A novel atmospheric pressure hydrolysis without stirring and combustion–calcination process for the fabrication of magnetic Fe3O4/α-Fe2O3 heterostructure nanorods

Atmospheric pressure hydrolysis without stirring and a combustion–calcination method were utilized to fabricate magnetic Fe3O4/α-Fe2O3 heterogeneous nanorods. First, the β-FeOOH nanorods were fabricated via hydrolysis, and the concentration of Fe3+, hydrolysis temperature, and hydrolysis time were optimized. The optimal fabrication conditions were as follows: a 0.1 M FeCl3 solution was hydrolyzed at 90 °C for 2 h. The average length and diameter of the β-FeOOH nanorods fabricated under the optimal conditions were approximately 216 and 58 nm, respectively. Subsequently, Fe3O4/α-Fe2O3 heterogeneous nanorods were fabricated via a combustion–calcination process. The volume of absolute ethanol, calcination temperature, and calcination time were investigated to optimize the fabrication conditions of Fe3O4/α-Fe2O3 heterogeneous nanorods under the following conditions: absolute ethanol: 50 ml; calcination temperature: 300 °C; and calcination time: 2 h. Magnetic Fe3O4/α-Fe2O3 heterogeneous nanorods fabricated under optimal conditions were characterized with an average length of 199 nm, an average diameter of 51 nm, a zeta potential of +17.2 mV, and a saturation magnetization of 13 emu·g–1.


Introduction
As a new generation of materials, nanomaterials have several novel properties that are different from those of traditional materials, including the quantum tunneling, surface, and quantum size effects [1]. Nanomaterials have broad applications, including crop production [2], medicine and health [3], aerospace [4], environmental [5], aviation and space exploration, resources and energy [6], and biosensors [7].
Magnetic iron oxide materials have been widely used as nanomaterials in science and technology. Magnetic iron oxide materials typically have three crystal types: magnetite (Fe 3 O 4 ), hematite (α-Fe 2 O 3 ), and maghemite (γ-Fe 2 O 3 ) [8]. Magnetic ferrite materials have been widely applied in biomedicine, reaction catalysis, and electronic components owing to their excellent magnetic and catalytic properties, low cost, simple preparation, and high chemical stability. α-Fe 2 O 3 is the most common polycrystalline compound in nature in the form of minerals [9]. It is widely found in rocks and soil and has antiferromagnetic or weak ferromagnetic properties at room temperature. Fe 2 O 3 exhibits good catalytic activity, strong stability, low cost, and several morphologies. Furthermore, α-Fe 2 O 3 is paramagnetic above 956 K (Curie temperature) [10], and has a rhombohedral structure consisting of O 2and Fe 3+ [11]. α-Fe 2 O 3 is simpler to synthesize than other forms of iron oxide, because it is an end product of other forms of iron oxide. Furthermore, it is highly stable under natural conditions. In contrast, Fe 3 O 4 [12] has ferromagnetic properties at room temperature. It differs from other Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
