Effect of the thickness reduction on the structural, surface and magnetic properties of α-Fe2O3 thin films

doi:10.1016/j.tsf.2016.03.052

Thin Solid Films

Volume 607, 31 May 2016, Pages 50-54

https://doi.org/10.1016/j.tsf.2016.03.052 Get rights and content

Highlights

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Hematite thin films with different thickness were deposited by RF sputtering technique.
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X-ray diffraction patterns confirm the formation of hematite phase in all samples.
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Hysteresis curve at 300 K shows the presence of a weak-ferromagnetic phase.
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XPS show the presence of Fe^2 + ions coexisting with Fe^3 + ions.

Abstract

Hematite (α-Fe₂O₃) polycrystalline thin films of different thicknesses were produced by thermal oxidation in air atmosphere from Fe metallic thin-films deposited by radio frequency (RF) sputtering technique. X-ray diffraction (XRD) patterns confirm the formation of hematite phase in all samples and indicate that the mean grain size decreases as the film thickness becomes thinner. Conversion electron Mössbauer spectroscopy (CEMS) spectra at room temperature show magnetic splitting (six line patterns). It is determined that the resonance peaks become broader and asymmetric as the film thickness decreases. This finding was associated with the structural disorder introduced by the thickness reduction. Magnetization as a function of the magnetic field curve obtained at 300 K shows the presence of a weak-ferromagnetic contribution, which was assigned to the large density of decompensated spins at the films surface. From the magnetization vs. temperature curves it has been determined that the Morin transition temperature (T_M) is shifted from ~ 240 K to ~ 196 K, meanwhile it becomes more broadened as the film thickness decreases. X-ray photoelectron spectroscopy (XPS) measurements show the presence of Fe^2 + ions coexisting with Fe^3 + ions whose population increases as the film becomes thinner. The density of chemisorbed oxygen increases as the film thickness is reduced in agreement with the results obtained from the other measurements in this work.

Introduction

Due to the grain size effects and novel physical properties of nanostructures, such as thin films, nanotubes, nanoparticles, nanorods, nanowires, among others, they are of great interest for theoretical studies and nanodevice applications. In this respect, hematite (α-Fe₂O₃) thin films, the most stable phase of iron oxide, with rhombohedral structure (space group R-3c), have been considered as a promising candidate in several areas of nanotechnology, due to its extraordinary chemical stability in oxidative environment, favorable optical band gap (~ 2.2 eV), abundance and low cost [1]. Among the applications of this material we highlight the works authored by Punnosse et al. which uses the magnetic properties of hematite thin films for hydrogen gas sensor application [2]; Chen et al. which uses the photocatalyst properties of hematite to remove organic dyes from wastewater [3]; and Sivula et al. which uses hematite as the photoanode in water splitting devices. Due to the increasing global demand for energy and the requirement of using cleaner and environmentally friendly sources of energy the last-mentioned application (photoanode for water splitting) is a promising approach of using solar energy to produce hydrogen from water splitting [4]. Hydrogen represents a highly-efficient and environmentally friendly fuel when compared to fossil fuels [5], [6]. Diverse methods have been used to produce hematite, either as nanoparticles or thin films, among them one can find the chemical vapor deposition method (CVD) [7], [8], DC reactive magnetron sputtering [9], sol-gel method [10], [11], among others. On the other hand, it is interesting to note that hematite is considered as an insulator with localized Fe^3 + ions. However, a slight deviation of hematite from the standard stoichiometry induces an enhancement of the photo-electrochemical properties. Moreover, it is well known that hematite has an extremely poor electric conductivity with a hole length of 2–4 nm [7]. This is why size reduction is so important while modifying the stoichiometry in order to enhance the electric conductivity. On the other hand, with respect to the magnetic properties, bulk hematite is weakly ferromagnetic between the Néel (~ 955 K) and the Morin (T_M ~ 260 K) temperatures [12]. Nevertheless, as well as the electric properties the magnetic properties are also strongly affected by the size reduction. For example, it is reported in the literature that the size reduction of hematite particles leads to an abrupt reduction of the Morin transition temperature [13]. However, the size reduction of the system can also induce the formation of core–shell structures and secondary phases. In this regard, the capability of controlling the physical properties of low-dimensional systems is of fundamental importance to design new devices. In this study, we report on a systematic investigation of the structural, hyperfine, magnetic and surface properties of polycrystalline hematite thin films of different thickness deposited by radio-frequency (RF) sputtering.

Section snippets

Experimental details

Thin films of iron with thickness varying from 10 to 50 nm were grown at room temperature by RF magnetron sputtering technique, using the sputtering targets of Fe onto 1 × 1 cm² glass substrate. The base pressure of the sputtering system was ~ 4 × 10^− 6 mbar and the argon pressure during the sputtering process was 3.6 × 10^− 2 mbar. After deposition, the Fe thin films were oxidized by a thermal treatment carried out in air atmosphere at 500 °C for 2 h. The thicknesses of the iron films were determined by

Results and discussion

The XRD spectra of the hematite thin films with different thicknesses (t) deposited onto glass substrate are shown in Fig. 1(a)_. The Bragg reflection peaks reveal the formation of the hematite phase, α-Fe₂O₃ (JCPDS No. 33-664), in all samples. As shown in the inset of Fig. 1(a) the linewidth (full width at half maximum) of the (104) diffraction peak shows a tendency to decrease as the film thickness increases. This effect can be attributed to the increase of the grain size and/or to changes in

Conclusion

In summary, in the presented study we reported on the systematic experimental study of hematite thin films successfully deposited by the sputtering technique. The structural characterization shows a clear increase of the mean grain size as the film thickness is increased in agreement with the square root dependence (< D > ~ t^1/2 ). No clear evidence of changes in the lattice parameter has been determined from our data analysis. The dependence of the magnetic moment, hyperfine parameters and surface

Acknowledgements

This work was financially supported by the Brazilian agencies CNPq, CAPES, and FAPEMIG.

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Effect of the thickness reduction on the structural, surface and magnetic properties of α-Fe2O3 thin films

Highlights

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgements

Int. J. Hydrog. Energy

Thin Solid Films

Mater. Lett.

Appl. Surf. Sci.

J. Catal.

J. Electron Spectrosc. Relat. Phenom.

Solar water splitting: progress using hematite (a-Fe2O3) photoelectrodes

Chemsuschem

Thickness dependent on photocatalytic activity of hematite thin films

Int. J. Photoenergy

Nanostructured hematite: synthesis, characterization, charge carrier dynamics, and photoelectrochemical properties

Energy Environ. Sci.

Nature

Growth and magnetic properties of oriented a-Fe2O3 nanorods

J. Phys. Chem. B

Effect of the thickness reduction on the structural, surface and magnetic properties of α-Fe₂O₃ thin films

Solar water splitting: progress using hematite (a-Fe₂O₃) photoelectrodes

Growth and magnetic properties of oriented a-Fe₂O₃ nanorods