Structural, optical and magnetic properties of the Fe₂TiO₅ nanopowders

Abstract

Iron titanate nanopowders with a particle size range of 48–70 nm could be obtained after calcinations of the dried gel at 900°C for 2 h. Fe₂TiO₅ indicates a ferrimagnetic–paramagnetic behaviour, as evidenced by using vibrating sample magnetometer at room temperature. In the temperature range of 25–300°C the empirical equation of the heat capacity C _p (J/mol K) = −692.328 + 1.39 T + 3.757 × 10⁷/T ² for Fe₂TiO₅ was determined from differential scanning calorimetry. Direct optical band gap of Fe₂TiO₅ was calculated using the Tauc model by UV-Vis diffuse reflectance spectroscopy. Band gap energy of Fe₂TiO₅ was determined as 1.95 eV.

Keywords:

• iron titanate
• pseudobrookite
• magnetic measurements
• heat capacity
• optical band gap

1. Introduction

Titanium-substituted iron oxides are widespread in nature and represent an important mineral resource for the commercial extraction of both iron and titanium 1. Mixed metal oxides have been widely investigated since they have interesting catalytic properties 2. Chemical properties of the active sites can be significantly modified by mixing oxides, in the form of solid solutions, or by supporting the oxide catalyst on another oxide 3,4. There has been growing interest in iron titanium oxide as a good candidate for new optical fibres 5–7, and as a sensor material for the detection of NO₂ 8 and gas sensing applications 9. The polycrystalline ferrites have been studied for several years due to their wide use as magnetic materials for telecommunications audio and video power transformers and many other applications. The crystallographic, electrical and magnetic properties depend strongly on the stoichiometry, preparation conditions and particle size 10–21.

Fe-titanates exist principally as three minerals, namely, ilmenite (FeTiO₃), pseudo-brookite (Fe₂TiO₅) and ulvöspinel (Fe₂TiO₄). These multifunctional oxides can be both ferrimagnetic and wide band gap semiconductors, and can be exploited in a variety of ways in radhard electronics, microelectronics and spintronics technologies 22.

The chemical synthesis of solid inorganic materials is making an increasingly important contribution to the development and manufacture of ceramic materials 23,24. It also plays a vital role in the development of fabrication techniques for the successful exploitation of various useful properties of ceramics 25. There is a growing realisation that a single-source precursor with different metals assembled in one molecule in the desired metal ratios can lead to nanometre level defined morphology and interesting physical properties 26,27.

In recent years, nano-structured iron–titanium mixed oxides with different Fe/Ti ratios were prepared by sol–gel methods under different preparative conditions with a mixture of iron–titanium oxides prepared in different calcination temperatures 28. Also polycrystalline Fe₂TiO₅ films were prepared on nesa silica glass substrates by the sol–gel method, and their photoanodic properties were measured in a three-electrode wet cell with an aqueous buffer solution of pH = 7 29.

The sol–gel technique has many advantages over other fabrication techniques, such as homogeneity, stoichiometry control, purity, ease of processing and controlling the composition, and ability to coat large and complex area substrates 30.

In this study, we chose one typical wet-chemistry synthesis method, stearic acid gel, to prepare pure Fe₂TiO₅ nanopowders. The value of optical band gap, empirical equation of heat capacity and magnetisation of the iron titanate were determined.

2. Experimental

Fe₂TiO₅ powders were prepared along a synthetic procedure, as summarised in Figure 1. Iron acetyl acetonate, tetra-n-butyl titanate and stearic acid were all analytical grade reagents. First, an appropriate amount of stearic acid was melted in a beaker at 73°C, and then a fixed amount of iron acetyl acetonate was added to the melted stearic acid and dissolved to form a transparent brown solution. Next, stoichiometric tetrabutyl titanate was added to the solution, stirring to form a homogeneous light brown solution, naturally cooling down to room temperature and drying in an oven for 12 h to obtain dried gel. Finally, the gel was calcined at 700°C, 800°C and 900°C temperatures in air to obtain nano-crystallites of Fe₂TiO₅.

Figure 1. Flowchart for the preparation of Fe₂TiO₅ nanopowders.

