Green synthesis of iron oxide nanoparticles using Hibiscus plant extract

Abstract

In the present work, hematite (α-Fe₂O₃) nanoparticles (NPs) were synthesized by using a simple and facile green synthesis by using Hibiscus plant extract. The used solution is composed of plant extract and 0.1 M of iron chloride mixture. The nanoparticles are grown at ambient temperature. The prepared nanoparticles morphology, structure and composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and high-resolution transmission microscopy (HRTEM). The synthesized nanoparticles are formed by agglomeration of spherical and mono-disperse grains. XRD analysis reveals that the grains are polycrystalline with an average particle size of 20 nm. The calcinated nanoparticles contain small traces of maghemite and hydroxide. A vibrating sample magnetometer (VSM) was used for the nanoparticle magnetic properties determination. The finding reveals the superparamagnetic behaviour of the synthesized α-Fe₂O₃ nanoparticles owing to a saturated magnetization (Ms) of 0.96 emu/g, and low remnant magnetization Mr of 0.06 emu/g.

KEYWORDS:

• hematite
• green chemistry
• nanoparticles

1. Introduction

Iron oxide Fe₂O₃ oxides have been one of the most extensively studied transition metal oxides, and it has four polymorphs phases: (1) α-Fe₂O₃ (hematite); (2) -Fe₂O₃ (magnetite); (3) γ-Fe₂O₃ (maghemite) and (4) ϵ-Fe₂O₃. The -Fe₂O₃ and ϵ-Fe₂O₃ phases are generally synthesized in the laboratory while α-Fe₂O₃ and γ-Fe₂O₃ are naturally formed [1]. Each polymorph possesses peculiar crystal structures and physical properties. Among them, hematite (α-Fe₂O₃) has many interesting properties, such as abundance low toxicity, environmental friendly nature, low-cost synthesis, chemical inertness and biocompatibility [2–8]. These properties make α-Fe₂O₃ an interesting transition metal oxide and motivate an intense research activity on its synthesis and applications in several technological areas such as sensor [9], wastewater treatment [10], drug delivery [11], magnetic materials coatings, [12], catalysts [13] and magnetic hyperthermia [14], suprecapacitors [15] and cancer treatment [16].

Iron oxide Fe₂O₃ nanoparticles have been synthesized by a large number of techniques including hydrothermal methods [17], sol–gel reactions [18], microwave [19], co-precipitation [20], sonochemical [21] and microemulsion [22]. Generally, chemical synthesis techniques involve occasionally toxic chemicals that might produce hazardous by-products. This issue is at the origin of the interest in developing clean, simple and ecofriendly process for nanoparticle synthesis. Thereafter, various green synthesis protocols, employing plant extract and other biological products, have been investigated. The use of plant extract microorganisms might be an eco-friendly alternative to the conventional methods for metal or metal oxide nanoparticle preparation. Plant parts like leaves, seeds, fruits and root extracts contain some phytochemicals that act as both reducing agents and capping or stabilization agents. Therefore, currently, green methods of synthesizing NPs are preferred over the physical and chemical methods due to the suppression of chemical solvents and chemical products that may be hazardous to human health and the environment besides the fact that it is a simple and low-cost process.

Various herbs, spices and plants containing antioxidants are tested for iron oxide nanoparticle synthesis. These antioxidants are responsible for metal ion reduction and aggregation of metal nanoparticles prevention. They act as a capping and reducing agent, yielding to the formation of stable nanoscale nanoparticles. For example, Desalegn et al. produced iron nanoparticles, at room temperature, using Mango peel extract [23]. Fruit extract from Cornelian cherry was used by Rostamizadeh et al. [24] to synthesize Fe₂O₃ nanoparticles. Markova et al. [25] synthetized iron–polyphenol nanoparticles using green tea extract. Ahmmad et al. [26] succeeded in the synthesis of pure α-Fe₂O₃ nanoparticles by using green tea leaf extract. -Fe₂O₃ nanoparticles were prepared by Prasad et al. by utilizing the leaf extract of Garlic Vine [27]. Phumying et al. have used Aloe Vera extract for the synthesis of Fe₂O₃ nanoparticles. Iron nanoparticles were prepared using Eucalyptus leaf extract [28]. Venkateswarlu et al. tested plantain peel to synthesize magnetite nanoparticles. By utilizing Tridax procumbens leaves extract, Senthil and Ramesh synthesized Fe₃O₄ nanoparticles [29]. Recently, Jamzad et al. [30] synthesized nanoparticles (α-Fe₂O₃) nanoparticles using an extract of Laurus nobilis L. leaves.

