1. Introduction

Nanoscale transition metal oxides (TMOs) attracted much research attention due to their potential applications. Among the different well studied TMOs, NiO with rock salt structure is an attractive material well known for its chemical stability. NiO has shown its potentiality in wide range of technological applications such as gas sensor [1], p-type transparent conductor [2], electrochromic material for smart windows [3], resistive random access memory [4] etc. Doping with TM could change properties of NiO which can be useful for different applications. NiO nanoparticles exhibit room temperature ferromagnetism at the expense of parent antiferromagnetic property when doped with Fe [5-8]. Mn doping led to appearance of superparamagnetism [9] whereas Co doping retains the parent antiferromagnetic property in NiO [10]. Even slightly changing the synthesis conditions led to the appearance of superparamagnetic properties in place of ferromagnetism in Fe doped NiO [11].

Change in optical band gap of NiO with doping has been reported [12-14]. Changing the band gap will affect different properties of material and these materials are necessary to build many technologically important devices [15].

In the present work, Ni_1-xFe_xO (x = 0, 0.01 and 0.02) nanoparticles synthesized by chemical co-precipitation method and the effect of Fe on the band gap of NiO is studied.

2. Experimental Methods

The Ni_1-xFe_xO (x = 0, 0.01 and 0.02) nanoparticles synthesized by chemical co-precipitation method as described elsewhere [8]. The evolution of band gap, refractive index and electron polarizability in Ni_1-xFe_xO samples were characterized by diffuse reflectance spectroscopy using an UV-Visible spectrophotometer (Simadzu, UV-2450) having an integrating sphere assembly.

3. Results and Discussion

Figure 1 shows the variation of absorption coefficient (α) with photon energy (hv) for Ni_1-xFe_xO (x = 0, 0.01 and 0.02) nanoparticles synthesized by chemical co-precipitation method. As indicated from the Fig. 1, the absorption intensity for all samples increased with increasing energy due to nanocrystalline nature of the material [16]. The evolution of optical band gap (E_g) of NiO with Fe doping can be extracted from the α using the following relation [17]:

(1)

where B is a materials dependent constant. The constant m takes the values of 1/2, 3/2, 2 or 3 respectively [18] for direct allowed, direct forbidden, indirect allowed or indirect forbidden transition. One can determine the value of E_g for a given material by extrapolating the linear portion of the vs. hv plot to The linear relation observed for (αhv)² vs. hv plot (Fig. 2) indicated that the Ni_1-xFe_xO nanoparticles exhibit direct allowed transition as was seen for NiO nanostructures [19].

Figure 1. Variation of absorption coefficient (α) with photon energy (hv) for Ni1-xFexO (x = 0, 0.01 and 0.02) nanoparticles

Figure 2. Variation of photon energy, hv for Ni1-xFexO (x = 0, 0.01 and 0.02) nanoparticles

The value of E_g of NiO decreased from 3.65 to 3.43 eV with increasing doping concentration from x = 0 to x = 0.02 (Fig. 3). It is well known that the carriers introduced with dopants which occupy the energy states near the band edges of the host semiconductor in a doped semiconductor [12]. NiO is a p-type material and the holes are expected to generate at the top of the valence band due to Fe doping. When Fe³⁺ occupies Ni site, two such Fe ions can lead to a vacancy at Ni site and each Fe substitution at Ni site led to a hole in Fe doped NiO matrix [5]. These holes cause the bandgap shrinkage [20] as is seen in our case. Similar observation has been reported for Al doped NiO [12]. These holes not only affects the optical band gap but also causes the emergence of ferromagnetic ordering at the expense of antiferromagnetic ordering of host NiO and possibly due to the double exchange interaction between Fe ions and free charge carriers [5]. The appearance of room temperature ferromagnetism in this system [5-8] could useful for the possible application of this material in spintronic devices.

Refractive index (n) of Ni_1-xFe_xO nanoparticles can be calculated from the E_g as per the following relation [21]:

(2)

where K = 3.3668 and C = -0.32234. The variation of ‘n’ with Fe doping concentration in NiO is shown in Figure 3. These values of n match closely with that of visible light transparent NiO thin film [22]. The ‘n’ is related to the electron polarizability of ions and local field inside the material. The values of classical electron polarizability and from E_g for Ni1-xFexO nanoparticles can be calculated as per the following relations [23, 24]:

(3)

and

(4)

where M and are molecular weight (g/mol) and density (g/cm³) of the substances, respectively. Figure 4 shows the evolution of and with different Fe doping concentration in NiO.

Figure 3. Variation of Eg and refractive index (n) in Ni1-xFexO (x = 0, 0.01 and 0.02) nanoparticles

Figure 4. Variation of and in (x = 0, 0.01 and 0.02) nanoparticles

4. Conclusions

The UV-visible spectroscopic characterization of Ni_1-xFe_xO nanoparticles showed Fe doping causes the shrinking of optical band gap of NiO. The refractive indices extracted using E_g values are enhanced with Fe doping in NiO nanoparticles. The values indicated that the Fe doped NiO nanoparticles are transparent to visible light. Our study thus indicates that the shrinking of band gap in Ni_1-xFe_xO samples could be useful for optical device applications.

ACKNOWLEDGEMENTS

The authors thank to Prof. N.C. Mishra, Utkal University, Bhubaneswar for his encouragement and support.