The formation of process and structural characterisation of Fe₂TiO₅ phases have been investigated by XRD, Fourier transform infrared spectroscopy (FTIR), EDX, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and the physical properties have been investigated by vibrating sample magnetometer (VSM), differential scanning calorimetry (DSC) and diffuse reflectance spectroscopy (DRS).

Powder X-ray diffraction patterns of the samples were recorded using X`pert pro Philips Diffractometer with Cu-Kα radiation (λ = 0.15418 nm) in the range from 10° to 80° (2θ) to examine the crystallisation and structural development of Fe₂TiO₅ powders.

The morphology of the product was studied by SEM(Philips XL30) and TEM (Zeiss EM900, 80 keV).

The FTIR analysis was performed using (Magna-IR 550 Nicolet) spectrometer by employing KBr pellet technique. The chemical composition of the product was tested by energy dispersive X-ray spectroscopy (EDS, Philips XL30). Magnetisation measurement is carried out using VSM (BHV-55, Riken, Japan) at room temperature.

The specific heat capacity was measured by scanning method and using a differential scanning calorimeter and DSC (PL DSC-polymer laboratory) in a pure nitrogen atmosphere. The sample (calcined at 900°C) was heated from 25°C to 300°C with a heating rate of 10°C min⁻¹. The UV-Vis DRS was performed on an AVASPEC spectroscopy.

3. Results and discussion

3.1. X-ray diffraction patterns

Figure 2 shows the XRD patterns of the Fe₂TiO₅ powders after heat-treatment from 700°C to 900°C in air for 2 h. At 700°C, the crystallisation of orthorhombic iron titanate phase (▴ marked peaks) began along with traces of hematite phase (▪ marked peaks) (Figure 2a). Further, by increasing the calcination temperature to 800°C, the hematite phase was decreased with an increase in the intensity of Fe₂TiO₅ phase (Figure 2b). The nanopowders were obtained after calcination at 900°C (Figure 2c).

Figure 2. X-ray diffraction patterns of Fe₂TiO₅ powders calcined at (a) 700°C; (b) 800°C and (c) 900°C.

However, at this temperature, the nanopowders displayed sharp and intense peaks indicating fine crystalline orthorhombic Fe₂TiO₅ phase. All the peaks corresponding to orthorhombic phase were well matched with database in JCPDS (file number: 76–1158).

3.2. Morphology of samples

X-ray diffraction has been performed on powder and the particles size L has been estimated with the Scherrer formula 31:

where K is the form factor (equal to 0.9), λ = 1.5418 Å, 2θ is the peak position and Δ (2θ) is the FWHM of the diffraction peak in terms of radians.

The particle size was calculated by Scherrer's formula for different calcination temperatures. The crystallite size of the powders calcined at 700°C, 800°C and 900°C were about 34, 27 and 34 nm, respectively.

The surface morphology of the obtained Fe₂TiO₅ nanocrystals was investigated by SEM images at 800°C and 900°C. SEM images in Figure 3(a) and (b) revealed that the surface morphology of Fe₂TiO₅ particles was quasi-spheric. Images show that all powders have agglomerated graining structure.

Figure 3. SEM images of Fe₂TiO₅ nanostructures synthesised at (a) 800°C and (b) 900°C.

Figure 4 is the TEM micrograph of Fe₂TiO₅ powders calcined at 900°C for 2 h. Image shows that all powders have agglomerated graining structure. The particle size was estimated in the ranges about 48–70 nm.

Figure 4. TEM image of Fe₂TiO₅ powders calcined at 900°C for 2 h.

TEM counting provides a larger average grain size than XRD. The reason for this apparent discrepancy is that the size provided by XRD corresponds to the average of the smallest undistorted regions in the material, whereas TEM counting is related to regions separated by more-or-less sharp contours in the TEM micrograph. Therefore, when dislocations are arranged in a configuration, which causes small orientation differences between two adjacent regions, the XRD size corresponds to the two separate regions, whereas in the TEM micrograph the two regions may seem to correspond to the same grain 32.

