Synthesis, surface modification, and characterization of Fe3O4@SiO2 core@shell nanostructure

Seham S. Alterary; Anfal AlKhamees

doi:10.1515/gps-2021-0031

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Synthesis, surface modification, and characterization of Fe₃O₄@SiO₂ core@shell nanostructure

Seham S. Alterary und Anfal AlKhamees

Veröffentlicht/Copyright: 3. Juli 2021

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Abstract

In recent times, nanoparticles have been the focal point of research in nanoscience due to their wide scope of potential applications in all fields of science. Iron oxide (Fe₃O₄) nanoparticles (NPs) show incredible magnetic saturation, stability, biocompatibility, and intuitive properties on the surface, which makes them ideal for being utilized in several ways. In the present study, Fe₃O₄ NPs were synthesized by co-precipitation and further coated with silica (SiO₂) to avoid aggregation. Synthesized nanoparticles (Fe₃O₄@SiO₂) were individually functionalized using glycine and malonic acid and characterized by various spectroscopies and microscopies techniques. XRD diffraction analysis showed that the presence of SiO₂ did not alter the diffraction pattern peaks, which represented the existence of Fe₃O₄. The presence of Fe₃O₄ and SiO₂ nanoparticles were further confirmed using EDS. Transmission electron microscope micrographs of the synthesized nanoparticles exhibited spherical shape and confirmed the increase in particle size after coating with SiO_2. Also, the analysis of dynamic light scattering showed that the particle size of Fe₃O₄@SiO₂ functionalized with malonic acid (229.433 nm) was greater than those functionalized with glycine (57.2496 nm). However, the surface area was greater in Fe₃O₄@SiO₂-glycine (104.8 m²/g) than Fe₃O₄@SiO₂-malonic acid (26.15 m²/g). The key findings suggest that the synthesized core-shell Fe₃O₄@SiO₂ nanoparticles are a promising candidate for a wide array of applications in the field of medicine and environmental science.

Keywords: core@shell; Fe3O4@SiO2 ; functionalization; nanostructure; surface modification

1 Introduction

In the recent decade, applications of nanotechnology have emerged with an increasing use of nanosized materials in various fields due to their unique properties at the nanoscale in comparison with their bulk counterparts [1]. Integration of nanotechnology with other sciences has resulted in the rapid development of novel research areas, which will improve and revolutionize technology in several industries [2].

Magnetite and Fe₃O₄ NPs provide a unique nanoplatform and play a major role in providing a broad range of applications owing to their important magnetic property of superparamagnetism. The phenomenon shows that ferromagnetic or ferrimagnetic nanoparticles would have zero magnetic remanence together with a high magnetic susceptibility. Among them, Fe₃O₄ NPs stand out due to their high biocompatibility, low toxicity, strong superparamagnetic property, and easy preparation process, and thus, they have attracted increasing attention [3,4] in the field of research. Fe₃O₄ NPs do have their limitations, such as rapid agglomeration, wide surface area, high chemical reactivity, and high surface energy, resulting in magnetism loss [5]. Therefore, appropriate surface modification of Fe₃O₄ NPs is required to avoid the aforementioned problems. The coating is the most common surface modification method to conjugate organic or inorganic materials onto the surface of iron oxide nanoparticles. This approach avoids not only the oxidation and agglomeration but also provides the possibility of further functionalization [6,7]. Functionalization of magnetic NPs boosts up their physicochemical properties, making them ideal candidates for catalysis or biomedicine purposes [8]. Many studies are conducted on the synthesis of Fe₃O₄ nanoparticles into composite structures with other materials. Chang et al. [9] and Sobhanardakani and Zandipak [10] have reported the formation of silica layers on the surface of stabilized magnetite nanoparticles using a surfactant [11]. Amorphous SiO₂ is the most representative material among potential candidates for constructing core-shell structures with Fe₃O₄. SiO₂ is used because of its inherent high thermal stability, physicochemical durability, and surface characteristics, which is because it retains a large number of surface hydroxyl groups [12,13]. This offers a mode to overcome the limitations encountered during the synthesis of Fe₃O₄ nanoparticles. Among the methods of synthesis used, chemical co-precipitation is the most common method both for the laboratory purpose and on a larger scale for industrial processes since it is highly advantageous. The process has easy processing operation, high yield product, low temperature, and time of reaction in comparison with methods like thermal decomposition and hydrothermal and utilizes inorganic reactants and environmental-friendly solvents such as water [14]. To make a core-shell structure, there is a major deficiency in using Fe₃O₄ nanoparticles obtained by co-precipitation due to the large surface-to-volume ratio, high surface energy, and magnetic dipole–dipole attractions between the particles, and magnetic nanostructures are highly prone to aggregation. To synthesize well-dispersed silica-coated Fe₃O₄ NPs, sol–gel method, the Stöber method, and microemulsion are the most common methods for coating the surface of Fe₃O₄ nanoparticles with silica [15,16].

