Synthesis and characterization of α-Fe2O3 hollow-spheres through a new hard-template strategy

doi:10.21203/rs.3.rs-1442975/v1

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Synthesis and characterization of α-Fe2O3 hollow-spheres through a new hard-template strategy

https://doi.org/10.21203/rs.3.rs-1442975/v1

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A new route to manufacture α-Fe₂O₃ hollow spheres with 142 nm in diameter was presented. The proposal is based on the methodology described by Yoshikawa in combination with the paper published by Kobayashi and using polystyrene spheres (PS with 200 nm in diameter) as a hard template. The transmission electron microscopy, Raman spectroscopy and UV-Vis analyses, indicated the intermediate stage with product formed by a spherical 12 nm shell (α-FeOOH) supported on the PS surface (YK200a sample). At the final stage, the α-Fe₂O₃ hollow spheres without PS template showed some traces of alpha iron hydroxide (YK200b sample). The detection of crystalline phase was possible by using X-ray diffraction combined and Raman spectroscopy.

hollow nanospheres

iron oxide

hard-template

polystyrene nanospheres

The use of nanomaterials is considerably due to the wide field of applications in devices, cosmetics and food [1]. In addition to this applications, the motivation due to unique properties achieved in systems formed by nanomaterials is is getting bigger. Incorporation of nanostructures in order to develop new products has been the motivation of different research groups. Some of these researches are already well established in three pillars; composition, size and shape [2]. The choice of a given material is related to the physical-chemical property achieved in relation to optical, magnetic, electrical, thermal, mechanical and rheological [3, 4]. The application of nanoparticles is well established, generating great interest in the development of new advanced nanostructured materials [5]. Synthesis of new nanostructured materials and the search for new methodologies is a trend today, and promises to play a very important role in future technologies. This, thanks to the properties allowing for versatility and applications [6].

Hollow nanomaterials, which an empty space in their interior, can be single-shell, multi-shell or rattle-type [7]. These materials are obtained in various sizes, different geometric shapes and compositions. Considered as a special class of nanostructured materials, search for their development has been increasing in recent decades [8, 9]. Caruso [10], used a hard-template strategy to synthesize submicrometric silicon oxide spheres. Due to their very high surface area and low density, these structures exhibit great potential and a wide field of application in sensing, chemical catalysis, rechargeable batteries, supercapacitors and energy production [11]. Incorporating specific characteristics to the structure of a nanometric shell can be used in a more targeted and specific way [12]. Fang [13], presented an improvement of a catalytic process applying hollow structures of alumin-silicates in catalysis activity, through the control of pore size.

Due to the confined environment of hollow structures, the search for new alternatives has aroused considerable interest for application in catalysis [14]. Structures of hollow semiconductor octahedrons based on CuInS₂, obtained by using the self-template synthesis, were used, aiming at the production of hydrogen through photocatalytic processes. Hollow structures of spherical morphology, composed by iron oxide, can also be observed in works demonstrating the application of these structures in lithium batteries [15]. Iron is one of the most abundant elements on earth [16] with high biocompatibility and magnetic properties.

For different applications, is necessary to achieve some characteristics such as: monodispersity, shape control, reproducibility, scalability and synthesis complexity. Magnetic resonance imaging (NMR) is a very important technique in medicine, and it is possible to observe biological events such as gene expression, metastasis at cellular and subcellular levels, and use of magnetic particles as contrast agents [17]. Magnetic particles have already been found in lung, liver and spleen, originating from contrast material for NMR imaging [18]. Chaves et al. [19] reported the accumulation of magnetic particles in mouse lungs for three months, reporting no cytotoxic association with the magnetic material. These particles are metabolized enzymatically at the cellular level, as reported by Robson and Nissim [20], attributing and highlighting the importance of ferritin and transferritin in the biodegradation process of these iron nanoparticles. The excellent biocompatibility, added to the confined environment (empty space) and the very low density, give the hollow iron oxide structures promising applications in controlled drug release, contrasting agent in NMR imaging, among other biological applications.

Hollow materials formed from magnetic phases have a promising application in hard disk, random memories (RAMs) or in biological applications, with controlled release of drugs and magnetohyperthermia [21]. Some works have already demonstrated the importance of the study and the unique characteristics of magnetic materials on a nanometric scale. Quan-Lin Ye [22] reported optical properties and magnetic anisotropy of Fe₃O₄ submicron hollow-sphere systems compared to their dense form, having thin film characteristics due to shell thickness. Streubel, et al. reported the fundamental magnetization states due curved geometries [23]. Magnetic characteristics and properties have already been demonstrated by Revoredo et al., demonstrating magnetic properties in magnetic nanohelmet arrays [24]. Satoru Kobayashi et al. [25] describe studies on Fe₃O₄ hollow spheres, showing the presence of vortex-like magnetic states describing first-order magnetization reversal. These phenomena in nanostructured materials with curved morphologies that present a one-domain magnetic have already been demonstrated both experimentally [26] and in theoretical works [27].

