Green synthesis of iron oxide nanoparticles using Hibiscus plant extract

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.

Iron oxide Fe 2 O 3 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, sim-ple 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 Fe 2 O 3 nanoparticles.Markova et al. [25] synthetized iron-polyphenol nanoparticles using green tea extract.Ahmmad et al. [26] succeeded in the synthesis of pure 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.

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.

Iron nanoparticles preparation
Iron oxide nanoparticles are prepared by mixing 10 ml of the Hibiscus extract with 90 ml FeCl 3 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 o C during 30 min and finally calcinated at 500°C during 2 h. Figure 1 illustrates the different steps of iron nanoparticle synthesis.

Results and discussion
The X-ray diffraction spectrum of the obtained nanoparticles is shown in Figure   [ [34][35][36][37].Recently, Khan et al. [18] have observed a mixture of α-Fe 2 O 3 and -Fe 2 O 3 in sol-gel prepared iron oxide after a heat treatment up to 600°C.Das et al. [38] have reported the transition of α-Fe 2 O 3 to Fe 3 O 4 phases using hydrothermal reaction after thermal heating in hydrogen/argon atmosphere at 300°C.The maghemite can be formed at low temperatures and by elevating the temperature; it transforms to αstable phase [39,40].As well, iron oxohydroxide FeOOH phase can be formed at low temperature up to 80 o C [41,42].
It is well known that the phase transition temperature of γ -Fe 2 O 3 to α-Fe 2 O 3 occurs at 400°C.This transition temperature can be influenced by various parameters, including lattice defects, particle size, surface phenomena and pressure, etc. [43][44][45].
The interspace distance d hkl , in a hexagonal structure such as the hematite phase, is given by the following relation: Knowing the relationship between d hkl and the diffraction angle θ , Equation ( 1) can be written as 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 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: 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.
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][47][48].
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-Fe 2 O 3 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-Fe 2 O 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-Fe 2 O 3 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].
The EDX spectrum (Figure 4b) confirms the presence of the main elements O and Fe in the asprepared 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].
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 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-Fe 2 O 3 (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 spincanting 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-Fe 2 O 3 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-Fe 2 O 3 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-Fe 2 O 3 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.

Conclusion
In the present work, hematite (α-Fe 2 O 3 ) 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 -Fe 2 O 3 and hydroxide FeOOH are present in the nanoparticle indicating their incomplete transformation to hematite α-Fe 2 O 3 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.

Figure 1 .
Figure 1.Different steps of iron oxide nanoparticles preparation.α-Fe 2 O 3 nanoparticles by using green tea leaf extract.β-Fe 2 O 3 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 Fe 2 O 3 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 Fe 3 O 4 nanoparticles [29].Recently, Jamzad et al. [30] synthesized nanoparticles (α-Fe 2 O 3 ) 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.

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 = 13.73Å, the bulk maghemite lattice parameter are a = b = c = 8.37 Å).

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

Figure 3 .
Figure 3. Raman spectrum of the synthesized nanoparticles.

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

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

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.