With this premise, the present study was carried out to use the Fe₃O₄ nanoparticle as a magnetic core coated by SiO₂ shell and then modified by organic and inorganic reagents to synthesize the Fe₃O₄@SiO₂ as an efficient nanomagnetic catalyst. Initially, the magnetic iron oxide nanoparticles (Fe₃O₄ NPs) were produced using the co-precipitation method and then coated with SiO₂. The coated particles are then functionalized with glycine and malonic acid.

2 Materials and methods

2.1 Co-precipitation method synthesized of Fe₃O₄ NPs

A volume of 500 mL of 1.5 M NaOH (BdH Laboratory Supplies) was added with a dropper with continual stirring at 80°C to the mixture of FeCl₃·6H₂O (LOBA Chemie; 0.04 mol, 10.8 g) and FeSO₄·7H₂O (UNI-CHEM; 0.02 mol, 5.5 g) dissolved in 50 mL of 0.5 M HCl (Salzsäure rauchend). Thereafter, the prepared Fe₃O₄ precipitant was collected with a magnet, washed several times with deionized water, and dried for 7 h at 50°C [17,18].

2.2 Synthesis of silica-coated Fe₃O₄ NPs (Fe₃O₄@SiO₂ NPs)

A volume of 10 mL of deionized water, 30 mL of absolute ethanol (CH₃CH₂OH ≥99.8%; Sigma-Aldrich), and 1 mL of ammonia (30% NH₃; Applichem Panreac) were added to 0.5 g Fe₃O₄ nanoparticles. This mixture was incubated in an ultrasonic tank for 30 min, after which we added 2.5 mL of tetraethylorthosilicate (TEOS ≥99.0%; Fluka) while stirring for 22 h. The coated Fe₃O₄@SiO₂ was dried at room temperature [19,20].

2.3 Surface modification of Fe₃O₄@SiO₂ NPs by glycine and malonic acid

A total of 0.15 g of Fe₃O₄@SiO₂ NPs was added to 7.5 mL of deionized water in ultrasonic tanks. Then, 0.15 g of glycine (WINLAB) or malonic acid (LOBA Chemie) was added to the solution by stirring at 90°C for 20 min. After the reaction was complete, the precipitate was washed three times with deionized water and dried under vacuum for 12 h at 50°C [21,22].

2.4 Characterization techniques

The FTIR spectra of Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-glycine, and Fe₃O₄·SiO₂-malonic acid nanoparticles were analyzed by Fourier transform infrared spectrophotometer (Perkin-Elmer Spectrum BX) to examine the functional chemicals groups of the prepared samples using the potassium bromide (KBr) disk method. The structural analyses of the collected samples were carried out using XRD and Bruker D5005 diffractometers using CuK5-007 radiation (5-007 = 1.5418 A₀). The average particle size of the prepared nanoparticles was calculated according to the Scherrer equation. The particle size distribution was investigated by a dynamic light scattering (DLS) technique using a Zetasizer (HT Laser, ZEN3600 Malvern Instruments, Malvern, UK). To determine particle size, morphology, and composition of Fe₃O₄, Fe₃O₄@SiO₂ and acquired samples were examined using a JEOL JSM-6300 SEM with an EDS and a JEM-200CX TEM. The Brunau–Emmet–Teller (BET) method was employed to measure the surface area of Fe₃O₄@SiO₂-glycine and Fe₃O₄@SiO₂-malonic acid nanoparticles with a Micromeritics Gemini 2360 surface area analyzer. Nitrogen gas molecules were adsorbed onto the solid surface, which allows measurement of the surface area of the nanoparticles.