The present work is an investigation of the iron oxide nanoparticles synthesis via green chemistry route by using, for the first time to the best of our knowledge, the Hibiscus plant extract.

2. Experimental details

Purchased Hibiscus flowers were scrupulously cleaned with tap water followed by distilled water rinsing to eliminate any contaminants and then dried in air. Flowers were cut and grinded. The Hibiscus extract was prepared by heating 10 g, at 60°C during 30 min, and the obtained flower powder in 100 ml of distilled water. The clear distilled colour changes to brown indicating the plant elements dissolution in the water.

2.1 Iron nanoparticles preparation

Iron oxide nanoparticles are prepared by mixing 10 ml of the Hibiscus extract with 90 ml FeCl₃ solution 0.1 M prepared with salt dissolution in distilled water. The formation of nanoparticles was marked by the appearance of intense black precipitate. Precipitate was collected by filtration. After washing, the collected nanopowder was dried in air at 60^oC during 30 min and finally calcinated at 500°C during 2 h. Figure 1 illustrates the different steps of iron nanoparticle synthesis.

Figure 1. Different steps of iron oxide nanoparticles preparation.

3. Results and discussion

The X-ray diffraction spectrum of the obtained nanoparticles is shown in Figure 2. The peaks located at 2θ =  33.16°, 35.56°, 40.87°, 49.43°, 53.96°, 62.46° and 64.02° are assigned to (104), (110), (113), (024), (116), (214) and (300) plans, respectively, originating from the rhombohedral hematite phase (JCPDS card no. 89-596.) indicating that the sample is well crystallized and mainly composed of α-Fe₂O₃ hematite phase. The peaks located at 29.7° and 32.12° are assigned to the (220) diffraction plane of maghemite γ-Fe₂O₃ and the (103) diffraction plane of α-FeOOH phase. The presence of these two peaks suggests that γ-Fe₂O₃ and α-FeOOH are initially formed [31–33] and not transformed to α-Fe₂O₃ hematite phase during the calcination step at 500°C. Several authors have noticed the formation of pure hematite with increasing the heating temperature [34–37]. Recently, Khan et al. [18] have observed a mixture of α-Fe₂O₃ and ϵ-Fe₂O₃ in sol-gel prepared iron oxide after a heat treatment up to 600°C. Das et al. [38] have reported the transition of α-Fe₂O₃ to Fe₃O₄ phases using hydrothermal reaction after thermal heating in hydrogen/argon atmosphere at 300°C.

Figure 2. XRD diffraction pattern of the synthesized hematite nanoparticles.

The maghemite can be formed at low temperatures and by elevating the temperature; it transforms to α-stable phase [39, 40]. As well, iron oxohydroxide FeOOH phase can be formed at low temperature up to 80^oC [41, 42].

It is well known that the phase transition temperature of γ-Fe₂O₃ to α-Fe₂O₃ occurs at 400°C. This transition temperature can be influenced by various parameters, including lattice defects, particle size, surface phenomena and pressure, etc. [43–45].

The interspace distance d_hkl, in a hexagonal structure such as the hematite phase, is given by the following relation: (1) $\left(\frac{1}{d}\right)=\frac{4}{3}\left(\frac{h^2+\mathrm{hk}+k^2}{a^2}\right)+{\left(\frac{1}{c}\right)}^2$(1) Knowing the relationship between d_hkl and the diffraction angle θ, Equation (1) can be written as (2) ${\sin }^2\theta =\frac{{\lambda }^2}{4}\left[\frac{4}{3}\left(\frac{h^2+\mathrm{hk}+k^2}{a^2}\right)+{\left(\frac{1}{c}\right)}^2\right]$(2)

The lattice parameter a can be then calculated using the diffraction angle 2θ = 35.56° assigned to the diffraction plane (110) the lattice constant a is then (3) $a=\frac{\lambda }{\sin \theta }$(3) While for the determination of the lattice parameter C, we have used the diffraction angle at 33.16° assigned to the plane (104) therefore the lattice parameter c can be calculated using the relation: (4) $c=\sqrt{\frac{12a^2.{\lambda }^2}{3a^2{\sin }^2\theta -{\lambda }^2}}$(4)

The lattice parameters for the hematite and maghemite are evaluated using Equations (3) and (4), the obtained results are regrouped in Table 1. The crystallite size of both phases was estimated using the Debye Sheerer formula. The obtained values are also shown in Table 1. Table 2 is a comparative representation of the hematite crystallite size obtained by different authors using various processes and plant extract.