Using a hard-template route, Yoshikawa and collaborators [28] synthesize hollow structures of iron and cobalt by direct chemical deposition, using 600 nm polystyrene (PS) particles as a template. The search for sizes smaller than 600 nm is important for magnetic applications [29, 30] and is not possible by the above metodology. Given this, and all the application possibilities involving hollow spherical structures, the present work investigate and develop a new coherent synthesis strategy in order to obtain hollow iron oxide structures with diameters smaller than 200 nm.

For the syntheses, FeCl₃.4H₂O (Sigma Aldrich – purity 99.9%), ammonium acetate (Dynamic purity 98%), urea, polyvinylpyrolidone (PVP) (MM10000) and 203 nm polystyrene spheres (PS), were used. The PS spheres were supplied by ThermoFisher Scientific, with product number 3020A, diameter of 203 ± 5 nm, size distribution less than 3% and 10.5 g/cm³ density. The synthesis strategy was by using the 200 nm diameter PS spheres as hard-template. The reactions were carried out by the solvothermal method using mini teflon reactors with a capacity of 30 mL and heating in a muffle furnace manufactured by EDG. Figure 1 show a squematic representation of preparation route and sacanning electron microscopy of the PS template used in the present work. The reaction was performed following the procedure described by Yoshikawa [28] with variations that correspond to the new methodology reported here. The combination of the two methodologies mentioned here gave the possibility to report a new way of making spherical shells of less than 150 nm in diameter, as we'll see further ahead.We started with the first experimental condition using water as solvent and iron III chloride as iron source. For this, urea was used in the presence of polyvinylpyrolidone PVP at 100°C for 20 h and heating rate of 1°C/min. This is because urea degrades at temperatures above 85°C, having as a product of this degradation carbonate and ammonia ions. With this, a gradual increase in pH is obtained, for a smoother growth of the bark on the particle. With this route particles are not precipitated, only dilution in Milli-Q water is made.

The methodology adopted is a new proposal based on Yoshikawa (Y) and Kobaiashy (K). PS spheres with approximately 200 nm in diameter were used and the product was prepared until the autoclave step (YK200a sample). In another sequence, the preparation was continued in the oven muffle to obtain hematite spheres (YK200b sample), Fig. 1. The YK200a and YK200b samples were the results of innovative proposal in the present work, sintetized according to a modification of Yoshikawa's work [28]. The modification of the original method was because the desired shells in our work were smaller (200 nm) than those obtained by Yoshikawa (600 nm). The modification was made based on the work of Kobayashi [25] with a different solvent. For this, 400 µL of an aqueous solution of 1% m/v concentration of PS particles was separated in 4 tubes of 2 mL Eppendorf type. Each tube containing 100 µL diluted in 1.5 mL of ethanol and precipitated in a centrifuge under 10000 rpm. The precipitate was dispersed in 5 mL of ethylene glycol and subjected to an ultrasonic bath for 10 min. Then, 10 mg of FeCl₃.4H₂O (0.05 mmol), 35 mg of ammonium acetate (0.5 mmol) were added, leaving it under magnetic stirring for 30 minutes. The system was transferred to a Teflon reactor and heated in a muffle furnace at 100°C for 20 hours, with a heating ramp of 0.5°C/minute. The reaction crude was precipitated in a centrifuge under 10000 rpm for 10 minutes and resuspended in water 3 times, followed by the same procedure 3 times with ethanol. The particles were resuspended and stored in ethanol (YK200a). In this step was obtained an iron-based composite shell supported on 200 nm diameter PS spheres.

Finally, the samples YK200a suspended in ethanol were dropped onto silicon substrate until the solvent was dried in a desiccator for 24 hours. After complete evaporation of the solvent, the substrate was heated in a muffle oven at 550°C for 4 hours with a heating rate of 20°C/minute (last step in Fig. 1). The final product of this new synthesis was obtained, calling the samples as YK200b. It is worth mentioning that, at this stage samples are spherical shells with an iron-based composition and without PS support.

Scanning electron microscopy analyzes were performed in a MIRA3 TESCAN model microscopes. Transmission electron microscopy analyzes were performed using a 120 kV TECNAI SPIRIT microscope, brand FEI, was used. For the characterization by X-ray diffraction, a diffractometer in low angle configuration, Rigaku brand SmartLab model, with Cu-Kα radiation (1.541874 Å), was used. Measurements were performed at a scan interval between 20° and 80°, with a step of 0.01° and an acquisition rate of 1 second. Reports by Sendova [31] show α-FeOOH (goethite) and α-Fe₂O₃ (hematite) using optical instruments. In our analyses, an iHR320 – Horiba equipment was used.

