Phase Transformation of Iron Oxide Nanoparticles from Hematite to Maghemite in Presence of Polyethylene Glycol : Application as Corrosion Resistant Nanoparticle Paints

This work proposes a simple method for the efficient and rapid synthesis of hematite (α-Fe 2 O 3 ) nanostructures based on simple heating method under ambient conditions. Polyethylene glycol (PEG) is employed as a structure directing agent in driving the morphology and phase transformation. Typically, Fe 2 O 3 nanoparticles of size below 50 nmwere synthesized at temperature around 500C. The morphology and mechanism of formation of the nanocapsules and then aggregation of nanocapsules to form larger size nanoclusters were studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. Interestingly, this work demonstrates the structural phase transformation of hematite (α-Fe 2 O 3 ) to maghemite (γ-Fe 2 O 3 ) upon addition of different amounts of PEG (say 0.066M, 0.133M, and 0.2M) and then heat treating at 500C.The prepared powders were used in nanoparticle paint preparation and applied as corrosion resistant coatings on iron samples. Polarization studies performed on the paint coatings made out of all the prepared samples showed size-dependent corrosion resistance. As the particle size decreases, the surface area increases and so the corrosion resistance also increases.


Introduction
From the last few decades, transition metal oxide nanoparticles have attracted tremendous interest due to their promising applications as electrode materials for rechargeable solidstate batteries [1,2], as efficient catalysts for fuel-cell reactions [3,4], and as nanoscale magnetic models for understanding nanomagnetism [5,6].Amongst these, iron oxide nanoparticles have received much attention due to their massive applications in the fields of information storage disks, ferrofluids, pigments, and medical applications like targeted drug delivery and cancer diagnoses [7][8][9].Iron (III) oxide is an inorganic compound with the formula Fe 2 O 3 and it is of one of the three main oxides of iron; the other two are rarely available iron (II) oxide (FeO) and naturally occurring iron (II, III) oxide (Fe 3 O 4 ).In steel industry, hematite is the main source of the iron for the production of steel and it is paramagnetic, readily attacked by acids, and reddish brown in colour.As hematite is the most stable and ntype semiconductor under ambient conditions, it is widely used as catalysts, gas sensors, and pigments due to its high resistance to corrosion and low cost.It can also be used as a starting material for the synthesis of maghemite (-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ), which have been intensively pursued for technological applications in the last few decades [10].However, hematite is also interesting material as it has a band gap of 2.3 eV; the conduction band (CB) is composed of empty d-orbitals of Fe 3+ and the valence band (VB) consists of fully occupied 3D crystal field orbitals of Fe 3+ with some admixture from the O 2p nonbonding orbitals [11].Several methods to synthesize iron oxide nanoparticles such as mechanochemical [12,13], wire electrical explosion [14],  [17,18].The prepared nanoparticles were used in paint preparation and were found to show excellent corrosion resistance as coatings.particles were taken onto a glass slab and then a crater is made in the middle of the powder, into the crater 1 mL of linseed oil is poured, and the mixture was mixed thoroughly with pallet knife.Along with this, binder is prepared by the addition of 2% of bee wax in the linseed oil.For this purpose, 0.4 g of bee wax is added to 19.6 mL of linseed oil and the mixture was heated at 60 ∘ C along with magnetic stirring, until all the wax gets dissolved in linseed oil and the colour of mixture turns out to be yellowish.This is cooled to room temperature and can be used as binder.Once the binder cools to room temperature, the pigment is taken onto glass slab; few drops of binder are added and ground well using a miller till the pigment gets mixed up with binder and feels like paint.The prepared paint is stored in plastic containers and was coated on thin sheets of iron on one side and tested for polarization curves.Figure 1 shows powder XRD patterns of all synthesized iron oxide nanoparticles.As can be observed, the iron oxide prepared in the absence of PEG (Fe 2 O 3 -0 PEG) and after heat treatment at 500 ∘ C showed pure hematite -Fe 2 O 3 phase, matching exactly with PDF-01-077-9926.The main peak of hematite at 33.15 ∘ is clearly observed.Other peaks at 24.1 ∘ , 35.6 ∘ , 40.8 ∘ , 49.4 ∘ , 54.1 ∘ , 57.4 ∘ (doublet), 62.4 ∘ , and 63.9 ∘ are found and indexed.However, the iron oxide synthesized in presence of PEG and heat treated at same temperature starts showing a phase transformation as observed from Figure 1; the main peak of hematite at 33.15 ∘ decreases in intensity and new peaks corresponding to maghemite (-Fe 2 O 3 ) appear at 30 ∘ and nearly 43.5 ∘ .Interestingly, the addition of 12 g (0.2 M) of PEG shows major phase of maghemite (-Fe 2 O 3 ) which can be well indexed and matched with PDF-00-039-1346 along with small amount of hematite -Fe 2 O 3 still remaining.The percentages calculated from Diffrac Eva software show 72.91% of maghemite (-Fe 2 O 3 ) and 27.09% of hematite (-Fe 2 O 3 ) (refer to supplementary information in Supplementary Material available online at http://dx.doi.org/10.1155/2016/1328463).Phase transformation as a function of temperature is a common phenomenon [17,18], but interestingly in the present work the phase transformation from hematite to maghemite phase is occurring on treatment with various amounts of PEG (0 M, 0.066 M, 0.133 M, and 0.2 M).The reason may be attributed to the fact that the phase transformation in iron oxides is often controlled by size-dependent thermodynamic relationships between initial and final product phases.The new phase is not thermodynamically favored until crystallites of the initial phase have assembled to certain critical size.These formed secondary structures underwent phase transformation, in which a new mineral phase begins forming at the interface between two surfaces of the initial phase [20].

