial aging stage (Supporting Information, Figure S4). After aging of 20-24 hours, the morphology was attained in its self-assemble super-structure to decrease the surface energy. [20] Hence, uniform morphology of the particles was achieved apparently in 96 hours. Also, it was noted that there was no effect on the growth of hematite particles when the aging time was increased beyond 96 hours, and it was completed after four days (96 hours) of aging at a temperature of 98 C. [17] As can be seen from the above discussion, experimental parameters strongly affected the morphological features of the precipitated iron oxide particles. Therefore, precise synthesis circumstances were discovered after extensive optimization of all these parameters to produce uniform particles of iron oxide. The oval shape particles with uniform morphological features were obtained under the optimized conditions of temperature, aging, and reactant concentration as captioned in Figure 1. The mechanism is simply a self-assembled system for the synthesis of iron oxide particles. After the initial nucleation, the particles were agglomerated in a specific size, and then an elongated structure was formed in the self-assembled super-structure. As a result, the free surface energy was reduced by driving force, which led to the elimination of interfaces among the nanoparticles. [20] Characterization The particles prepared under the optimized conditions of experiments ( Figure 1) were monodispersed, whereas the recipe protocol for the particles shown in Figure 1D was reproducible; therefore, selected for further characterization.
The spectrum of FT-IR was recorded in the 4000-400 cm À1 range for inspection of various chemical groups adsorbed on the surfaces of the synthesized particles. The spectrum (Figure 2A) for the synthesized particles ( Figure 1D) showed absorption bands at various locations due to different means of vibrations of many groups on the test material particle surfaces. The bands observed at 418 cm À1 , 486 cm À1 , and 584 cm À1 were assigned the stretching vibrations of (Fe-O) metal oxide bonding, while the peak at 1023 cm À1 was also referred to as the bending vibration of Fe-O. [21] Also, it was found from the I-R spectrum that the adsorption bands at 1615 cm À1 , 1416 cm À1 , 3234 cm À1 , 3415 cm À1 , and 3558 cm À1 resembled the stretching and bending vibration of the OH À group of water molecules. [21,22] The band at 2353 cm À1 assigned that the atmospheric carbon dioxide was absorbed on the surface with less intensity. [23,24] The FT-IR results showed the occurrence of water and structural hydroxyl groups absorbed on the surface of the test material because the product was prepared in the aqueous solution.
The phase purity, crystal structure, and composition of the as-prepared nanoparticles ( Figure 1D) were further assessed through the X-ray diffraction technique (XRD). The obtained sharp peaks pattern of X-ray diffractogram in Figure 2B at different 2h values revealed that the synthesized particles were crystalline and rhombohedral (hexagonal) in structure. The characteristic peaks of iron oxide were detected at 2h values of 33.  214), and (300), respectively. All the significant and intense diffraction peaks were identified by JDX-3500 software which showed that the synthesized material was nano-crystalline with lattice parameters a ¼ 5.032 nm and c ¼ 13.752 nm. The peaks were matched with the diffraction standard data of ICDD No. 11053. [25] The XRD pattern for the as-produced particles confirmed the configuration of monophasic hematite, as no additional peaks of other phases were detected in the XRD detection limit, which showed the pure and singlephase nature of the as-prepared particles as a-Fe 2 O 3 .
The average crystallite size of the particles was calculated by using a well-known Debye-Scherer equation (Equation (2)), [26] i.e., where, D is the diameter of particle's crystallite size, K is the shape constant factor (the value is 0.89), k is the wavelength of the X-rays incident beam used, b is the broadening of the diffraction line measured in half radians to its maximum intensity (FWHM) and h is the Diffraction Bragg's angle. The average crystallite size of a-Fe 2 O 3 particles from the XRD data ( Figure 2B) calculated by the Scherer equation was found to be 18.7 nm. Other researchers also reported the average size, i.e., 14-20 nm, of hematite nanoparticles prepared by the forced hydrolysis process in the presence of various surfactants. [20] Phase transformation and decomposition of the tested sample during the heat treatment were examined through TGA analysis (Supporting Information, Figure S5). Thermal analysis of the selected as-prepared sample (SEM, Figure  1D) was carried out at the heating rate of 10 C per min in the temperature range of about 30 C-1000 C in the air atmosphere. The observed total weight loss was about 4.2% and correlated with the theoretical weight loss (4.5%), calculated from the mentioned reaction: The weight loss indicated that the water was physically absorbed on the surface of particles and weak enough, which showed only a minor change in the heating chamber temperature, which led to its desorption. [19,27] Another experiment was done on the additional powder of the synthesized iron oxide nanoparticles to authenticate the above heat-mediated results. In this, a weighed sample was calcined up to 1000 C-1200 C in the well programmable furnace for a stay time of 1 hour with a rate of 5 C/ min À1 . The calculated weight loss after calcination (4.22%) was nearly close with the results of TGA (4.2%) and with the weight loss calculated theoretically (4.5%) from Equation (3).
