Abstract

The work reports the convenient method for synthesis of iron oxide hematite (α-Fe₂O₃) nanoparticles by hydrothermal route. The nanoparticles were synthesized at very low temperature of 1600C under magnetic stirring in stainless steel autoclave with a Teflon liner. Ferric nitrate, DI water and ammonia were used as reactants. Structural, Phase, morphological studies and optical properties of the synthesized particles were studied by powder X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), UV Visible and Raman spectroscopy. XRD studies confirm that hematite nanoparticles have a clear mesoporous structure with a bulk density around 1gmcm-3. FESEM images depicted an average size of around 70 nm. The band gap of the as-synthesized α-Fe₂O₃ nanoparticles was found about 1.99eV.

Keywords: Hydrothermal synthesis; Α-Fe₂O₃ Nanoparticles; Morphological studies; Optical properties; X-Ray diffraction; Field emission scanning electron microscopy; Hematite; Electrode materials; Nanowires; Plates; Cubes; Rods; Spectrometer

Introduction

Iron oxide nanoparticles are very useful materials because of their cherishing and potential applications, abundance, low processing cost, stability, environmental friendly-features, and biocompatibility [1]. In recent years, α-Fe₂O₃ has extensively applied in catalysts, gas sensors, pigments, optical and electromagnetic, drug delivery and so on because of their enhancing properties attributed to their various structures [2]. Iron oxide nanoparticles have been synthesized by various methods but developing facile and environmentally friendly synthesis methods is therefore essential [3]. Hematite (α-Fe₂O₃) has a band gap of 1.9-2.2 eV and can act as a very good semiconductor catalyst [4]. The changes in the band gap of the material during the synthesis process can be helpful for further improvements in its biomedical applications and optical properties [5]. The recent developments in nanosized materials show diverse uses such as lithium rechargeable batteries, super capacitors, magnetic materials, photo catalytic degradation and electrode materials [6]. The oxides of iron appear in three common forms namely hematite, maghamite and magnetite, out of which, hematite (α-Fe₂O₃) is the more applicable oxide because of its better thermal stability under extreme conditions and environmental friendly features [7]. Hematite (α-Fe₂O₃) nanostructures have been observed in many shapes such as nanowires, plates, cubes, rods, particles and tubes. Keeping in view large range of shape dependent uses, various synthetic ways, such as, sol-gel method [8-10], thermal decomposition method, co precipitation method, solvothermal reduction and hydrothermal method [11-13] have been developed to prepare hematite nanostructures of different sizes and shapes. For example, Ghosh et al. [14]. synthesized single phase α- Fe₂O₃ nanopowder with an average particle size in the range of 20-30nm and BET surface area of 35-20m2/gm by combustion method at low temperature. J.P. Tu et al. [15]. synthesized uniform Fe₂O₃ nanorods by combustion synthesis method which showed optimal electrochemical performances. S. Mukhergee et al. [16] synthesized α-Fe₂O₃-reduced graphene oxide (nanocomposites (NCs) and bare α-Fe₂O₃ nanoparticles (NPs) by a simple sol–gel method useful for industrial applications. Juliano Toniolo et al. [17] synthesized ultra fine nanocrystalline iron oxide (containing hematite and magnetite phases) by solution combustion process. Prabhu et al. [18] reported surfactant- assisted combustion method for the synthesis of α-Fe₂O₃ nanocrystalline powders. Also, Peizhi Guo et al. [19] achieved controlled synthesis of hematite nanorods and microcapsules for studying magnetic and sensing properties. So, the emphasis was pointed to synthesize hematite nanoparticles and study their character and various properties for further scope and applications in material science, biology and engineering. In this communication, we successfully synthesized α-Fe₂O₃ nanoparticles by simple hydrothermal method and studied structure, morphology and the optical properties.

Experiment

The synthesis procedure begins by preparing a mixture of ferric nitrate (5.4g), DI water (60ml) and ammonia (to maintain pH at 9) under magnetic stirring for 1 hour. The solution was kept at a temp of 1600C in a Teflon lined Autoclave for about 12 hours. The resulting precipitate was filtered and dried. The sample weighing about 0.78gm was crushed to form a fine powder and taken for Characterization. The Phase quality & structural studies of the asprepared samples were done with the help of X-ray diffractometer (Model D-8 Advance & Bruker AXS- XRD) with Cu-Kα radiations of wavelength 1.54056Å and the morphological studies were studied by field emission scanning electron microscope FESEM (Hitachi S4800). The band gap and optical properties were recorded by using the Perkin Elmer model Lamda 950 UV-Vis Spectrometer.

Instrumentation

The instrument used for synthesis was a stainless-steel autoclave with a Teflon liner as shown in (Figure 1).

