Preparation and characterization of Fe3O4 nanoparticles via a hydrothermal process with propanediol as the solvent

Fe3O4 nanomaterials have received great attention in various technology fields. However, the limitations are Fe3O4 is easy to agglomerate and obtaining Fe3O4 nanoparticles of tunable magnetism and controllable size, and well-dispersed ability remains a challenge. In this study, a simple hydrothermal process with propanediol as the solvent was used to prepare Fe3O4 nanoparticles. In the optimization of preparation conditions, three key factors (hydrothermal temperature, hydrothermal time, and solvent volume) were optimized by x-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The results showed that the magnetism and the phase content of the prepared Fe3O4 were controllable during the optimization process. The optimum hydrothermal temperature was 170 °C, hydrothermal time was 18 h and solvent volume was 40 ml. The elemental composition, surface morphology, and magnetic properties of Fe3O4 nanoparticles were characterized. The prepared Fe3O4 nanoparticles exhibited superparamagnetic properties and high crystallinity, with an average particle size of 20 nm, a specific surface area of 84.756 m2 g−1, a pore volume of 0.265 cm3 g−1, and saturation magnetization (Ms) of 129.38 emu g−1.


Introduction
The preparation and research of magnetic nano-functional materials have attracted extensive attention due to their unique optical and electrochemical properties [1,2], good biocompatibility [3,4], and magnetic separation characteristics [5]. Among them, magnetic Fe 3 O 4 nanomaterials have been widely studied as substrates for various technical applications [6,7]. Fe 3 O 4 nanoparticles and Fe 3 O 4 nanocomposites are widely used in biosensing, catalysis, targeted drug delivery, and environmental remediation research [8][9][10]. Fe 3 O 4 nanomaterials have many advantages such as large specific surface area, low toxicity, superparamagnetism, and easy functionalization [11]. However, obtaining Fe 3 O 4 nanoparticles of tunable magnetism, controllable size, and well-dispersed ability remains a challenge, which limits its practical application. Therefore, it is necessary to develop a simple and general method to prepare Fe 3 O 4 products with desirable crystal structure and magnetic properties, and uniform morphology. Among the common methods for the preparation of Fe 3 O 4 nanomaterials, such as microemulsion [12], co-precipitation [13], sol-gel [14], and hydrothermal [15,16], the hydrothermal method is well developed, which lacks the need for calcination and complex pretreatment, and the products have good crystallinity.
In this work, a simple hydrothermal process with propanediol as solvent was selected to prepare Fe 3 O 4 nanoparticles [17]. The resulting Fe 3 O 4 nanoparticles were optimized by XRD and VSM, and the optimum reaction temperature, reaction time, and reaction volume were verified. These results confirmed the successful preparation of superparamagnetic Fe 3 O 4 nanoparticles (Ms = 129.38 emu g −1 ) with high crystallinity and Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. tunable magnetism, whose average particle size was 20 nm, the specific surface area was 84.756 m 2 g −1 , and the pore volume was 0.265 cm 3 g −1 .

Preparation of Fe 3 O 4 nanoparticles
The Fe 3 O 4 nanoparticles were prepared via a hydrothermal process. Firstly, 1.5 g of FeCl 3 and 4.1 g of CH 3 COONa were added to 40 ml of C 3 H 8 O 2 , and the solution was ultrasonically dispersed in C 3 H 8 O 2 for 10 min. The well-dissolved mixture was transferred to a glass conical flask and heated in a 95°C water bath for 30 min under vigorous stirring. Subsequently, according to the reported literature, the hydrothermal method was used for the preparation of magnetic Fe 3 O 4 nanoparticles. The stirred mixture was then transferred to a Teflon-lined autoclave and heated at 170°C for 24 h. The solid products obtained were separated by centrifugation or an external magnetic field, washed with ethanol and water three times, and then vacuum dried overnight at 90°C to obtain Fe 3 O 4 nanoparticles.

The optimization of preparation conditions
When magnetic Fe 3 O 4 nanoparticles are applied in practical fields, appropriate morphology, crystal properties, and magnetic properties are very important to improve the specific surface area and avoid unnecessary agglomeration. Therefore, the influences of hydrothermal temperature, hydrothermal time, and hydrothermal pressure (solution volume) in the autoclave were changed successively to investigate the crystal and magnetic properties of the nanoparticles. The optimum preparation conditions were determined by XRD and VSM experiments. Firstly, based on the original experimental conditions, the hydrothermal temperatures were set as 110°C, 130°C, 150°C, 170°C, and 190°C, respectively and the optimum temperature was selected. Under the optimum temperature, the hydrothermal time was set as 3 h, 6 h, 12 h, 18 h, and 24 h, respectively and the optimum time was chosen. Subsequently, the effect of solvent volumes (20, 40, 60, and 80 ml) was examined in the same way.

