SONOCHEMICAL SYNTHESIS OF HEMATITE NANOPARTICLES

Hematite nanoparticles were prepared by a procedure consisting in sonication of μ3-oxo trinuclear iron(III) acetate of composition [Fe3O(OOCCH3)6(H2O)3]NO3∙4H2O, {Fe3O}NO3 as iron source, in strong basic conditions followed by thermal treatment at 600 ̊C. The formation of the hematite was confi rmed by IR spectroscopy, X-ray powder diffraction and Raman spectroscopy while, the shape and size of the nanoparticles and their agglomeration were evidenced and estimated on the basis of the images taken with TEM techniques.

There are various forms of iron oxides [7].They differ in composition, in the valence of Fe and, above all, in crystal structure.The most stable and wide spread oxide phase of iron is α-Fe 2 O 3 , where Fe has the lower Gibbs free energy [8].Hematite has hexagonal structure of the corundum type with a close-packed oxygen lattice in which twothirds of the octahedral sites are occupied by Fe(III) ions.Bulk hematite is antiferromagnetic till Morin temperature (TM) (~260 K).Between TM and Neel temperature (TN) (~ 950K) it is weak ferromagnetic and above TN the hematite is paramagnetic [9,11].These transitions are infl uenced by particle size, shape and crystallinity [9][10][11].Particles smaller than 16 nm have a superparamagnetic behaviour at room temperature [12].
Hematite nanoparticles proved to be effective catalyst in numerous reactions such as the decomposition of soot and NO x in diesel exhausts [13], oxidation of CO [14], photocatalytic degradation of salicylic acid [15], and Fischer-Tropsch synthesis [16].An iron-based solid catalyst is also normally used as a Lewis acid catalyst and/or support in homogeneous and heterogeneous catalysis [17].Hematite nanoparticles could be applicable in water treatment technology for removing metal [18].Hematite has a bandgap of 2-2.2 eV thus being semiconductor suitable for photocatalytic water-splitting with hydrogen formation [19].
Our approach in this study is to obtain hematite nanoparticles using μ 3 -oxo homotrinuclear {Fe 3 O}NO 3 acetate as an iron source and easy sonochemical route as a synthesis procedure.The obtained product was characterized by adequate techniques (FTIR, energy-dispersive X-ray spectroscopy, Raman spectroscopy, wide angle X-ray spectroscopy, transmission electron microscopy), in order to evaluate the formed structure.

Equipments
An Energy Dispersive X-Ray system (EDX) available on Environmental Scanning Electron Microscope (ESEM) type Quanta 200 was also used for qualitative analysis and elemental mapping.
The infrared spectra were registered on a Bruker Vertex 70 FT-IR instrument, in transmission mode, in the 300-4000 cm -1 range (resolution 2 cm -1 , 32 scans), at ambient temperature.
The Raman spectra were recorded with a Renishaw InVia Refl ex spectrometer, equipped with a 632.8 nm HeNe laser as excitation source.A 50x objective lens with NA= 0.75 of a Leica DM 2500M microscope was used to focus the laser beam on the sample and collect the backscattered Raman signal.The investigated spectral region was 100-1000 cm -1 , at low incident laser power selected in order to avoid sample degradation.For a high signal to noise ratio, the exposure time and accumulation number were optimized.
Transmission Electron Microscopy (TEM) investigation was made with Hitachi High-Tech HT7700 Transmission Electron Microscope operated at 100 kV accelerating voltage in high contrast mode.The samples were prepared on carbon coated copper grids of 200 mesh size.Microdrops of the nanoparticles dispersed in water (0.1 %) were placed on the grids, and then solvent was removed in vacuum.
Scanning electron microscope (SEM) images were acquired with an electronic microscope (ESEM) type Quanta 200 operating at 30 kV with secondary and backscattering electrons in high vacuum mode.
Wide Angle X-rays Diffraction (WAXD) was performed on a Bruker-AXS D8 ADVANCE diffractometer, with Bragg Brentano parafocusing goniometer.Scans were recorded in step mode using Ni-fi ltered Cu Kα radiation λ =0.1541 nm.The working conditions were 40 kV and 30 mA tube power.The Bruker computer software Eva 11 and Topaz 3.1 were used to plot and process the data.