Morphology

In Fig. 2 are presented SEM micrography by secondary electrons of YK200a sample. From these data an histogram with diameter distribution is also displayed and to have a more complete view, TEM bright-field images of these samples are also shown. This result, in contrast to the one obtained by Yoshikawa's methodology, shows an indicative of the success of our proposal, in order to manufacture hollow spheres supported in PS of 200 nm in diameter, thus suggesting the total covering of the PS cores. These results led us to further investigate these conditions perform transmission electron microscopy. In the TEM image presented in Fig. 2 it is possible to confirm the result, showing spheres with a very uniform coating.

The samples were measured and the images were quantified, for this was constructed particle diameter measurements on the images obtained by scanning electron microscopy and was represent in Fig. 2. In this figure is observed a distribution with a mean value of 225 nm, and comparing to the 203 diameter of the PS particles, an average thickness of 21 nm, was obtained. In order to adquire more appropriate thickness values of the material on the PS surface the transmission electron microscopy images was showed. Figure 2 presents the inner diameter of 185 ± 8 nm, which represents a different value from the PS spheres initially used (203 ± 5 nm). Taking the difference between the average internal and external values of these two measurements, it is possible to state that the deposited shell has an average thickness of 12 nm (10.5 nm by SEM and 12 nm by TEM). From the related results of morphology, the proposal was aceptable for coating PS spheres with uniform cover.

After that, the final step was to remove the PS template by a controled calcination. Initially, the spheres were suspended in ethanol at an appropriate concentration and deposited by dripping onto the silicon substrate, then dried at room temperature. Subsequently, the substrate containing YK200a was subjected to heat treatment at 550°C for 4 h, to ensure total degradation of the organic core (PS) (Fig. 1). As the result, the hollow sphere of sample named YK200b was obtained. A heating rate of 20°C/min was used, because this rate cannot be too soft so that the core does not lose its spherical shape.

Figure 3 shows a scanning electron microscopy image for YK200b sample. It is possible to observe well-formed hollow spheres with well-defined regularity. This result is encouraging as it is the particles of YK200a, after a severe heat treatment (at final step in Fig. 1). Initially was expect the deformed or collapse spheres because of this hard hod step. This was defined by our treatment conditions, suitable for eliminating the PS support. On this image, diameter measurements were taken from the spheres and the result is in the histogram, Fig. 3. The histogram obtained from the SEM data and the TEM image show that the mean value of the spheres is very similar. In the adjustment on these data, an average diameter of 142 ± 9 nm was obtained, with a reduction in the average diameter of initial PS spheres (200 nm). With the above description, a very important part of the discussion was concluded, which is the morphological analysis and size estimation of the hollow spheres. Another important issue already resolved at this point is that the YK200b sample does not show PS signals, according to the contrast shown in the TEM images in Fig. 3.

Composition and atomic ordering

The composition of above spheres are the question that has not been resolved up to this point in the manuscriptand, so was performed an initial study by EDX analysis. For the characterization of the hollow sphere without PS (YK200b), an elemental mapping analysis was performed and compared to the same result on the sample with PS core (YK200a). At this stage, we only seek to understand the real absence of the PS core in the YK200b (Fig. 3), after the YK200a heat treatment. Figure 4 shows iron and carbon maps from YK200a sample. Through the images generated by the carbon signal, the existing PS nuclei in the YK200a was demostrated. From the map generated by the characteristic line from iron, questionable is the coating of the spheres. With a thickness of approximately 20 nm, the sample – electron beam interaction and the contribution of other species can attenuate the signal coming from the coating, generating the result presented here. This issue is clarified after the YK200b mapping analysis, in which the PS was removed by heat treatment. In Fig. 5, from the map generated by the iron and in the map generated by the oxygen, that a coating on the YK200b sample with an iron-based phase containing oxygen, exist. Furthermore, the secondary electron micrograph shows the defined edges of a possible hollow contour. This result had already been observed in Fig. 3, showing that there was elimination of PS and that the phase is iron and oxygen is predominat.