Characterization of magnetic Fe 3 O 4 nanoparticles
To verify the preparation of magnetic Fe 3 O 4 nanoparticles, the properties of the nanoparticles prepared under the selected conditions were investigated, including SEM, TEM, EDS, XRD, and BET. According to SEM (figure 1(A)) and TEM ( figure 1(B)) results, the prepared nanoparticles were a nearly spherical shape, with uniform size and an average particle size of about 20 nm. EDS spectrum (figure 1(C)) confirmed the presence of Fe and O elements, where the atomic percentage (at%) of Fe was 37.69% and that of O was 51.68%. The presence of Au peak is due to gold spraying to increase the conductivity. The XRD pattern of the nanoparticles compared with the Fe 3 O 4 standard card (JCPDS NO.75-0449) was shown in figure 1(D). The diffraction peaks at 30.1°, 35.8°, 43.5°, 57.5°, 63.1°, and 74.8°corresponded to the diffraction peaks of the standard card, indicating that the prepared nanoparticles were composed of Fe 3 O 4 and had a high crystallinity. The specific surface area calculated according to the BET equation was 84.756 m 2 g −1 , pore volume was 0.265 cm 3 g −1 proving that the Fe 3 O 4 nanoparticles prepared by exploring the hydrothermal method have the advantages of large specific surface area and pore volume (Supplementary information, figure S1). Figure 2(A) presented the XRD patterns of the products at different hydrothermal temperatures (110°C, 130°C, 150°C, 170°C, and 190°C) and showed the crystal properties of the as-prepared Fe 3 O 4 nanoparticles. With the increase in temperature, the characteristic peaks of Fe 3 O 4 appeared at 170°C, and the diffraction peak intensity at 35.8°increased. The peak of 35.8°was a typical diffraction peak of Fe 3 O 4 , which confirmed the formation of the inverse spinel structure Fe 3 O 4 [18]. When the hydrothermal temperature was lower than 170°C, there was no typical Fe 3 O 4 characteristic peak, indicating that the solid product prepared at low temperatures may not contain Fe 3 O 4 . With the increase in temperature, the diffusion ability of reaction atoms was enhanced, so the grain size will be increased. According to Debye Scherer's equation [19], when the full width at half maximum (FWHM) of characteristic peaks is narrower, the grain size is larger. Figure 2(A) showed consistent results, the FWHM of 190°C was narrower than that of 170°C, hence the grain size was larger.

Effect of hydrothermal temperature on Fe 3 O 4 nanoparticles
In figure 2(B), the hysteresis loop of the sample was characterized and the Ms value was obtained. The results indicated that the products generated at low temperatures were nonmagnetic material, suggesting that Fe 3 O 4 could not be generated at low hydrothermal temperatures. Although both 170°C and 190°C showed typical Fe 3 O 4 characteristic peaks, the hysteresis loops indicated that the magnetism at 170°C was lower than that at 190°C . Strong magnetism will aggravate the agglomeration of the product, which is not conducive to practical application. To reduce self-agglomerations, 170°C was chosen as the subsequent reaction temperature. The prepared Fe 3 O 4 nanoparticles had a coercivity of 0 and had no remanence, showing superparamagnetic properties [20]. This property allows the nanoparticles to be easily dispersed and to be easily recovered by magnetic fields for practical applications. In addition, the results proved that the magnetism of the Fe 3 O 4 nanoparticles prepared using the hydrothermal process was tunable, which confirmed that our work can provide a general method for the preparation of Fe 3 O 4 nanoparticles with favorable magnetic properties.

Effect of hydrothermal time on Fe 3 O 4 nanoparticles
The hydrothermal time was set as 3 h, 6 h, 12 h, 18 h, and 24 h, respectively, and the hydrothermal temperature was 170°C. The XRD spectra and hysteresis loop of the materials obtained under different hydrothermal time were depicted in figure 3. The XRD results were consistent with hysteresis loops. The grain size, the intensity of XRD characteristic peak, and the Ms values were positively correlated with the hydrothermal time, which may be due to the continuous growth of Fe 3 O 4 crystals with the lengthening of hydrothermal time. When the hydrothermal time was 12, 18, and 24 h, the color of the generated product was black, and the characteristic peak attributed to Fe 3 O 4 appeared. However, the crystallinity at 12 h was unfavorable. At 18-24 h, the XRD peak position of the nanoparticles was consistent with the standard card. In addition, the crystallization degree and the peak intensity of Fe 3 O 4 nanoparticles prepared by hydrothermal reaction for 18 h were preferred, and the Ms value was lower (129.38 emu g −1 ). The exploration of the hydrothermal time revealed its important effects on the formation of Fe 3 O 4 nanoparticles, and 18 h was selected as the optimal hydrothermal time.

Effect of solution volume on Fe 3 O 4 nanoparticles
Different solution volumes will lead to different internal pressures in the Teflon lining, resulting in different shapes or crystalline properties of the nanomaterials [21,22]. Therefore, the effect on solvent volumes (20-80 ml) was examined. As shown in figure 4(A), as the solvent volume increased, the XRD characteristic peak intensity of the nanoparticles gradually decreased, and the grain size kept decreasing. According to the hysteresis loops in figure 4(B), the Ms value of the nanoparticles increased from 20 ml to 60 ml and decreased at 80 ml. The Ms value at 20 ml was the lowest (75 emu g −1 ). However, only a very small amount of Fe 3 O 4 nanoparticles was  generated at 20 ml, which was uneconomical and environmentally unfriendly. Since the Ms value at 40 ml was low and the production was appropriate, the volume of solvent was selected as 40 ml.

Conclusions
In this study, a hydrothermal method was used to prepare magnetic Fe 3 O 4 nanoparticles. By exploring the effects of key factors on the grain size and magnetic properties of nanoparticles, the optimal conditions were selected and the final prepared Fe 3 O 4 nanoparticles were characterized. The conclusions are as follows: (1) Firstly, the hydrothermal temperature, hydrothermal time, and solvent volume were changed successively to screen the optimum products. The results of the XRD and hysteresis loop showed that the optimum hydrothermal temperature was 170°C, hydrothermal time was 18 h and solvent volume was 40 ml.
(2) The elemental composition, surface properties, and magnetic properties of Fe 3 O 4 nanoparticles were characterized by SEM, TEM, EDS, VSM, BET, and XRD. These results confirmed the successful preparation of superparamagnetic Fe 3 O 4 nanoparticles (Ms = 129.38 emu g −1 ), whose average particle size was 20 nm, the specific surface area was 84.756 m 2 g −1 , and the pore volume was 0.265 cm 3 g −1 .

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).