Preparation of hematite nanoparticles (NPs)
The μ 3 -oxo heterotrinuclear {Fe 3 O} acetate ([Fe 3 O(CH 3 COO) 6 (H 2 O) 3 ]NO 3 ·4H 2 O) (1.00 g, 1.38 mmol) was dissolved in 5 mL distilled water and ultrasonicated for 5 min at room temperature.To this, 5 mL of 25 M NaOH solution was added.The obtained mixture was ultrasonicated for another 30 min (UTR200, 200w, 24KHz).The purifi cation of the product was done by washing with distilled water until the pH reaches neutral value.After that it was calcinated for 10 h at 600 ºC.The fi nal product was obtained as a red fi ne-crystalline mass.

Results and discussion
Iron oxide, NPs, have been prepared by a procedure consisting in the decomposition of μ 3 -oxo trinuclear iron(III) acetate, {Fe 3 O}NO 3 , by sonication in strong alkaline aqueous medium (pH=11.7),followed by neutralization and calcination, as is illustrated in Scheme 1.The water molecules are eliminated and acetate groups are decomposed.It is assumed that during these processes the Fe-O bonds in iron(III) oxide molecules are preserved.The oxide molecules are agglomerated in nanoparticles (NPs) of different sizes.

Scheme 1. A graphical representation of the pathway leading to iron oxide nanoparticles.
The infrared spectrum of nanoparticles, NPs, as compared with that for {Fe 3 O}NO 3 cluster, (Figure 1) reveals the almost complete disappearance of acetate and nitrate anions, water, and iron-ligand vibrations present in the spectrum of iron acetate.The bands at 552, 474, 446, 384 cm -1 could be assigned to Fe-O vibrations, characteristic for hematite according to literature data [32].The presence of iron and oxygen in the prepared material is clearly revealed by EDX analysis (Figure 2).The peak with a low intensity which corresponds to carbon, is attributed to the substrate.
TEM images (Figure 3) were taken on water-dispersed nanoparticles sprayed on carbon coated copper grid and glass substrate, respectively.From Figure 3a it can be observed that particles have irregular shape.Images were processed with ImageJ 3.0 [33] to obtain the derived histogram (Figure 3b) and, according to these, the agglomerated particles size are in the range 10 -40 nm with the main diameter of about 20 nm.Wide Angle X-ray powder diffraction was measured on the obtained material at room temperature in the range 20 -70 (2θ ˚).The found XRD patterns (Figure 4) revealed good crystallinity and the peak assignment made according to literature data corresponds to well established structure of hematite as shown in Table 1 [34].Appling Scherrer formula [35], the crystallite size was calculated as being 15 nm.The Raman spectrum of nanoparticles is shown in Figure 5.In this spectrum the signatures specifi c for hematite are visible: there are two A 1g modes (222 and 493 cm -1 ) and four E g modes (241, 289, 404, 607 cm -1 ), and longitudinal optical (LO) E u mode (656 cm -1 ) [36,37].Thus, the data from the Raman spectrum confi rmed the presence of α-Fe 2 O 3 (hematite) as it was identifi ed through WAXD.

Conclusions
Sonochemistry followed by thermal decomposition has been proved to be an effi cient route to iron oxide nanoparticles starting from μ 3 -oxo trinuclear iron(III) acetate as a metal source.The chemical nature of the reaction medium (basic pH) together with some physical factors (e.g., temperature), favored the formation of the hematite species of the iron oxide as has been demonstrated by WAXD analysis and confi rmed by Raman and FTIR spectroscopy.The size of agglomerated nanoparticles estimated on the basis of TEM images was found to be in the range 10-40 nm, but applying Scherrer formula, the crystallite size was calculated as being 15 nm.