2.5 Statistical analysis

All experiments were performed in triplicate. Results were expressed as means ± standard deviation (n = 3). The size of nanoparticles was measured from the scanning electron microscope (SEM) and transmission electron microscope (TEM) pictures using ImageJ software.

3 Results and discussion

3.1 FTIR analysis

The chemical compositions of Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-glycine, and Fe₃O₄@SiO₂-malonic acid nanoparticles were characterized using FTIR spectroscopy as shown in Figure 1. The FTIR results confirmed that the nanoparticles were composed of iron oxide; the Fe₃O₄ nanoparticles exhibited well-defined peaks at 577, 974, 1,622, and 3,118 cm⁻¹. The peak at 577 cm⁻¹ indicates the presence of the Fe–O bond. This peak was observed in all prepared samples, indicating the presence of Fe₃O₄ nanoparticles in all preparations. The occurrence of a peak of 974 cm⁻¹ is due to the presence of the nitrate group. In addition, the peaks at 1,622 and 3,118 cm⁻¹ were due to the bending vibration of absorbed water and O–H stretching, respectively [23,24]. The Fe₃O₄@SiO₂ spectrum exhibits specific bands positioned at 1,080, 1,082, 1,400, and 3,141 cm⁻¹. The 1,080 cm⁻¹ peak corresponds to Si–O–Si (siloxane) stretching vibrations, while the 1,082 cm⁻¹ corresponds to Fe–O–Si stretching vibrations, confirming the presence of SiO₂ layers in the nanoparticles. The low-intensity peak at 1,400 cm⁻¹ corresponds to Fe–O stretching, and the band at 3,141 cm⁻¹ corresponds to O–H stretching. These results strongly suggested that the Fe₃O₄ nanoparticles were successfully coated with SiO₂ [25,26]. The Fe₃O₄@SiO₂ was loaded with glycine and malonic acid individually, which was also confirmed using FTIR spectroscopy. The functionalization with glycine yielded specific peaks in the Fe₃O₄@SiO₂-glycine sample at 465, 597, 892, 1,030, 1,087, 1,327, 1,406, 1,620, 3,206, and 3,400 cm⁻¹. Carboxylic group peaks appeared at 465, 598, and ∼700 cm⁻¹. C–C–N symmetric and asymmetric stretching vibration bands appear at 892 and 1,030 cm⁻¹, respectively. NH₃ rocking and CH₂ twisting bands appeared at 1,087 and 1,327 cm⁻¹, respectively. The band at 1,620 corresponds to CO₂ asymmetric stretching, and the bands at 3,206 and 3,400 cm⁻¹ correspond to the O–H stretching vibration and intramolecular C–H stretching vibration, respectively [27]. The malonic acid-specific bands appeared at 956, 1,083, 1,369, 1,564, 1,722, and 3,425 cm⁻¹. The peak at 956 cm⁻¹ corresponds to the C–C–C stretching vibration, and the peak at 1,083 cm⁻¹ corresponds to CH₂ rocking vibration. The O–H bending and C–O stretching bands appear at 1,369 cm⁻¹. The C═O stretching vibration appears at 1,722 cm⁻¹. Finally, the band at 3,425 cm⁻¹ indicates O–H stretching [28,29,30].

Figure 1

FTIR spectra of the synthesized nanoparticles.

3.2 XRD analysis

The composition of the prepared nanomaterial was primarily identified using XRD. XRD patterns for Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-glycine, and Fe₃O₄@SiO₂-malonic acid are depicted in Figure 2. Peaks at 35.6°, 43.29°, 57.5°, and 62.82° were assigned to (311), (400), (511), and (440) reflections of Fe₃O₄, respectively (Figure 2a) [31,32]. This suggests that the Fe₃O₄@SiO₂ nanoparticles were synthesized successfully without damaging the crystal structure of the Fe₃O₄ core [33]. The broad peaks around 23° in Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-glycine, and Fe₃O₄@SiO₂-malonic acid (Figure 2b–d, respectively) correspond to the amorphous shell of silica on the surface of the nanoparticles. In Figure 2c, the Fe₃O₄@SiO₂-glycine pattern, glycine-specific peaks appear at 29.85°, 30.45°, and 57.3° that correspond to (200), (111), and (302) reflections, respectively [34]. The (311) peak of the highest intensity was used to evaluate the diameter of Fe₃O₄ particles, approximately 21 nm. The patterns (COD 9006318) are consistent with the planes. Furthermore, diffractograms show no peak shifting upon coating and functionalization, indicating that the crystalline phase and the stability of magnetite nanoparticles were persevered. This further illustrates that the acid did not significantly affect the crystallinity or the crystal structure [35].