Moreover, from the weight loss reaction, the activation energy initiated by the thermal process of the iron oxide nanoparticles was also calculated using the Coats Redfern equation (Equation (4)). [28] ln À ln 1 À a=T 2 The decomposed mass (a-values) of the original material was calculated by the following mentioned equation (Equation (5)) [29] from the weight loss data (TGA): where, W i ¼ Initial weight W f ¼ Final weight W t ¼ Weight at the time For calculation of activation energy, the experimental data was plotted as ln [Àln(1-a/T 2 )] vs. 1/T (Equation (4)) for each step (Supporting Information, Figures S6 and S7) and the observed activation energy values were 1.2 kJ.mol À1 , 1.5 kJ.mol À1 , and 2.3 kJ.mol À1 for the weight loss in the temperature range of 25-50 C, 50-500 C, and 750-900 C, respectively. In the first step, a smaller activation energy value indicated that the weight loss occurred thermally due to the loss of physically adsorbed water molecules.

Synthesis of magnetite (Fe 3 O 4 ) nanoparticles
Two routes were employed for the production of magnetite nanoparticles. In the first method, the hematite nanoparticles synthesized by forced hydrolysis (SEM Figure 1D) were transformed to magnetite particles through controlled calcination at high temperatures (800 C-1200 C). It was practically indicated that the high treatment of temperature converted the as-prepared powder into black color, which displayed phase transformation of Fe 2 O 3 nanoparticles at high temperatures. [19,30] The SEM images of the obtained particles ( Figure 3A-C) revealed that the morphology of the particles retained their shape integrity up to 800 C ( Figure  3A), and no mass change was observed up to this temperature, showing the stability of hematite nanoparticles. [31]  While at high temperatures (1000 C, 1200 C), the grain boundaries came closed, and numerous particles were fused, resulting in the agglomeration and forming larger size particles ( Figure 3B and C).
At 1000 C and above, the extensive growth of grains occurred, which pointed to the endothermic nature of the aggregation progression of particles. [22,32] The particles calcined at elevated temperature ( Figure 3C) were designated as Magnetite-I in further study.
The second route for the synthesis of magnetite was coprecipitation. But it has several disadvantages, like irregular morphology, extensive agglomeration, and particle size distribution. Therefore, the modified co-precipitation method was used in this study to overcome these deficiencies and explore an economical and simple route for the synthesis of monodispersed magnetite nanoparticles. For this purpose, a known concentration of aqueous solutions of FeCl 2 and ammonium hydroxide were taken and mixed in definite ratios under magnetic stirring at several temperatures (30 C-90 C) without adding any surfactant. The pH of the mixture was increased with the addition of ammonium hydroxide (25%) and the reactant solution was aged for different periods of time. At the end of the experimental time, the air was also bubbled through the resulting dispersals for a specific period (oxidation precipitation). It was cooled to room temperature, and then the precipitant was isolated from the mother liquors by filtration process through the membrane filter. It is noteworthy that it is a very facile and effective method, and the mechanism of precipitation can be expressed as follows [33] : Mechanism; Following the above mechanism of the reaction, Fe(OH) 2 was converted from the hexagonal structure into a spherical magnetite structure under the precise oxygen gas atmosphere. Fe(OH) 2 precipitated out in basic solution, then the development of FeOH þ took place with the dissolution method, then the slow oxidation process occurred in the air to magnetite nanoparticles. [34] The preliminary experiments showed that the concentration of reactant composition mixture, stirring rate, aging time, pH, and temperature affected the morphology of the synthesized particles of magnetite. To obtain uniform morphology of magnetite particles, optimization of these reaction parameters was carried out, which meaningly affected the morphological structures of synthesized particles.