SEM and EDX.
The purpose of study was to measure the size and morphology of the prepared nanoparticles from SEM measurements.Figure 2 shows the SEM images of samples which were prepared in presence of various amounts of PEG.Interestingly, it was observed that the particles prepared in the absence of PEG showed irregular morphology including spherical particles whose size was larger when compared to other samples, although existing in the nanorange.When prepared with 4 g (0.066 M) and 8 g (0.133 M) of PEG, the morphology of particles is capsule shaped and is uniform in size and shape.With 4 g (0.066 M) of PEG, nanocapsules of length of 62 nm and cylindrical diameter of 24 nm and in presence of 8 g (0.133 M) of PEG, nanocapsules of decreased size with length of 52 nm and cylindrical diameter of 20 nm were obtained.However, in presence of excess amount of PEG (12 g, 0.2 M), the capsules got sewed up to form angular nanostructures which get interconnected to form rhombic nanostructures which get converted into nanocluster with an average length nearly equal to 101 nm and average width of 40 nm as shown in Figure 2(d).The hypothetical mechanism of capsules getting converted into nanocluster is depicted in Figure 3.Although the average particle size is different under various conditions, the nanoparticles formed have an average particle size of less than 100 nm.
EDX analysis was used to identify the elemental composition of the samples and performed on different sites of each sample.The results showed that the samples consist of iron and oxygen, and no sign of impurity was detected and the composition exactly matches with Fe 2 O 3 .

Corrosion Inhibitor Coatings.
The corrosion rate and corrosion resistance of iron samples coated with prepared paints involving different iron oxide powders (0 M, 0.066 M, 0.133 M, and 0.2 M PEG) were studied.The polarization curves (Figure 4) clearly depict that it is the size and not the structure which influences the corrosion resistance behavior.If it would be structure which influences the corrosion resistance behavior, then the sample coated with Fe 2 O 3 -12 PEG being a cubic structure with more close-packing should show high corrosion resistance but it is observed that the paint prepared with Fe 2 O 3 -8 PEG has high corrosion resistance and low corrosion rate compared to other paints; this is due to the smaller size of particles in Fe 2 O 3 -8 PEG.Due to the smaller size, the surface area is high, so Fe 2 O 3 -8 PEG exhibit good surface properties and protect the surface from corrosion (Table 2).Figure 4  resistance and high corrosion rate.Obviously, the reason is large particle size compared to other samples.From Fe   5).

Conclusion
This paper outlines the simple method to synthesize nanocrystalline and -iron oxides by precipitationcalcination method.It was observed that different morphologies of iron oxide were precipitated by varying amounts of PEG, which is acting as a structure directing agent.Interestingly, powder X-ray studies showed a phase transformation from rhombohedral hematite (-Fe 2 O 3 ) to cubic maghemite (-Fe 2 O 3 ) when the amount of PEG is increased and calcined at the same temperature (500 ∘ C).Scanning electron micrographs clearly shown at low PEG content nanocapsules were formed, which aggregate to form nanocluster on addition of 12 g (0.2 M) of PEG.However, this agglomeration of nanocapsules is also responsible for phase transformation.These prepared iron oxide nanomaterials are further used in paint preparation and were found to be better protective coatings on iron samples when compared to microsized iron oxide particle coatings.

3 Figure 1 :
Figure 1: Powder XRD patterns of Fe 2 O 3 nanoparticles at different concentrations of PEG.
2 O 3 -0 PEG paint to Fe 2 O 3 -8 PEG paint corrosion resistance increases.However, in the paint prepared out of Fe 2 O 3-12 PEG, corrosion resistance decreases and the reason is aggregation of nanocapsules to form nanoclustered large particles.The polarization curves (Figure4) depict that as the size increases the corrosion resistance decreases.To explore more, the paints were prepared with microsized -Fe 2 O 3