forms of iron oxide in that its structure contains both bivalent and trivalent iron. Fe 3 O 4 is easy to magnetize; therefore, it exhibits a high magnetic response when affected by an external magnetic field. Fe 3 O 4 is a metastable oxide of iron oxide because it can be oxidized to α-Fe 2 O 3 at temperatures above 673 K [13]. Fe 3 O 4 nanoparticles have become a research hotspot for magnetic separation materials because of their high specific surface area, excellent biocompatibility, quantum size effect, and easy surface modification. However, Fe 2 O 3 has weak magnetism, limiting its further application. Simultaneously, the strong magnetism of Fe 3 O 4 causes it to agglomerate. To solve this problem, magnetic Fe 2 O 3 /Fe 3 O 4 heterogeneous nanomaterials have been developed [14,15], which exhibit a series of advantages such as stable performance, strong tolerance, and low toxicity. In addition, the saturation magnetization is between that of Fe 2 O 3 and Fe 3 O 4 , which further improves the directional transport ability [16].
From the perspective of polymer modification, nanomaterials with a large aspect ratio can provide polymers with excellent rigidity. Compared with three-dimensional spherical particles of the same size, two-dimensional rod-shaped nanoparticles have a longer circulation time and better transport and penetration performance in porous tissues, such as tumor sites. Moreover, the large surface area of the nanorods can result in stronger interactions [17]. Hence, rod-shaped nanoparticles have important application prospects in tumor imaging, diagnosis, and targeted drug delivery.
Various methods can be used to prepare ferrite nanomaterials, which can be roughly divided into liquid- [18], solid- [19], and gas-phase methods [20]. Among them, the liquid-phase method is the most widely used, including the coprecipitation [21], sol-gel [22], microemulsion [23], thermal decomposition [24], and hydrothermal methods. The coprecipitation method has attracted the attention of several studies because of its lack of pollution and controllable particle size. The sol-gel synthesis method can control the composition, microstructure, purity, and shape of ferrite by adjusting the sol concentration, stirring rate, and annealing temperature. Thermal decomposition, which is economical and environmentally friendly, is the simplest method for synthesizing ferrite. The properties of ferrite nanomaterials are closely related to their preparation method. Owing to their excellent physical and chemical properties, ferrite nanomaterials have been widely used in biomedicine, electronic components, reaction catalysis, and absorbing materials. Although ferrite nanomaterials are magnetically recyclable and easy to separate, their catalytic activity is not high; therefore, there are significant opportunities for further development.
In this study, atmospheric pressure hydrolysis without stirring and the combustion-calcination method were introduced for the fabrication of magnetic Fe 3 O 4 /α-Fe 2 O 3 nanorods, and the effect of all the factors in the process was determined. Some studies have investigated the effect of the solubility product of divalent metal hydroxides on the size and magnetism of nanoparticles formed in the process of co-precipitation [25]. Moreover, it has been reported that the synthesis of pure magnetite nanoparticles by the co-precipitation technique proved that the initial pH, temperature of the iron salt solution, and the final pH are key parameters in the controllable preparation of nanoparticles [26]. The atmospheric hydrolysis method is simple to operate with low cost raw materials. Compared with pure Fe 3 O 4 or Fe 2 O 3 nanomaterials [27], the magnetic properties of Fe 3 O 4 /α-Fe 2 O 3 heterostructure nanorods are moderate, do not agglomerate easily, and are convenient for later applications.

