Shape-Evolution and Growth Mechanism of Fe 3 O 4 Polyhedrons

1 Innovation & Application Institute, Zhejiang Ocean University, Zhoushan 316022, China 2State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China 3State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute of Precious Metal, Kunming 650106, China

From a crystallographic viewpoint, Fe 3 O 4 has a cubic inverse spinel structure with oxygen anions forming a facecentered-cubic (FCC) close-packed structure.As a FCC crystal, a general sequence of surface energies may hold,  {111} <  {100} <  {110}, which indicates that the Fe 3 O 4 crystals usually exist with {111} lattice planes as the basal surfaces [16].The difference in , the growth rate in ⟨100⟩ to that of ⟨111⟩, results in a series of polyhedral shapes; however, the factors controlling this difference remain controversial.Zhang [18].Some polymers or additives were also fabricated to control the Fe 3 O 4 nanocrystals [19,20].The results from Kumar et al. indicated that the shapes and sizes of Fe 3 O 4 are controlled by adjusting the concentrations of the precursor with an invariable base medium [21].Most of the views indicate that the pH value of the precursor solution is the key factor affecting the morphologies of the products [16].Therefore, it remains necessary to systematically study the shape-evolution process and determine the underlying shape-evolution mechanisms for Fe 3 O 4 particles.
In this study, dodecahedral, octahedral, hexagonal, and triangular plates were synthesized using a simple solutionphase route, and the shape evolution of Fe 3 O 4 was systematically investigated by controlling the different conditions.Our results will enrich the controlling methods and clarify their evolution mechanism of Fe 3 O 4 polyhedrons.In a typical synthesis of Fe 3 O 4 polyhedrons, 1.7 mmol EDTA was added to an 8.0 mL aqueous solution of 1.0 mmol FeCl 3 ⋅6H 2 O under vigorous magnetic stirring.Then, N 2 H 4 ⋅H 2 O (85%) and NaOH were rapidly added to the mixture.The entire mixture was intensively stirred for another 30 min to obtain a homogeneous solution, which was sealed in a stainless-steel autoclave and maintained at 220 ∘ C for 10 h followed by natural cooling to room temperature.The resulting black solid products were rinsed with distilled water and absolute ethanol several times and finally dried in a desiccator at 60 ∘ C for 4 h for further characterization.

Experimental Section
Characterization.The products were characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM).XRD patterns were recorded with a Rigaku D/MAX 2200 VPC diffractometer using Cu K radiation ( = 0.15045 nm) and a graphite monochromator.SEM images were taken with a FEI Quanta 400 Thermal FE environmental scanning electron microscope.Samples were coated with gold before the SEM analysis.X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 spectrometer to characterize the particle surfaces using an energy analyzer in the pass energy mode at 20 eV and an Al K line applied as the excitation source.