Results and Discussion

X-ray Diffraction (XRD) Studies

(Figure 2) shows XRD patterns of α - Fe₂O₃ samples for 2ө ranging from 200 to 800. From the XRD pattern, it can be identified that the position of the diffraction peaks is in appreciable agreement with hexagonal structure α -Fe₂O₃. Presence of diffraction peaks in XRD spectra of the samples at 2θ=24.05°, 33.15°, 35.61°, 40.78°, 49.50°, 53.92°, 62.62°and 64.02° are in good agreement with the corresponding (012), (104), (110), (113), (024), (116), (214) and (300) diffraction planes of α-Fe₂O₃ (JCPDS card 33-0664). No other crystalline phases of Fe₂O₃ or impurities are present, indicating a quite high purity phase of α-Fe₂O₃. The (104) peak in the XRD pattern is the highest among all, which indicates that the material prefers to exists in the particular direction among various orientations. The sharpness of the diffracted peaks indicates the appreciable crystallinity of the material. The points of all the maxima match with the peaks that is characteristic of the hematite phase (JCPDS card 33-0664). Since there is no other diffraction line observed corresponding to other phases, which indicate that, the sample is composed of pure phase of hematite. The average crystallite size of α- Fe₂O₃ nanoparticles can be calculated from the XRD data using Debye– Scherrer formula:

where β is the width of the observed diffraction peak at its half maximum intensity (FWHM), K is the shape factor, which takes a value of about 0.9 and λ is the X-ray wavelength (Cu Kα radiation equals to 1.54056 Å).

Morphological studies:

The morphological studies of the as synthesized nanoparticles were carried out by FESEM (Hitachi S4800). (Figure 3) shows the FESEM images of iron oxide nanoparticle sample. It revealed good uniformity in the distribution of particles [20]. The particle size of the synthesized nanoparticles ranges from 50nm to 90nm with an average size of 70nm. It can be observed that particles are having hexagonal shapes and foamy agglomerated particles with a wide distribution. The voids in their structure are formed which may be attributed to the fact that particles tend to aggregate and accumulate at the temperatures of the hydrothermal synthesis process. It is also evident that the material possesses good porosity and the grain boundaries are quite visible and well defined [21]. Many empty spaces can be seen in between the particles which may be useful in diffusion of gas on the α-Fe₂O₃ surface and consequently enhancing the sensing properties of the material [22]. The bulk density of the as synthesized particles is 1gm cm-3 which is highly important to show outstanding properties in nanoscale. There is a difference observed in the crystallite size calculated using X-ray diffraction and particle size obtained by FESEM images which may be due the fact that the particles composed of several crystallization domains are observed by X-rays while whole particle is observed with FESEM

UV Visible Spectroscopy:

The absorption spectra of the samples were recorded by Perkin Elmer model Lamda 950 UV-Vis Spectrometer in the wavelength range of 200-700nm. (Figure 4(a)) shows the UV– Vis absorption spectra of the prepared samples. It can be seen that there is high absorption in the UV region as compared to visible or infrared region. UV-Vis spectra results may attribute to different types of electronic transitions like interactions between magnetically coupled Fe (III) ions or Ligand field transitions or may be oxygen–metal charge transfer excitation from O (2p) nonbonding valence band to Fe (3d) Ligand field orbital [23]. There were three sharp absorption bands observed for sample at a wavelength of 450nm, 494nm and 560nm. The second and more prominent peak is due to result of 6A1→4T1 (4P) charge transfer and pair excitations [24]. However, a broad absorption band appeared at around 600nm. Optical absorption coefficient has been calculated in the wavelength region of 200–800nm.The band gap energies of the synthesized nanoparticles were calculated by Tauc Mott (TM) relation which relates absorption coefficient and the incident photon energy of semiconductors as:

Where α is the absorption coefficient, ‘a’ is a constant, and n is equal to 2 for allowed direct transitions and 0.5 for indirect transitions. (Figure 4(b)) shows the Tauc plots of samples and the indirect band gap is determined by extrapolating the linear portion of the plot and the band gap value is 1.99 eV which is equal to 624 nm.

Raman Spectroscopy:

Raman spectroscopic studies are helpful in differentiating the various phases of iron oxide and the hematite phase spectrum is different from those of common impurity phases, like magnetite and maghamite. (Figure 5) shows the Raman spectrum of the synthesized sample with the peaks observed at 87, 162, 200, 234, 355 and 421 cm^-1. The results are comparable with previous reports [25]. The Raman peaks observed at 93 cm^-1 and 117cm^-1 is assigned to the A1g mode whereas 314 cm^-1 has been assigned to the Eg modes [26, 27]. The peak at 445 cm^-1 has been attributed to the presence of Fe₂O₃ nanocrystals [28].

Acknowledgements

The authors would acknowledge the Department of Science and Technology, New Delhi for the financial support under the NanoMission Project at NIT Srinagar. Further the support of Prof C.N.R Rao at IIT Madras and Prof. Shubra Singh at Anna University Chennai is highly acknowledged for providing characterization instruments.

Conflict of Interest

The authors confirm that this article content has no conflict of interest.