Table 1. Lattice parameters, inter-plane spacing and crystallite size of the phase (the hematite bulk lattice parameter are a = b = 5.07 and c = 13.73 Å, the bulk maghemite lattice parameter are a = b = c = 8.37 Å).

	θ	dhkl (Å)	a(Å)	b(Å)	c(Å)	D (nm)
Hematite	33.13	2.70	5.04	5.04	13.9	20
Maghemite	29.8	2.99	8.40	8.4	8.4	12

Table 2. A comparative table of the reported values of the crystallite size of the hematite phase prepared by different methods and plant extract.

Preparation method	Crystallite size (nm)	Refs.
Commercial hematite	120	[61]
Hydrothermal classic	54.7	[63]
Precipitation from ferric nitrate and hydrochloric acid	37 nm	[64]
By coprecipitation technique with the assistance of chelating specialist EDTA	20.68 nm	[65]
Co-precipitation method	44 nm	[20]
Green synthesis extract of green tea (Camellia sinensis) leaves	33 nm	[51]
Green synthesis of iron oxide nanoparticles by the aqueous extract of Laurus nobilis L. leaves	21.5 nm	[30]
Pomegranate	100–200	[66]
Banana peel	10–25	[67]
Eucalyptus	20–80	[68]
Oolong tea	4–50	[69]
Hibiscus extract	20 nm	Present work

The Raman spectrum of the prepared nanoparticle is shown in Figure 3. As shown the spectrum is composed of the active optical (TO) mode vibration peaks characteristic of the hematite phase located at 227.4 and 50 cm⁻¹ assigned to A_1g modes and four peaks located at 292.8, 409, 609.8, 664.3 cm⁻¹ attributed to Eg modes [46–48].

Figure 3. Raman spectrum of the synthesized nanoparticles.

Due to the antiferromagnetic of hematite, excited spin can have a collective vibration, forming the so-called magnon. Therefore, the intense peak at 1315 cm⁻¹ is ascribed to two-magnon scattering [46].

All peaks of the synthesized nanoparticles were found to be in accordance with the observed frequencies of α-Fe₂O₃ nanoparticles. The minor peak shifts originate from the variation in the size and shape in different nanoparticles. The broadness of the peak indicates the low size of the crystallite. It is commonly known, in nanoparticles, that as the particle size decreases, the peak lines become broader and shift towards lower wavenumbers [49,50].

This is consistent with the XRD analysis confirming the main composition of the prepared nanoparticles with α-Fe₂O_3. The presence of the maghemite cannot be easily detected since the peak of the Raman vibration characteristic of maghemite is 227, 289, 408 and 610 cm⁻¹ coinciding with the hematite phase peaks position.

The SEM image and EDX composition of the synthesized nanopowder are represented in Figure 4(a–c). As seen in the SEM image (Figure 4a), an agglomeration of spherical nanoparticles is observable. Similar nanoparticles features were reported in iron oxide nanoparticles α-Fe₂O₃ prepared using an aqueous extract of Psoralea corylifolia seeds [51], Salvadora persica aqueous extract [39], Camellia sinensis (green tea) extract [52] and aqueous root extract of Arisaema amurense [53].

Figure 4. SEM image (a), EDX analysis (b) and elemental mapping (c) of the prepared hematite.

The EDX spectrum (Figure 4b) confirms the presence of the main elements O and Fe in the as-prepared hematite nanoparticles. The elemental mapping (Figure 4c) indicates the uniform dispersion of oxygen and iron elements in the nanoparticle. The nanoparticles are composed of 33% of oxygen and 67%of Fe this composition ratio indicates the lack of oxygen regarding iron.

The presence of Fe and O is also confirmed by the FTIR spectroscopy, as shown in Figure 5, the peak located at 592 cm⁻¹ is assigned to stretching vibration of the Fe–O bond [54].

Figure 5. FTIR absorption spectrum of the prepared nanopowder.

To have an insight on nanoparticle shape and arrangement, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) studies were carried out. The TEM and HRTEM of the synthesized nanoparticles are shown in Figure 6(a–c). The TEM image confirms the formation of nanopowder with the agglomeration of almost equal spherical nanoparticles. The grain size has a narrow distribution centred on 60 nm (Figure 6b). The HRTEM image (Figure 6c) confirms the crystalline nature of the prepared nanoparticles, the interspace associated with the (104) intense diffraction plane is visible.