Effect of NaOH Concentration.
According to many reports in the literatures [16,17], the pH value has a great effect on the formation of Fe 3 O 4 polyhedrons.To verify this claim, different conditions were implemented with NaOH additions of 2.0 mmol, 3.75 mmol, 5.0 mmol, and 12.5 mmol with a fixed dosage of hydrazine hydrate (1.5 mL).The powder X-ray diffraction (pXRD) patterns in Figure 1 match well with JCPDS Card No. 89-3854 for cubic Fe 3 O 4 with  =  =  = 8.395 nm, confirming that all of the products are cubic-structure iron oxides.However, it is difficult to distinguish the -Fe 2 O 3 and Fe 3 O 4 phases only from the XRD patterns because of their similarity.XPS analysis (Figure 2) was performed to further ascertain the phase of the synthesized typical products (NaOH = 5.0 mmol).The binding energies relating to Fe2p 3/2 and Fe2p 1/2 are approximately 710.6 and 724.9 eV, respectively.The Fe 2+ and Fe 3+ signals are forming the same peak, which can be deconvoluted in the two peaks ( ∼ 708 and 710 eV).No satellite peak between the Fe2p 3/2 and Fe2p 1/2 peaks is observed, and these are in good agreement with the results for Fe 3 O 4 reported in the literature [22].Therefore, the XPS results also confirmed the composition of Fe 3 O 4 .The morphologies of four samples were further examined via SEM observation, and the results are presented in Figure 3.The sample synthesized with NaOH = 2.0 mmol mainly consisted of truncated octahedrons (Figure 3(a)).Octahedrons were obtained when NaOH = 3.75 mmol (Figure 3(b)).Continually increasing the amount of alkali to 5.0 mmol, the products are mainly dodecahedral structures (Figure 3(c)).Some irregular spherical-like crystals were obtained when the addition of alkali was increased to 12.5 mmol (Figure 3(d)).The sizes of the four products decreased upon increasing the alkali content, which should result from the fast nucleation and growth rate along different facets [23].In addition, the results also reveal that, upon increasing the amount of OH − in the solution, the Fe 3 O 4 shapes transform into high-energy dodecahedrons.The SEM images of the samples (Figure 4) reveal that the morphology evolution of samples was controlled from plate to octahedron to truncated dodecahedron and eventually to a complete dodecahedron upon increasing the dosage of N 2 H 4 ⋅H 2 O from 0.8 mL to 2.5 mL.Herein, the alkaline contents in the solution were all adjusted for pH = 9.Therefore, the experimental results indicate that the dosage of the reducing agent can also bring the shape evolution of Fe 3 O 4 polyhedrons.Xuan et al. observed that the Fe 3+ and reductant concentration of vitamin C directly affected the final morphology in the preparation of Fe 3 O 4 [24].Xu et al. also observed that an appropriate EG/H 2 O ratio in the solvent is crucial for the formation of polyhedral nanocrystals [1].Our results are consistent with these reports and indicate that a high concentration of reductant favors the formation of high-energy surface polyhedrons.

The Effect of Concentration of Fe 3+ in the Starting
Materials.Further, the effect of Fe 3+ concentration in the starting materials was also studied for shape evolution of Fe 3 O 4 through four other experiments according to the similar synthesis process; that is, the amount of FeCl 3 ⋅6H 2 O is 0.05 mmol or 2.5 mmol differently.The SEM images of the as-obtained samples corresponded to Figure 5.Under the condition of the low monomer concentration ([Fe 3+ ] = 0.5 mmol), the as-obtained products are dodecahedrons (Figure 5(a)).[Fe 3+ ] was increased to 2.5 mmol at fixed pH = 9; some irregular polyhedrons and many small particles are found, which probably shows there are two different growth routes in the process.The irregular and big polyhedrons should be the result of Ostwald ripening, and many small particles are generated probably due to the fast nucleation rates.
Finally, we studied the effect of EDTA on the morphology at pH = 9, and the results are presented in Figure 6.In the presence of EDTA, the morphologies of the products are micrododecahedrons about 3 m (Figure 6(a)).The morphologies changed from micrododecahedrons to nanooctahedrons in the absence of EDTA, although the nanooctahedrons are not perfect (Figure 6(b)).This result shows that EDTA not only alters the shapes of products, but also favors the formation of the bigger crystals, which is probably because EDTA has a strong coordinating ability with Fe 3+ and forms a very stable complex ( formation = 10 25 ), which reduces the nucleation and growth rate of Fe 3 O 4 crystals.Based on the experiment results, we can summarize the shape evolution mechanism of Fe 3 O 4 polyhedrons as in Table 1.When the NaOH concentration was high, the growth rates on various planes are fast that the differential growth is not significant, leading to a spherical shape.Except for the spherical-like structure, the typical shapes of Fe 3 O 4 in this paper also include octahedrons, dodecahedrons, and plates.The shapes and facets of the as-synthesized Fe 3 O 4 crystals are sketched in Figure 7 [17,25,26].In general, the facet growth rates were governed by the intrinsic surface energy.As a FCC crystal, a general sequence of surface energies may hold,  {100} <  {111} <  {110} in spinel oxides [23].However, from the perspective of the growth kinetics, it is widely accepted that the growth rate of crystal facets can be controlled by experiment conditions, such as concentrations of primary materials, reaction temperature, and the surfactants.The as-grown crystal morphology is dominated by the slow-growing faces because the fast-growing faces may grow out and not be represented in the final crystal habit.