Experimental
2.1. Fabrication of β-FeOOH nanorods 5.41 g FeCl 3 ·6H 2 O (0.1 M) was placed in 200 ml distilled water with magnetic stirring. The obtained homogeneous solution was then transferred to a round flask, heated at 90°C for 2 h, and hydrothermally reacted without magnetic stirring. The resulting suspension was centrifuged to obtain the sediment. β-FeOOH nanorods were successfully fabricated by rinsing, drying, and grinding. The effects of initial Fe 3+ concentration (0.05, 0.1, 0.2, 0.3, and 0.5 M), hydrolysis time (2, 4, 6, 8, 10, 12, and 16 h), and hydrolysis temperature (60, 70, 80, and 90°C) were investigated to optimize the fabrication process. First, various concentrations of Fe 3+ were heated at 90°C for 2 h in a water bath to prepare the β-FeOOH nanorods. Subsequently, the solution with the optimal concentration of Fe 3+ was hydrolyzed at 90°C for various periods. Finally, the effect of the hydrolysis temperature on the β-FeOOH nanorods was investigated.

Fabrication of magnetic Fe 3 O 4 /α-Fe 2 O 3 heterostructure nanorods
The combustion-calcination process was employed to fabricate Fe 3 O 4 /α-Fe 2 O 3 heterogeneous nanorods. 5.41 g FeCl 3 ·6H 2 O (0.1 M) was rapidly dissolved in 200 ml ultrapure water and heated in a water bath at 90°C for 2 h. The suspension was then centrifuged and alternately rinsed six times with absolute ethanol and water. After drying for 12 h, the precipitate was mixed with varying volumes of absolute ethanol (20, 30, 40, 50, and 100 ml) to optimize the fabrication conditions. Subsequently, the mixture composed of the precipitate and absolute ethanol was placed in a crucible and then ignited, which was evenly dispersed by ultrasound. When the flame extinguished and the combustion product cooled to room temperature, the combustion product was calcined at 300°C for 2 h in a programmed temperature-controlled furnace [28]. Hence, a sample group was prepared to investigate the amount of ethanol. Subsequently, the products prepared using the optimal amount of ethanol were calcined at various temperatures (200, 250, 300, 350, and 400°C) for 2 h. After optimizing the amount of ethanol and calcination temperatures, the calcination times (0.5, 1, 2, 3, and 4 h) were investigated to determine the optimal fabrication conditions. The products obtained using the optimal amount of ethanol calcinated at the optimal temperature, were then calcinated for various periods. Magnetic Fe 3 O 4 /α-Fe 2 O 3 heterogeneous nanorods were successfully fabricated after grinding. In summary, the amount of absolute ethanol, calcination temperature, and calcination time were varied to optimize the fabrication process.

Characteristics of β-FeOOH and magnetic Fe 3 O 4 /α-Fe 2 O 3 nanorods
The phases of the β-FeOOH and Fe 3 O 4 /α-Fe 2 O 3 nanorods were characterized by x-ray diffraction (XRD). Their morphologies were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A vibrating sample magnetometer (VSM) was used to measure the magnetic properties, and the zeta potential of the Fe 3 O 4 /α-Fe 2 O 3 nanorods was measured using a laser particle size analyzer.  figure 1(B) indicates that the products had rod-shaped structures with good dispersion. After the measurements, the average length and diameter of the nanorods were approximately 216 and 58 nm, respectively. The reason for the rod-shaped structure may be that FeCl 3 was heated and hydrolyzed to produce Fe(OH) 3 and HCl, and then Fe(OH) 3 was further decomposed to produce β-FeOOH. HCl was easily volatilized during heating, owing to its low boiling point, resulting in rod-shaped β-FeOOH that was not corroded. Figure 1(C) shows that β-FeOOH had a rod-like structure and uniform size, and the average length and diameter were approximately 218 and 59 nm, respectively, which were consistent with the SEM morphology ( figure 1(B)).

Effect of reaction conditions on β-FeOOH nanorods 3.2.1. Effect of Fe 3+ concentration
The SEM morphologies of the β-FeOOH nanorods with various initial Fe 3+ concentrations and their relative yields are shown in figure 2. It is evident from figure 2(A) that when the initial Fe 3+ concentration was 0.05 M, the fabricated products were a mixture of nanoparticles and nanorods with significant agglomeration. The reason for this phenomenon may be the small amount of β-FeOOH caused by the low initial Fe 3+ concentration, which limits the formation of the rod-like structure. As clearly shown in figures 2(B)-(E), the resulting products were all rod-like structures with uniform size when the concentration was 0.1-0.5 M. However, the yield of β-FeOOH became lower with an increase in the initial Fe 3+ concentration (figure 2(F)). Simultaneously, the amount of FeOOH generated by hydrolysis was limited. When the initial Fe 3+ concentration was increased, the morphologies of the β-FeOOH nanorods remained similar. Hence, the optimal initial Fe 3+ concentration was determined as 0.1 M. Figure 3 shows the SEM morphologies of β-FeOOH nanorods heated at 90°C for varying hydrolysis times and their relative yields. The products were all rod-like structures upon investigation of hydrolysis time. The β-FeOOH nanorods exhibited a uniform size after less than 12 h. However, when the time was extended to 16 h, the nanorods agglomerated significantly, and the dispersion of the materials was poor. The plethoric β-FeOOH grains and long growth time caused by the extended hydrolysis time may be explained by the large size of the nanorods and their poor dispersion. The yields of the β-FeOOH nanorods after 12 h were not significantly different. When the hydrolysis time was 16 h, the yield increased slightly, indicating that the hydrolysis time had little impact on the yield when the hydrolysis time was less than 12 h. Hence, a hydrolysis time of 2 h was selected as the best condition owing to it being time-independent.  figure 4(E), exhibiting an upward trend with increasing temperature. This may be attributed to the accelerated speed of hydrolysis and the increase in β-FeOOH grains. Because the yield was affected by temperature, an optimal temperature of 90°C was selected.