Figure 6. (a) TEM, (b) particle size distribution and (c) HRTEM of the synthesized nanoparticles.

Figure 7 shows the magnetic hysteresis loop recorded in synthesized nanopowder. The prepared powder exhibits a superparamagnetic material behaviour characterized by a saturation magnetization Ms of 0.96 emu/ g, low remnant magnetization Mr of 0.06 emu/g and low coercivity Hc of 18.9 Oe. The obtained saturation in the prepared hematite is largely higher than that reported for bulk α-Fe₂O₃ (0.3 emu/g) [55]. The presence of maghemite with hematite, as deduced from XRD analysis, may cause the increase in the synthetized nanoparticle's magnetization since the hematite phase has lower magnetization than the maghemite one. It is well known that the particle crystallinity and size control among each other, the magnetic behaviour of ferrite nanoparticles. This is attributed to the surface spin-canting effect [56], below 30 nm of size the obtained nanoparticles show a superparamagnetism behaviour [57]. Recently, Miri et al. [39] have also observed the superparamagnetic behaviour in a-Fe₂O₃ nanoparticles prepared using Salvadora persica aqueous extract with an Ms = 1.5 emu/g. Narayanan et al. [53] have noted the ferromagnetic behaviour of α-Fe₂O₃ nanoparticles synthetized via green chemistry by using root extract of Arisaema amurense, they measured a saturation magnetization (Ms) at 1.25 emu/g and remnant magnetization (Mr) at 0.50 emu/g and coercivity (Hc) at 330 G. The observed superparamagnetic behaviour in the green synthetized nanopowder suggests their potential biomedical application in drug delivery [58] MRI contrast agent [59] and in hyperthermia for cancer treatment [60]. In Table 3, the values of the parameters, saturated and remnant magnetization and the coercivity fields reported by different authors are regrouped for comparison to our results. The obtained saturated magnetism (Ms) of the synthesized iron oxide nanoparticles is higher than the commercial α-Fe₂O₃ nanoparticles (Ms = 0.6 emu/g) [61]. This can be attributed to the superparamagnetic properties of the synthesized nanoparticles due to their small grain size leading to the single magnetic domain in the nanoparticles [62]. However, the obtained magnetic parameters are comparable to the reported ones in iron oxide nanoparticles prepared by green chemistry using different plant extracts.

Figure 7. H–M hysteresis loop of prepared nanopowder.

Table 3. A comparative table of magnetic properties values: saturation (Ms), remnant (Mr) magnetization and coercivity field (Hc) of iron oxide reported by different authors.

	Ms (emu g−1)	Mr (emug−1)	Coercivity Hc (G)	Refs.
Iron oxide (Fe3O4) nanoparticles coated with polymer ligand	45			[70]
α-Fe2O3By hydrothermal	2.8	0.12	22	[62]
α-Fe2O3 thin film prepared on liquid–vapour	18	0	0	[71]
Mesoporous silica nanocomposites/ferrite	4.1		16.3	[72]
Commercial α-Fe2O3-NPs	0.6			[62]
α-Fe2O3 nanoparticles co-precipitation method.	31.28	2.16	127	[73]
α-Fe2O3 nanoparticles biosynthesized using the Salvadora persica aqueous extract	1.8			[39]
α-Fe2O3 nanoparticles green synthesized using root extract of Arisaema amurense	1.25	0.5	330	[74]
α-Fe2O3 nanoparticles green synthesized using flower extract of Hibiscus	0.96	0.06	0	Present work

4. Conclusion

In the present work, hematite (α-Fe₂O₃) nanoparticles were synthesized by using a simple and facile green synthesis by using Artemisia plant extract. The structural analysis reveals the synthesis success of hematite nanoparticles with 20 nm of average size. Small amounts of maghemite γ-Fe₂O₃ and hydroxide FeOOH are present in the nanoparticle indicating their incomplete transformation to hematite α-Fe₂O₃ after calcination at 500°C. The magnetic properties study reveals the superparamagnetic behaviour of the synthetized nanoparticles. The obtained nanoparticles have a saturated magnetization (Ms) of 0.96 emu/g, and low remnant magnetization Mr of 0.06 emu/g suggesting, thereafter, their potential biomedical applications.

Acknowledgements

The authors gratefully acknowledge technical and financial support from the Ministry of Education and King Abdulaziz University, Jeddah Saudi Arabia.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research work was funded by institutional Fund Projects under grant number (IFPRC-197-130-2020).