$Figure 2 XRD diffraction patterns of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2-glycine, and (d) Fe3O4@SiO2-malonic acid nanoparticles.$

Figure 2

XRD diffraction patterns of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂-glycine, and (d) Fe₃O₄@SiO₂-malonic acid nanoparticles.

3.3 TEM analysis and particle size

The size and the morphology of the synthesized nanoparticles were characterized by TEM. TEM images of the prepared Fe₃O₄ and the Fe₃O₄@SiO₂ nanoparticles are shown in Figure 4. The as-prepared Fe₃O₄ nanoparticles exhibited spherical shapes and confirmed the increase in particle size after coating with SiO₂ as shown in Figure 3a. TEM micrograph (Figure 3b) showed spherical and rod-shaped particles with the SiO₂ layer after modification, and this layer was considered to consist of SiO₂, which is in good agreement with the previous FTIR and XRD results. The core (Fe₃O₄) and shell (SiO₄) structures of the nanoparticles were very clear in TEM images. The increase in particle size after coating with SiO₂ was further confirmed using TEM micrographs (ImageJ software), where the sizes of Fe₃O₄ and Fe₃O₄@SiO₂ nanoparticles were estimated to be 40.37 ± 2.97 nm and 53.20 ± 4.52 nm, respectively (Figure 3c).

Figure 3

TEM images of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, and (c) nanoparticle size distribution of the synthesized nanoparticles.

DLS analysis was employed to confirm the particle size and hydrodynamic diameters and to monitor the aggregation behavior of the synthesized nanoparticles. The DLS particle size distribution results indicated that the synthesized Fe₃O₄ NPs and core-shell have size distribution with a monodispersity index (PDI) of 0.260 and monodispersity index 0.279, respectively. It is reported that PDI values greater than 0.7 indicate that the sample has a very broad size distribution [36]. The smaller value of the PDI index suggests that nucleation is fast compared to the particle growth, and also that the secondary nucleation step is absent. Therefore, systematic measurements of the average size and PDI of the synthesized NPs confirmed the long-term stability of the tested systems. It can be summarized that the synthesized nanoparticles were stable for at least 4 months [37]. The average particle size (diameter, nm) of the nanoparticle after being functionalized with glycine was 57.2496 nm and that with malonic acid was 229.433 nm. This could be explained by the difference in the number of atoms in malonic acid versus glycine [13]. DLS measurements indicated an increase in the mean diameter compared to TEM results due to the interference of the dispersant into the hydrodynamic diameter [38,39].

3.4 SEM and EDS analysis

Figure 4 shows the SEM micrographs and the chemical composition of the synthesized nanoparticles. The synthesized Fe₃O₄ nanoparticles and Fe₃O₄@SiO₂, which were obtained via encapsulating magnetite nanoparticles in a shell of silica, are near spherical and possess a core/shell structure. EDS analysis demonstrated the chemical composition of the nanoparticles, and the percentage of Fe and O elements in Fe₃O₄ nanoparticles was 64.39% and 35.61%, respectively (Figure 4a). Moreover, the chemical composition of O, Si, and Fe in the Fe₃O₄ nanoparticles coated with SiO₂ was 45.8%, 10.09%, and 44.1%, respectively (Figure 4b). In the EDS spectrum, the presence of weak Si peaks and strong Fe peaks shows thin silica shell formation on the surface of Fe₃O₄ NPs [40].

Figure 4

SEM images (left) and EDS (right) analysis for (a) Fe₃O₄ and (b) Fe₃O₄@SiO₂ nanoparticles.

The size of the nanoparticles was measured from the SEM micrograph using ImageJ software. The Fe₃O₄ and Fe₃O₄@SiO₂ particle size was 20.68 ± 0.75 nm and 75.16 ± 15.75 nm, respectively. The increased size of the Fe₃O₄ nanoparticles is explained by the SiO₂ coating. The nanoparticles exhibited a spherical shape, which was corresponding (Figure 4) to the TEM micrographs (Figure 3).