Growth and Evolution Mechanism of the
The plate-like particles expose the {100} and {111} basal surfaces, which are with lowest surface energy among all of polyhedrons in this paper, so this structure can be obtained under the lowest concentration of reducing agent.Many results have reported that a difference and change of cation site occupancy in spinel oxide result in some different  [32].Therefore, in this paper, it is suggested that the generating rate of Fe(II) reduced from Fe(III) probably affects the growth speed along different facets, which further determined the shapes of the final products.At the same pH value, the morphological evolution of Fe 3 O 4 from octahedron into high-energy dodecahedron was observed upon increasing the dosage of hydrazine because the hydrazine hydrate as a strong reducing agent increased the generating speed of Fe(II).At the same dosage of hydrazine hydrate, OH − enhanced the hydrazine reduction ability by improving the solution chemical potential, further improving the reducing rate of Fe(III) into Fe(II) [33].Therefore, we propose that a high reducing rate of Fe(III) into Fe(II) favors the formation of the dodecahedral shape, in turn favoring the octahedron formation.

Conclusions
In this study, the morphological evolution of Fe 3 O 4 was systematically investigated.The platy, octahedral, dodecahedral, and spherical Fe 3 O 4 were synthesized through tuning the pH of the system, dosage of hydrazine hydrate, precursor concentration, and the presence/absence of EDTA.Based on the experimental results, both the nucleation and growth rate and the generating rate of Fe(II) affect the shapes and sizes of as-synthesized products.The slow nucleation and growth rate favor the formation of low energy plate-like products, and the spherical crystals were obtained as the result of extremely fast nucleation and growth rate.The fact that the different terminated planes are mainly composed of different Fe atoms and Fe(III)/Fe(II) ratios probably makes the reducing rate of Fe(III) into Fe(II) control the evolution of shape from decahedrons to octahedrons.
2 H 4 ⋅H 2 O Dosage.Using the above-mentioned route and fixing the pH at 9, the effect of the N 2 H 4 ⋅H 2 O dosage was investigated in the range of 0.8-2.5 mL.
Different Polyhedrons.Fe 3 O 4 octahedrons are surrounded by eight {111} facets, and dodecahedral crystals expose the {110} basal surfaces.The top and bottom surfaces of Fe 3 O 4 triangular plates are bounded by {111} facets, and the three sides are bounded by {100} facets[25].The exposed facets of hexagonal plates are the same as those of the triangular plates, which can be considered as the result of the alternating arrangement of six triangular plates.

Figure 7 :
Figure 7: The typical morphologies in our work and sketches of the corresponding crystal structures.
et al. fabricated Fe 3 O 4 octahedrons and suggested that ethylenediaminetetraacetic acid (EDTA) provides the octahedral chemical environments for Fe 3+ ions [17].Chen et al. reported that the reaction temperature and amount of N 2 H 4 ⋅H 2 O play important roles in determining the sizes and morphologies of the Fe 3 O 4 nanocrystals Preparation.Iron trichloride (FeCl 3 ⋅6H 2 O, Guangdong Guanghua Chemical Co., Ltd.), EDTA [(C 10 H 16 O 8 N 2 ),

Table 1 :
[31]experiment condition and growth mechanism of typical Fe 3 O 4 morphologies.Fe 3 O 4 has an inverse spinel structure with the formula (Fe 3+ )A(Fe 2+ Fe 3+ )BO 4 .Tetrahedral sites (A site, T d ) are occupied by Fe 3+ , whereas octahedral sites (B site, O h ) are occupied by equal numbers of Fe 2+ and Fe 3+ in Fe 3 O 4 bulk[23].The shape-dependent occupancy of the cation sites was recently measured using X-ray magnetic circular dichroism (XMCD) spectra.Cheng observed that the different terminated planes are mainly composed of different Fe atoms and Fe(III)/Fe(II) ratios[31].Hu et al. reported the effect of the cobalt doping concentration on the crystalline structure Co  Fe 3-x O 4 nanoparticles