The effect of ammonium hydroxide concentration on particle uniformity was checked by monitoring its concentration (10%-25%) in the reaction mixture (Supporting Information, Figure S8). It was observed that a higher concentration of NH 4 OH (25% À 22%) resulted in the gelatinous material. [35] The slow and continuous release of OHions at a minor concentration of NH 4 OH efficiently increased the value of pH of the solution, which made the size of the particle controllable and produced Fe 3 O 4 nanoparticles from the Fe 2þ hydrolyzed ions (Fe(OH) 2 ). [36,37] It  slowly helped in the pH increase of the solution up to 12-13, which facilitated the uniform evolution of the as-prepared magnetite particles.
The morphology of the particles was also controlled by the concentration of iron salt solution, which had greatly affected the particle morphology (Supporting Information, Figure S9). It was examined that at a higher concentration of the salt solution (0.5 À 0.2 mol/L), the Ostwald ripening of small particles was converted into large-size poly-dispersed particles. [38] It also indicated that spherical particles progressively grew bigger and became thick at the high concentration of iron chloride, which facilitated the development of mainly formed particles, which improved the crystal nucleus [39] and produced agglomeration. [17,38,39] Uniform small-size particles were produced at low (0.1-0.05 mol/L) salt concentration. [36] Similarly, to examine the variations of the morphology of particles, the consequences of air bubbling (oxygen) were also observed. The molecular oxygen controlled the size of particles through the oxidation of Fe(II) ions. [40] It was reported that the synthesis of magnetite nanoparticles thermodynamically from Fe(II) ions under the air atmosphere in an aqueous solution was favorable energetically.
To investigate the influence of temperature on the reaction mixture, many experiments were carried out at various temperatures (30 C-90 C), in a definite aging time for the identified proper concentration of ferrous chloride and ammonium hydroxide in controlled stirring deprived of using any shape modifier or surfactant. The air oxidation was done for 1 hour, which caused the complete conversion of particles into Fe 3 O 4 nanoparticles. It was observed that the particle size was increased when the medium temperature was increased up to 90 C. [34] It pointed out that at elevated temperatures, the growth process was sped up compared to the process of nucleation. Furthermore, the hydrophobic contact among the surfaces of recently prepared particles caused irregularity in the size distribution of particles, which ended them agglomerating in the last stage. [41] From the above experiments, the concentration of ferrous chloride, ammonium hydroxide, air bubbling flow rate, temperature and stirring rate (Supporting Information, Figure S10) were widely optimized for the generation of uniform magnetite particles. Therefore, the particle systems shown in Figure 4 were generated with uniform particle morphology, while the recipes for Figure 4E and G were reproducible under the captioned experimental conditions, and these particle systems were designated for further study.