3.5 BET surface area analysis

The BET method was used to determine the specific surface area of the synthesized particles. The method is based on the adsorption of gas on the surface of the particles and measuring the amount of adsorbed gas at known pressure. By analyzing a known amount of particles, the specific surface area can be found. It has particular importance for adsorption, heterogeneous catalysis, and reactions on surfaces [41]. The BET method was carried out to estimate the specific surface area and porosity of the different synthesized particles. The pore size was determined to be about 1.59 nm and was nearly the same in both samples in terms of pore width and diameter. The BET surface area of the synthesized Fe₃O₄@SiO₂-glycine and Fe₃O₄@SiO₂-malonic acid was 104.8 and 26.15 m²/g nm, respectively, which could be related to their average particle size. The analysis in the present study determined that the smaller particle had a higher surface area [42,43,44]. The adsorption capacity of nanomagnetite relies on the availability of pore sites and the time taken for the adsorption process. The reduction in the pore size after adsorption could be attributed to blockage of the pores due to adsorption. Furthermore, the reduction in pore size after adsorption may be due to the blockage of the pores due to adsorption, which is in consensus with the findings of a previous study [45,46].

4 Conclusion

The key findings of the present study showed a successful synthesis of Fe₃O₄@SiO₂ nanoparticles by the co-precipitation method, followed by surface modification. The presence of functional groups was confirmed by FTIR analysis, and XRD patterns of samples confirmed the formation of Fe₃O₄, Fe₃O₄@SiO₂, and amorphous silica structures whose core-shell structure was indicated by TEM. Also, electron microscopy clearly demonstrated the impact of surface modification on morphology as well as the shape and the size of nanoparticles, as nanoparticles increased in size from about 20.68 ± 0.75 nm to 75.16 ± 15.75 nm after SiO₂ coating. After surface modification, a further increase in size was also observed in the Fe₃O₄@SiO₂-glycine and Fe₃O₄@SiO₂-malonic acid, to 57.2496 and 229.433 nm, respectively, where the surface area was increased to 104.8 and 26.15 m²/g, respectively. Taken together, synthesized core-shell nanoparticles could provide a promising tool to be used for biomedical applications or for photocatalysis in the degradation of organic pollutants in wastewater water treatment.

Acknowledgments

The authors extend their appreciation to the Deputyship grant for Research & Innovation, “Ministry of Education in Saudi Arabia for funding this research work through the project number IFKSUHI-2020-135.”

Funding information: This research project was supported by a grant from the “Research Center of the Female Scientific and Medical Colleges,” Deanship of Scientific Research, King Saud University.
Author contributions: Seham S. Alterary: conceptualization, formal analysis, investigation, validation, resources, data curation, writing – original draft, writing – review and editing, visualization, supervision, funding acquisition, and project administration; Anfal AlKhamees: methodology, formal analysis, data curation, and writing – original draft.
Conflict of interest: The authors declare that there is no conflict of interest or state.
Data availability statement: All samples used in this research are available from the authors.

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Received: 2021-03-02

Accepted: 2021-04-06

Published Online: 2021-07-03

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/gps-2021-0031

Schlagwörter für diesen Artikel

core@shell; Fe3O4@SiO2; functionalization; nanostructure; surface modification

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BY 4.0

Synthesis, surface modification, and characterization of Fe3O4@SiO2 core@shell nanostructure

Artikel

Abstract

1 Introduction

2 Materials and methods

2.1 Co-precipitation method synthesized of Fe3O4 NPs

2.2 Synthesis of silica-coated Fe3O4 NPs (Fe3O4@SiO2 NPs)

2.3 Surface modification of Fe3O4@SiO2 NPs by glycine and malonic acid

2.4 Characterization techniques

2.5 Statistical analysis

3 Results and discussion

3.1 FTIR analysis

3.2 XRD analysis

3.3 TEM analysis and particle size

3.4 SEM and EDS analysis

3.5 BET surface area analysis

4 Conclusion

Acknowledgments

References

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Synthesis, surface modification, and characterization of Fe₃O₄@SiO₂ core@shell nanostructure

2.1 Co-precipitation method synthesized of Fe₃O₄ NPs

2.2 Synthesis of silica-coated Fe₃O₄ NPs (Fe₃O₄@SiO₂ NPs)

2.3 Surface modification of Fe₃O₄@SiO₂ NPs by glycine and malonic acid