3.3. EDS and IR spectra

EDS spectrum of the as-synthesised nanopowder confirms that our product consists of iron and titanium elements. No other phase was shown to be present in the prepared iron titanate product. The formation of Fe₂TiO₅ is further supported by the FTIR spectra, as shown in Figure 5. In this spectrum the Fe₂TiO₅ powders calcined at 900°C showed peak at 493 cm⁻¹which is assigned to the Fe–O absorption band and the strong peak at around 660 cm⁻¹ can be assigned to Fe₂TiO₅, corresponding to the formation of iron titanate 28. The broad absorption observed at 3439 cm⁻¹ for Fe₂TiO₅ nanostructure was assigned to the stretching vibrations of –OH group of adsorbed water 33.

Figure 5. FTIR spectra of the Fe₂TiO₅ powders calcined at 900°C.

3.4. Magnetic properties

Iron titanates are reported as ferrimagnetic materials 22, but specifically an anti-ferromagnetic ordering with weak ferromagnetism has been predicted for Fe₂TiO₅ 34. Comparison of the hysteresis loops (Figure 6) measured at room temperature with typical curves obtained from mixed magnetic systems shows a ferrimagnetic–paramagnetic behaviour of the high-pure Fe₂TiO₅ nanopowders 35. On the other hand, XRD patterns show rhombohedral Fe₂O₃ (as impurity). The magnetic behaviour of α-Fe₂O₃ (rhombohedral) and γ-Fe₂O₃ (cubic) is obviously different. Single-crystal α-Fe₂O₃ is paramagnetic, whereas γ-Fe₂O₃ is ferrimagnetic at room temperature 36. Therefore the ferrimagnetic–paramagnetic behaviour of the sample originated from Pseudobrookite and hematite phases, respectively.

Figure 6. VSM curves of Fe₂TiO₅ powders calcined at 900°C.

3.5. Heat capacity

The titanium-based oxides are well-known owing to their excellent dielectric, piezoelectric, pyroelectric and photostrictive properties 37,38. Fe₂TiO₅ is a ferroelectromagnetic material in which both ferroelectric and magnetic ordering co-exist simultaneously 39. Piezoelectric and pyroelectric phenomena coexist in ferroelectric materials 40 whereas pyroelectric and piezoelectric properties are related to heat capacity 41. The temperature dependence of the heat capacity of Fe₂TiO₅ determined by the scanning method is shown in Figure 7. In the temperature range between 25°C and 300°C, the empirical equation for the Cp of Fe₂TiO₅ is determined from the experimental values as follows:

Figure 7. Heat capacities of Fe₂TiO₅ by DSC.

3.6. Optical properties

The absorption coefficient and optical band gap of a material are two important parameters by which the optical characteristics and its practical applications in various fields are judged. Figure 8 displays absorption versus wavelength behaviour of iron titanate calcined at 900°C by UV-Vis DRS.

Figure 8. (Colour online) Absorption versus wavelength behaviour of Fe₂TiO₅ at 900°C by DRS.

The nature of the optical inter-band transition and value of the energy gap Eg can be determined using the Tauc model:

where α is the absorption coefficient, A is the edge width parameters independent of photon energy, hν is the energy of incident photon and E _g is the band gap of the material. For direct band gap determination, plot of (αhν)² versus hν is presented in Figure 9. Band gap value was obtained by extrapolating the straight portion of the graph on hν axis at (αhν)²= 0, as indicated by the solid line in Figure 9 41,42.

Figure 9. Tauc plot exhibiting (αhν)² versus photon energy (hν) for the calculation of direct band gap of Fe₂TiO₅ by extrapolating on hν axis α = 0.

The value of direct band gap for iron titanate came out to be 1.95 eV.

4. Conclusion

This study has demonstrated the feasibility of the synthesis of pure Fe₂TiO₅ powders using a wet-chemistry synthesis method using stearic acid gel. Well-crystallised Fe₂TiO₅ nanopowders could be synthesised at 900°C for 2 h. In addition, the microstructures of Fe₂TiO₅ powders have been evaluated using TEM and the grain sizes are shown to vary between 48 and 70 nm. This study also showed that the synthesised Fe₂TiO₅ has indicated a ferrimagnetic–paramagnetic behaviour, as evidenced by using VSM at room temperature. Moreover, the empirical equation for the C _p of Fe₂TiO₅ has been determined from DSC. The band gap energy of Fe₂TiO₅ was determined by DRS too.

Acknowledgements

The authors thank the Iranian Nanotechnology Initiative for supporting this work.