The question related to the composition of the iron and oxigen based shell can be answered by using X-ray diffraction (XRD), in YK200a and YK200b. The diffractograms showed a crystalline phase in YK200b, which has the characteristic peaks corresponding to hematite phase (Fe₂O₃), Fig. 6. The data has been refined to coincide with ICSD letter Ref. Code: 01-073-0603. The phase has a rhombohedral structure with space group R-3c and lattice parameters of 5.0342 Å and 13.7483 Å. The data corresponding to YK200a did not show a diffraction peak, possibly due to the presence of the PS sphere or because the shell being formed by amorphous material. As we can see in Fig. 2, the contrast in the TEM image indicates that there is little crystallinity in the layer over PS. Due to the reaction method the possibility that there is iron hydroxide exist, however, XRD measurements do not show de corresponding signals. In some works in the literature on characterization of iron hydroxide, similar measures have been reported [32], withou XRD peaks.

The shell of YK200b is formed by hematite and the composition of YK200a is not determined. The Raman spectra in Fig. 7 show a comparison between the measurements for YK200a and YK200b, using the laser in low power. The measurement for YK200a show a small peak at 301 cm^− 1, corresponding to the iron hydroxide [33, 34]. Thus, we have so far, a slight indication of the existence of this compound in the composition of YK200a, but the result require additional measures. The data from YK200b shows peaks at 226, 293 and 411 cm^− 1, which correspond hematite [31], corresponding to the XRD data. Along with these peaks were observed peaks at 246 and 301 cm^− 1 indicative of the iron hydroxide presence. α-FeOOH will show peaks around 239, 300, 390, and 476 cm^− 1 [34]. Studies reported that, at high temperatures, mixtures of α-FeOOH and α-Fe₂O₃ (hematite) are solid products of the hydrolysis of Fe³⁺ in aqueous solutions. Thus, we concluded that, in addition to hematite, there is a small part of YK200b shells still formed by α-FeOOH traces, in addition to α-Fe₂O₃.

It should be noted that in the preparation of the YK200b, the YK200a sample was used as starting material submited to a heat treatment at 550°C. Thus, the formation of α-FeOOH in the two samples must have occurred in the preparation step with an autoclave at 100°C. With this, the composition of YK200a is iron hydroxide and the signal from both XRD and Raman spectroscopy are compromised by the presence of a large volume of PS.

Measurements by Fourier Transform Infrared Spectroscopy (FTIR) in Fig. 8 shows the results taken on the uncoated PS spheres and YK200a. The PS peaks remain as superposed to the YK200a results and in addition to these corresponding to the PS, other peaks can help us understand the composition of the YK200a shell. According to literature reports [34], at 3400 cm^− 1 a peak related to water should appear and for 3200 cm^− 1 Fe-OH (Fe-OH stretch), the latter, even if it were present, would be difficult to be observed due to the water band. To continue our analysis, the curve in Fig. 8 shows a magnification of the measurement in a small wavenumber interval from 1000 to 600 cm^− 1. In this figure, two intense peaks at 892 and 792 cm^− 1, which can be attributed to α-FeOOH. With this was reinforced the conclusion that the coating on the YK200a PS sphere is constituted by iron hydroxide. The manufacturing of this advanced material that exhibit promising application for magneto optical applications and lituim energy storage devices [35–39].

The proposal to combine Yoshikawa’s and Kobayashi’s methodologies is efficient to conform hollow spheres of hematite. The new synthesis route includes different solvent, which leads to more uniform and thinner layer formation than reported in the literature for this size (200 nm approximately). This method has not been used in hard templates. The two reported samples (YK200a and YK200b) are taken from different steps. The later application of material defines where to stop during the methodology, the researcher can choose one (iron hydroxide for energy applications, for example) or another (hematite for magneto-optical applications, for example).

A new method to prepare hematite hollow spheres using PS spheres of 203 nm as a template was reported. The proposal to combine Yoshikawa’s and Kobayashi’s methodologies is efficient to conform hollow spheres of hematite. For the intermediate preparation stage, before the PS elimination, an iron hydroxide shell on the template surface is formed. This was confirmed by Raman and FTIR spectroscopy measurements. Even though the hematite phase is formed at the final step of the process, Raman measures showed traces of iron hydroxide in the hollow shell without PS. Highlighting the manufacturing of this advanced material that exhibit promising application for magneto optical applications and lituim energy storage devices.

Conflict of Interest: The authors declare that they have no conflict of interest.

Acknowledgments

The authors are grateful to the Brazilian Agencies: CAPES; CNPq; FINEP and FACEPE. The authors are grateful to the Multiuser Center of the Physics Department at UFPE, CHICO Laboratory in the Fundamental Chemistry Department CCEN, the UFAL’s Microscopy Laboratory and the Microscopy Laboratory of Materials Science Program at CCEN.

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No competing interests reported.

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Synthesis and characterization of α-Fe2O3 hollow-spheres through a new hard-template strategy

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Introduction

Materials And Methods

Results And Discussion

Morphology

Composition and atomic ordering

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