Optical Characterization of NiO Doped Fe 2 O 3 thin Films Prepared by Spray Pyrolysis Method

In this paper, the Fe 2 O 3 thin film were prepared with various ratios doping of NiO by spray pyrolysis method on glass substrate temperature 400 ºC. The initial solution was including a 0.1 M/L for both NiCl 2 and FeCl 3 diluted with redistilled water and a few drops of HCl. The effect of NiO-doping on optical properties were studied. UV-Visible spectrophotometer in the range of (300-900) nm used to determine absorbance spectra. The transmittance increased with increasing NiO content in NiO:Fe 2 O 3 thin films, same behavior of extinction coefficient and skin depth. The energy gap increased from 2.45 eV before doping to 2.86 eV after 3% NiO-doping. While the reflectance, absorption coefficient, and refractive index are decreased with increasing NiO content in Fe 2 O 3 thin films.


INTRODUCTION
Iron oxides exist in many forms in nature, with magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), and hematite (α-Fe 2 O 3 ) being probably the most common [1]. These three oxides are also very important technologically. The iron oxide (Fe 2 O 3 or hematite) is a semiconductor material with a narrow band gap (approximately 2.2 eV) and very good electrochemical stability in water. These properties make this material especially attractive when used as a photoanode to split the water into oxygen and hydrogen by sunlight [2][3][4]. Iron oxide (α-Fe 2 O 3 or hematite) photoanode has received considerable attention on account of its abundance, stability and environmental compatibility, as well as its suitable band gap and valence band edge position. The preparation of hematite thin films was first reported by Bard and Hardee using CVD method [5][6]. Recently, Fe 2 O 3 is found to have large third-order non-linear optical susceptibility and faster response time showing potential applications in optical computing [7].

EXPERIMENTAL
Thin films of Fe 2 O 3 doped by various ratios of NiO have been prepared by chemical pyrolysis technique, this technique is widely used for the large-scale production of films owing to its low production cost and simplicity of operation. A laboratory designed glass atomizer was used for spraying the aqueous solution, which has an output nozzle about 1 mm. The films were deposited on preheated cleaned glass substrates at a temperature of 400 °C. A 0.1 M for both NiCl 2 (Sigma Aldrich UK) and FeCl 3 (Merck Chemicals Germany) diluted with redistilled water and a few drops of HCl. Volumetric concentration of 1% and 3% was achieved the optimized conditions have been arriving at the following conditions; spray time was 10 s and the spray interval 2 min was kept constant, the carrier gas (filtered compressed air) was maintained at a pressure of 10 5 Nm -2 , and distance between nozzle and the substrate was about 30 cm ±1 cm.
Thickness of the sample was measured using the weighting method and was found to be around 400 nm. Optical transmittance and absorbance were recorded in the wavelength range (300-900 nm) using UV-Visible spectrophotometer (Shimadzu Company Japan).

RESULTS AND DISCUSSIONS
The transmittance spectra were recorded by UV-Visible spectrophotometer in the range of (300-900) nm on glass substrate temperature for NiO:Fe 2 O 3 thin films. The measured transmittance spectra of the NiO:Fe 2 O 3 thin films are shown in Fig. 1. Form this figure, it can show that the transmittance increases with increasing wavelength and there is Sharpe fall in the wavelength of 600 nm, and clearly increases of transmittance with increasing NiOdoped in the Fe 2 O 3 thin films.

International Letters of Chemistry, Physics and Astronomy Vol. 45 51
The reflectance (R) obtained from the expression [18]: Where A is the absorbance and T is the transmittance. The measured reflectance spectra of the NiO:Fe 2 O 3 thin films are shown in Fig. 2. Form this figure, it can show that the reflectance decreases with increasing NiO-doped in the Fe 2 O 3 thin films. The absorption coefficient (α) was directly obtained from the absorbance (A) with wavelength (λ) values by using the relation [19].
Where t is the thickness. Fig. 3 represent the plot of absorption coefficient vs. wavelength. From this figure, it can show that the absorption coefficient decreases with increasing NiO-doped in the Fe 2 O 3 thin films, and gradually increased with increasing wavelength.

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ILCPA Volume 45   The extinction coefficient (k) of the films was calculated using the expression [21]:

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Where α is the absorption coefficient. The plot of extinction coefficient versus photon energy for the NiO:Fe2O3 films deposited at different content of NiO is shown in Fig. 7. It observes that the extinction coefficient increases for doped film than that of undoped films. With an increase in wavelength for all the films, the extinction coefficient decreases sharply until the wavelength of 600 nm to become the decreases gradually. In general, the k values increases with increasing doping ratio of NiO in NiO:Fe2O3 thin films.
The refractive index is an important parameter for optical materials and for photoelectronics applications [22]. Thus, it is important to determine optical constants of the films and the complex optical refractive index of the films. The refractive index of the films was determined from the following relation [23]: Where R is the reflectance and k is the extinction coefficient. The plot of refractive index versus wavelength for the NiO:Fe 2 O 3 films is shown in Fig. 8. It observes that the refractive index decreased with increasing NiO content in the NiO:Fe 2 O 3 thin films. It has been well established that the refractive index is closely correlated with film density; a higher refractive index implies a denser film.  The skin depth (χ) represents the electromagnetic wave will have amplitude reduced after traversing a thickness can be calculated by using the following relation [24]:

CONCLUSION
The Fe 2 O 3 thin film prepared with various ratios doping of NiO by spray pyrolysis method on glass substrate temperature 400 ºC. UV-Visible spectrophotometer was used to recording the transmittance spectra. From studying the optical properties we found the transmittance increased with increasing NiO content in NiO:Fe 2 O 3 thin films, same behavior appears for extinction coefficient and skin depth. The energy gap increased from 2.45 eV before doping to 2.86 eV after 3% NiO-doping. While the reflectance, absorption coefficient, and refractive index are decreased with increasing NiO content in Fe 2 O 3 thin films.