Optical and electrochemical characteristics of sol–gel deposited iron oxide films

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Abstract

Homogenous, crack free iron oxide films are prepared by the sol–gel spin coating technique from a solution of iron iso-propoxide and isopropanol. The films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-visible (UV–Vis) spectroscopy and cyclic voltammetry (CV). XRD of the films showed that they had an amorphous structure. The optical constants refractive index (n) and extinction coefficient (k) were measured by scanning spectrometer in the wavelength range of 390–990 nm. The n and k values were found n =2.3±0.01 and k =0.2±0.002 at 650 nm. The electrochemical behavior investigated in 0.5 M LiClO4 propylene carbonate (PC) electrolyte-CV examinations showed good rechargeability of the Li+/e insertion extraction process beyond 300 cycles. Spectroelectrochemistry showed that these films exhibit weak cathodic coloration in the spectral range of 350–800 nm.

Introduction

Iron oxide has been widely used in several fields such as magnetic recording, photoelectrochemistry, energy storage, electrochromism, catalysis and sensors 1, 2, 3, 4. Iron oxide films are prepared by a large variety of techniques which include chemical vapor deposition (CVD) 5, 6, 7, radio frequency (rf) sputtering 8, 9and wet chemical deposition techniques 10, 11, 12, 13. Capital intensive equipment is needed for physical deposition techniques; and it is difficult to form large-area film. The wet chemical techniques have much lower capital intensiveness. The wet chemical deposition techniques consist of electrodeposition, and sol–gel [14]. Among the wet chemical deposition techniques, the sol–gel process has become one of the succesful techniques for preparing metal oxide films 15, 16. The sol–gel process can exhibit a number of advantages over the physical deposition techniques. One advantage of the sol–gel process is that it has the ability to tailor the film microstructure. The uniformity of the films prepared by this method is very satisfactory [16]. The process is an economical method for the preparation of single or composite metal oxide films as well as a relatively easy way to form large-area films.

Amorphous iron oxide films can be colored weakly with an applied electric field 10, 11. In a previous publication [11], we studied the electrochromism of the sol–gel deposited iron oxide films. The iron oxide films showed poor cyclic stability. In the present work, we attempt to improve the cyclic stability of the amorphous iron oxide films. For this purpose we used different solution synthesis, compared with the previous works.

Section snippets

Preparation of coating solutions

Iron oxide films were fabricated by the sol–gel process. The choice of starting materials and solvent is very important for high quality film in the sol–gel processing. Iron oxide precursor was prepared by using iron iso-propoxide (Fe(O-i-Pr)3, Gelest). Isopropyl alcohol (IPA, Aldrich) and nitric acid (HNO3, Aldrich) were used as a solvent and catalyst, respectively. For the stabilization of solution, triethanolamine (TEA, Aldrich) was used as a complexing additive. One molar TEA was mixed with

Formation and chemical analysis of the coatings

The heat treatment temperature dependence of the film thickness is shown in Fig. 2. The best uniformity was obtained at the heat treatment temperature of 250°C. All films investigated in this study were heat treated at this temperature.

The profile measurements of the iron oxide films indicate that the average thickness of a single layer densified coating is 83 nm. The coatings in this study were deposited using four coating cycles. The thickness of the resulting film is 200 nm. Fig. 3 shows the

Conclusion

Amorphous Fe2O3 films can be prepared by the sol–gel process using iron iso-propoxide as a precursor. The sol–gel spin coating technique was found to be an effective way for making iron oxide films with good optical quality. The best uniformity was observed at a spinning rate of 1600 rpm for the films heat treated at 250°C. The surface morphology examination by SEM suggests that films show uniform surface texture.

The lithiation and delithiation process in the Fe2O3 film resulted in a noticeable

Acknowledgements

We gratefully acknowledge the financial support of Istanbul Technical University Research Foundation (ITU-ARF). We also thank Daniel Vienneau of Nanoptix Inc. for his helpful assistance in the optical characterization.

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