Characterization of magnetic Fe 3 O 4 /α-Fe 2 O 3 heterogeneous nanorods
The SEM morphology of the magnetic Fe 3 O 4 /Fe 2 O 3 heterogeneous nanorods calcined at 300°C for 2 h with 50 ml absolute ethanol ( figure 5(A)) indicated that the heterogeneous nanomaterials exhibited rod-like structures with good dispersion. The average length and diameter was 199 and 52 nm, respectively, which were slightly lower than those of the FeOOH nanorods. The reason for this phenomenon may be that the nanorods appeared more stable with an increase in calcination temperature. Therefore, the size of the nanorods decreased with decreasing specific surface area and surface energy. The TEM image of the heterogeneous nanorods in figure 5(B) indicates that the rod-like structure was loose because of the rapid decomposition of FeOOH under high-temperature calcination. After the measurement, the average length and diameter of the magnetic Fe 3 O 4 /α-Fe 2 O 3 nanorods were approximately 198 and 49 nm, respectively, which were consistent with the SEM morphology. Figure 5     anhydrous ethanol was so large that it affected the ignition operation, and the combustion time was excessive, which increased the solvent and time related costs. With a change in ethanol content, the coercivity exhibited no clear change, however the area of the hysteresis loop first decreased and then increased. Therefore, 50 ml anhydrous ethanol was determined as the optimal volume for fabricating magnetic Fe 3 O 4 /Fe 2 O 3 heterologous nanorods. Figure 7 Figure 7(B) shows the hysteresis loops of the magnetic Fe3O4/α-Fe 2 O 3 heterologous nanorods calcined for different periods. The saturation magnetization of magnetic Fe 3 O 4 /α-Fe 2 O 3 nanorods increased slightly when calcinated for less than 2 h. This may be because Fe 2 O 3 was reduced to a greater extent by the reducing materials generated from the incomplete combustion of ethanol with longer calcination times, leading to an improved saturation magnetization of magnetic Fe 3 O 4 /α-Fe 2 O 3 heterologous nanorods. However, the saturation magnetization of the as-fabricated nanorods decreased when they were calcined for more than 2 h. The reason for this phenomenon may be that the reduced Fe 3 O 4 was re-oxidized to α-Fe 2 O 3 when calcined for a longer time, thus weakening the magnetic properties of the heterologous nanorods. With increasing calcination time, the coercivity exhibited no clear change, however the area of the hysteresis loop first increased and then decreased. Therefore, 2 h was selected as the optimal calcination time for the fabrication of the Fe 3 O 4 /α-Fe 2 O 3 nanorods.

Effect of calcination time and temperature
In summary, the optimal fabrication conditions for magnetic Fe 3 O 4 /α-Fe 2 O 3 heterologous nanorods were as follows: anhydrous ethanol: 50 ml; calcination temperature: 300°C; and calcination time: 2 h.

Conclusions
The fabrication and optimization conditions of β-FeOOH nanorods and magnetic Fe 3 O 4 /Fe 2 O 3 heterologous nanorods were introduced, and the following conclusions were made.
(1)The atmospheric pressure hydrolysis process was used to fabricate β-FeOOH nanorods. Various effects including the concentration of Fe 3+ , hydrolysis time, and hydrolysis temperature were optimized for 0.1 M FeCl 3 hydrolyzed at 90°C for 2 h. All the obtained β-FeOOH nanomaterials fabricated under the optimized conditions were rod-like structures with uniform size, and the average length and diameter were approximately 216 and 58 nm, respectively.
(2)Magnetic Fe 3 O 4 /α-Fe 2 O 3 heterogeneous nanorods were successfully fabricated via a combustioncalcination process. A calcination temperature of 300°C, 50 ml of absolute ethanol, and calcination time of 2 h were determined as the optimal fabrication parameters. The average length and diameter of the fabricated heterogeneous nanorods were approximately 199 and 51 nm, respectively, which were lower than those of the β-FeOOH nanorods. Moreover, the saturation magnetization was 13 emu·g -1 , exhibiting a superparamagnetic nature.