At high temperatures (70 C-90 C), the morphology of the particles remained stable in an aqueous solution for a more extended time period, deprived of using any stabilizing agent compared to the particles synthesized at low temperature and could be used for biomedical applications. [42] The better dispersibility in an aqueous medium may be due to the maximum absorption of ammonium ions on the surface of particles and increased zeta potential, which led to more stability of the products in pure water. [42] Thermal analysis (TGA/DTA) TGA of the as-prepared magnetite nanoparticles (SEM, Figure 4E and G) was examined in the range of 30-800 C at 10 C/min heating rate in the air atmosphere. The TGA/ DTA curves ( Figure 5A and B) demonstrated that the particles prepared at 70 C ( Figure 4E) showed 2% weight loss in the temperature range of about 40 C to 460 C. In contrast, 1.7% loss was noticed in the same range of temperature for the high-temperature synthesized particles (90 C, SEM Figure 4G). The observed weight loss was due to the loss of physically absorbed water molecules on the surface of material up to 100 C while the weight-loss differences between the two samples (2% and 1.7%) synthesized at 70 C and 90 C, respectively, was ascribed to the variations in the size of the particles. Therefore, the particles synthesized at 70 C ( Figure 4E) adsorbed more water molecules due to the smaller particle size as compared to the molecules adsorbed by the material synthesized at 90 C ( Figure 4G). [43] Furthermore, no significant weight loss or gain was noted at 460 C-800 C, which showed the crystalline fine magnetite phase in this temperature range. [44] Besides, the TGA graph ( Figure 5) showed an increase in the weight at 150 C to 300 C of the sample. This rare behavior in the case of the synthesized material may be due to the oxidation of the sample. [45] It was also reported that the surface adsorbed oxygen penetrated the core of the material, which was then released at higher temperatures. [31] The total weight loss experimentally calculated from TGA plots (Supporting Information, Figure S11) of the samples synthesized ( Figure 4E and G) was 2% and 1.7%, respectively, which can be interrelated with the theoretical weight loss calculated from the subsequent reactions (Equations (7) and (8) The weight loss (2.2% and 1.8%) calculated theoretically from the equations was agreed well with the observed experimentally weight loss of about 2.0% and 1.7%, respectively, which confirmed the accuracy of the proposed equations mentioned above for the heat-treated samples. The absence of exo/endothermic peaks in the DTA curve for the samples prepared at 70 C and 90 C ( Figure 5) showed that the physically adsorbed weakly bonded water molecules desorbed in the temperature of the heating chamber with the minor change. [19,27] Moreover, the thermal weight loss reactions of iron oxide material and its activation energy were calculated via equation [4] (Supporting Information, Figure S12) as stated above. The activation energy of the synthesized materials ( Figure  4E and G) was 1.14 kJ.mol À1 and 1.5 kJ.mol À1 , respectively for the weight loss in the temperature range of 40-460 C. These values demonstrated that the magnetite synthesized at a higher temperature (90 C) exhibited more excellent stability while the powder synthesized at a lower temperature (70 C) exhibited lesser stability. Thus, it was confirmed from the values of activation energy that the dispersibility and stability of the as-prepared particles, at higher temperature (90 C, SEM Figure 4G), was good which directed to the high particle crystallinity as associated with the particles at low temperature (70 C, Figure 4E). [42] For obtaining anhydrous and pure magnetite, the particles (70 C and 90 C) were calcined at 800 C according to the TGA results. After calcination of the particles ( Figure 4E and G), the resulting dry powders were chosen as Magnetite-II and Magnetite-III, respectively, in further study. The magnetite nanoparticle systems prepared by two different methods (Figures 3C and 4E and G), i.e., Magnetite-I, Magnetite-II, and Magnetite-III, were characterized by various techniques.

Fourier transforms infrared spectroscopic analysis
The FT-IR analysis of the magnetite particles is shown in Figure 6. The spectrum of Magnetite I ( Figure 6A) showed bands at 420 cm À1 , 478 cm À1 , 585 cm À1, which matched with the the Fe-O stretching vibration of Fe 3 O 4 . [26,44] It confirmed that a high calcination temperature of 1200 C caused the transformation of hematite particles into magnetite nanoparticles (Supporting Information, Figure S13). It was determined from the present discussion that above 1000 C, hematite nanoparticles were converted into magnetite nanoparticles through the process of calcination for 4 hours stay time. Still, in some cases, peaks for Fe 2 O 3 were also shown in combination with the magnetite at higher temperatures. [45] Similarly, the FT-IR analysis of Magnetite-II and Magnetite-III spectra were analyzed in the same conditions ( Figure 6B and C). The critical bands of these lines were detected at 397 cm À1 , 420 cm À1 , 446 cm À1 , and 485 cm À1, which was Fe-O bending vibrations for the magnetite nanoparticles. [26,34,44] In addition, Magnetite-I (FTIR, Figure 6A) shows the OH bending and stretching vibration bands, showing its hygroscopic nature. Figure 6B and C further demonstrated that the OH bending and stretching vibration bands are absent in Magnetite II and III. Inspection of Magnetite I and Magnetite III revealed that regardless of the variations of the synthesis pathway, slight variations were noted in the IR spectral lines. Like, nearly changes were also observed in the frequencies of OH À and Fe-O bending vibrations, which showed that the composition of the samples might also affect the spectral profile besides the synthesis route.

