Preparation and characterization of magnetic α-Fe2O3/Fe3O4 heteroplasmon nanorods via the ethanol solution combustion process of ferric nitrate

An ethanol solution combustion process of ferric nitrate for preparing magnetic α-Fe2O3/Fe3O4 heteroplasmon nanorods was introduced. The influencing factors, including the solvent type and the calcination conditions, were discussed. Anhydrous ethanol was considered to be the most suitable solvent for the preparation of α-Fe2O3/Fe3O4 heteroplasmon nanorods, and the optimal calcination time was determined to be 2 h. By changing the calcination temperature, α-Fe2O3/Fe3O4 heteroplasmon nanorods with different phase compositions could be obtained, and the mechanism was explained in detail. The results indicated that the rapid combustion method could achieve the controlled preparation of α-Fe2O3/Fe3O4 heteroplasmon nanorods, which provided a general preparation approach for α-Fe2O3/Fe3O4 heteroplasmon nanomaterials.


Introduction
The preparation of functional nanomaterials has become a research hotspot because of their unique properties. Among them, hematite (α-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) have attracted extensive attention. α-Fe 2 O 3 nanomaterials are chemically stable, non-toxicity, and environmental-friendly [1][2][3]. However, its saturation magnetization (Ms) is low, which requires solving the recycling of materials [4]. And pure α-Fe 2 O 3 has poor electrical conductivity and sluggish surface kinetics, which limit its application [5]. Nanoscale Fe 3 O 4 is widely used in catalysis, sensors, biomedical fields, owing to its good biocompatibility and unique electromagnetic properties [6][7][8]. Nevertheless, the magnetic Fe 3 O 4 nanomaterials are in a high energy state, which are prone to agglomeration and are easily oxidized.
Heteroplasmon nanomaterials are obtained by combining inorganic materials of different properties into the same particle [9]. With the deepening of the research, it is found that the heteroplasmon nanocomposites can realize functional integration or provide new properties through the coupling of different components, thus solving the functional limitations of single-component nanomaterials [10]. For example, electric fields and voltages could manipulate the magnetic properties of α-Fe 2 O 3 /Fe 3 O 4 , which don't occur in pure α-Fe 2 O 3 and Fe 3 O 4 [11]. Fe 3 O 4 @α-Fe 2 O 3 core-shell structures exhibit better photocatalytic performance than α-Fe 2 O 3 nanoparticles and can be magnetically recycled [5]. What's more, the α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanocomposites have potential applications in photocatalysis, electrochemistry, dye adsorption, drug loading, and other fields [12][13][14][15]. Therefore, it is of great significance to prepare α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanocomposites with crystal uniformity and ideal composition.
On the preparation of α- Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. oxygen was removed by nitrogen protection. These techniques are usually difficult to be controlled and high energy consumption. To overcome the disadvantages of other methods, a facile and safe preparation method for the preparation of α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods was proposed, namely the ethanol solution combustion process of ferric nitrate, which has the advantages of easy operation, low cost, environmental protection, convenience to industrialization, short fabrication period.
Herein, we prepared magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods via the ethanol solution combustion process of ferric nitrate, and the influences of solvent types, calcination conditions on crystal structure, phase composition, and magnetic properties were investigated to realize the controllable preparation of magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods.