X-ray diffractometric analysis (XRD)
The crystallinity and phase purity of synthesized iron oxide (Magnetite) particles were further assessed by the powder Xray Diffraction (XRD) method, and the patterns obtained are shown in Figure 7. The XRD analysis of Magnetite-I ( Figure 7A) showed the as-prepared nanoparticles (SEM Figure 3C) were fully transformed into magnetite phase at elevated temperature (1200 C) (Supporting Information, Figure S14). The chief intensity of the peak was used to calculate the percentage of Fe 3 O 4 particles in the mixture sample by comparing the experimental peak intensity with the reference one. From the XRD pattern, it was found the series of distinguishable peaks at 33 [45] showed an excellent index to inverse cubic pure spinel structure of magnetite.
Similarly, the particles designated as Magnetite-II and Magnetite-III were also analyzed by the XRD technique ( Figure 7B and C) to examine the particle phase purity, crystallinity, and composition. The main and strong peaks of Magnetite II ( Figure 7B) were matched with the data of ICDD card no. 19-0629. [46] The characteristic and intense peaks of particles located at 2h range of 30.37 , 35.81 , 43.47 , 53.02 , 57.36 , 63.02 , and 67.91 with the consistent reflections of (220), (311), (400), (422), (511), (440), and (531). At the same time, the X-ray peaks for the Magnetite-III particles ( Figure 7C) were matched with the standard diffraction data of ICDD card no.   [11,12] and peaks at 2h were at 30.20 , 35.40 , 43.14 , 53. 52 , 56.98 , 62.58 , which indexed as (220), (311), (400), (422), (511), and (440), respectively. It confirmed the planes of magnetite were crystalline in nature. [12] No other impurity peak was found for both samples, which indicated single-phase magnetite particles; the wide diffraction peaks proposed a smaller crystallite size of the Fe 3 O 4 nanoparticles. [11] The average crystallite size (Table 1) of all nanoparticles was calculated through Scherer's equation. The crystallite size of the heat-treated samples (800 C, 1000 C, and 1200 C) were 18.7 nm, 20.5 nm, and 22.6 nm, respectively. It indicated the thermal treatment of the particles up to 800 C did not change the crystalline structure as the same size (18.7 nm) was observed for the synthesized hematite nanoparticles. [47] While the crystallite sizes significantly increased (22.5 nm, 22.6 nm) with the increase in calcination temperature, i.e., 1000 C and 1200 C, respectively. It was because of the sintering of crystallites at elevated temperatures, as reported previously. [48] The crystallite size of Magnetite-II and Magnetite-III were also calculated by the same procedure, which was found to be 10.86 nm and 11.76 nm, respectively ( Table 2). It pointed out that temperature was the main factor that strongly influenced the development and nucleation mechanism of the nanoparticles. As the temperature increased, it increased the agitation process, which facilitated the rapid growth of the crystallites. [48] These results concluded that the size of crystallite of iron oxide nanoparticles (Fe 2 O 3 , Fe 3 O 4 ) was greatly dependent on the composition of the pioneer of the reacting solution and the applied conditions of experiments.

Conclusions
Uniform hematite and magnetite nanoparticles were successfully prepared by Forced hydrolysis and controlled co-precipitated method.  Table 2. Crystallite size of the Magnetite-II, and Magnetite-III displayed in Figure 4E and G. The monodispersed particle systems were synthesized without using any surfactant or organic additives as having been used by various researchers for the synthesis of the same particle systems. The synthesis strategies utilized in this work were simple, economically feasible, and environmentally friendly compared with the research groups of Kongsat and Cursaro. [10,13] SEM study showed that the morphology of the prepared nanomaterial was strongly dependent on the pragmatic technological parameters. Control over the reaction conditions led to the reproducible recipe for the synthesis protocol of uniform particles of iron oxide in different morphologies. Characterization of the synthesized selected samples inveterates the phase transition of the particles and crystalline structure at high temperatures. The calcination of synthesized iron oxide nanoparticles at varied temperatures (800 C-1200 C) increases the crystallite size, led to aggregation and phase transition of the particles.