Experimental
8.080 g Fe(NO 3 ) 3 ·9H 2 O was added in 15 ml alcohol solvent. Then, the mixture was stirred for 2 h using the magnetic stirrer at room temperature. The obtained homogeneous solution was ignited. The precursor was obtained after the solution was completely burned. After that, the precursor was calcined in an air atmosphere. Finally, the magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods were collected by grinding the calcined products. figure 1(a)) and TEM ( figure 1(b)) were employed to examine the morphologies of the α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods prepared with 15 ml ethanol and calcined at 400°C. The average diameter of the rodshaped structures with uniform morphologies was about 8 nm, and the average lengths up to 34 nm. As can be seen from the EDS spectra in figure 1(c), the nanorods were only composed of iron and oxygen elements, and there was no impurity residue. The percentage of iron element was 42.34%, which was between pure α-Fe 2 O 3 (40%) and pure Fe 3 O 4 (42.86%), proving the existence of two phases. Figure 1(d) showed the BET specific surface areas were 19.3 m 2 g −1 , and the pore volume was 0.131 cm 3 g −1 . Figure 1(e) showed the TG-DSC curves of magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods, it could be divided into three weight loss stages. The weight loss of 4.59% occurred below 100 ℃ was assigned to the evaporation of water absorbed by the material, with an endothermic peak at 53.2 ℃ [17]. The major weight loss of 13.07% occurred from 100 ℃ to 400 ℃ with an exothermic peak at 261℃, which could be attributed to the decomposition of NO 3 2- [18], corresponding to the FTIR results (figure 4(e)). A slight mass loss was in the range of 400 ℃ to 800 ℃ with an exothermic peak at 467.6 ℃, which could be ascribed to the combustion decomposition of the few carbon skeletons remaining in the materials [18]. Photoluminescence (PL) could explore the optical properties of the material. The PL spectra of the magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods with the excitation wavelength of 226 nm was shown in figure 2(a). A strong ultraviolet emission peak was detected at 387 nm, which may be attributed to the excitonic PL. The excitonic PL spectrum was mainly caused by oxygen vacancies and defects on the surface of the material. Small size nanomaterials had more oxygen vacancies and defects, leading to high exciton generation probability and strong PL signal [19,20]. The Raman spectrum was shown in figure 2(b) to further confirm the structural composition of the heteroplasmon nanorods. The peaks at 222 cm −1 , 288 cm −1 , 411 cm −1 , 489 cm −1 , 608 cm −1 were attributed to α-Fe 2 O 3 , and the peak at 670 cm −1 was attributed to Fe 3 O 4 [12], which proved that α-Fe 2 O 3 phase and Fe 3 O 4 phase exist in the heteroplasmon nanorods.   Figures 3(a), (b) showed the XRD patterns and hysteresis loops (M-H curves) of the α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods prepared with different solvents. Although the Ms of α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods prepared using n-butanol as a solvent was 195.8 Am 2 kg −1 , due to the long carbon chain of n-butanol, the solubility of ferric nitrate was poor, leading to a lot of impurities in the combustion process, and low yield. When the solvent was methanol and isopropanol, the Ms were small, which was not conducive to the recovery by the external magnetic field. The Ms of anhydrous ethanol as solvent was 57.8 Am 2 kg −1 , while that of n-propanol as solvent was 62.66 Am 2 kg −1 . Considering its non-toxicity, low cost, and good performance, anhydrous ethanol was the most suitable solvent.

Influence of key factors on α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods
The effects of calcination time were investigated (figures 3(c), (d)). The shape of the diffraction peaks changed not significantly, but the intensity of the diffraction peaks changed greatly with the extension of calcination time, indicating that the calcination time mainly affected the growth degree of the crystals under the constant calcination temperature. As shown in figure 3(d), the Ms raised with the calcination time and then decreased, and the maximum Ms was 57.5 Am 2 kg −1 at 2 h. Oxidation was inevitable as the calcination time was extended, so the Ms decreased when calcination time was more than 2 h.  4(b)). The increase of calcination temperature lead to the conversion of Fe 3 O 4 into α-Fe 2 O 3 , resulting in the decrease of magnetic properties. When the calcination temperature reached 400 ℃, the phase ratio of Fe 3 O 4 decreased to 9.6%, and the Ms of α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods decreased to 57.5 Am 2 kg −1 . When the temperature was equal to and higher than 450 ℃, the pure α-Fe 2 O 3 nanocrystals were obtained with good crystallinity, and low Ms (figure 4(c)). The above results indicated that the target phase ratio nanorods could be obtained by controlling the calcination temperature.

Conclusions
An ethanol solution combustion process of ferric nitrate had developed to prepare magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods. Anhydrous ethanol was the most suitable solvent, and the calcination time was determined to be 2 h. The influence mechanism of calcination temperature was described in detail. When the calcination temperature was 200°C-400°C, the products were typical magnetic α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods; and with the increase of temperature, the proportion of Fe 3 O 4 in α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods decreased from 43.5% to 9.6%. The control preparation of α-Fe 2 O 3 /Fe 3 O 4 heteroplasmon nanorods was realized.