Nickel Sub-lattice Effects on the Optical Properties of ZnO Nanocrystals

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The crystallite size may be calculated using Scherrer’s formula;

Where is the wavelength of the X-rays, θ is the diffraction angle, D is the crystallite size, k is the shape factor (i.e. 0.94 for Lorentzian line profiles and small crystals of uniform size), β is the full width at half maximum (FWHM) of the peak. The calculated parameters are shown in Table 1.

The expansion of the lattice constants of Ni-doped ZnO nano-particles (Table 1) indicated that Ni ions systematically substituted into the ZnO matrices ^[28]. Morphology of the undoped.ZnO nano-particles have been investigated using scanning electron microscopy (SEM), as shown in Figure 2. The spectrograph of the sample shows the growth of undoped ZnO nano-particles. It indicates that the growth is not exactly in uniform grain size but in average it is observed like hexagonal nanostructure. Figure 3 shows the EDS spectra which has confirmed that the synthesized sample contain only Zn and O without any impurity phase rather carbon which is due to the carbon tape used at the time of EDS analysis.

Table 1. Calculated parameters of undoped and Ni doped ZnO Nano-particles

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Figure 2. SEM micrograph of undoped ZnO

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Figure 3. EDS spectra of undoped ZnO

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3.1.2. Compositional Analysis

The formation of the wurtzite ZnO structure in undoped and doped ZnO nanocrystalline powders is further confirmed by FTIR spectra as shown in Figure 4. The FTIR spectra were recorded in the range of 400–4000 cm⁻¹ for all samples. The position and number of absorption bands not only depend on crystal structure and chemical composition but also on crystal morphology. The peaks around 1336 and 1537 cm^-1 are attributed to the symmetric and asymmetric C=O stretching vibration modes, while the peak at 441 cm^-1 is attributed to the stretching E2 (LO) mode which is typical for the ZnO wurtzite structure. In Figure 4, appearance of small peak at 1000 cm^-1 is attributed to C-H stretching. A small hump observed at 1900-2051 cm^-1 revealed the presence of O-C-O which appeared due to the atmospheric absorption at the preparation time of sample in non-vacuum system. A strong absorption peaks in all undoped and Ni-ZnO samples have been observed at 3331cm^-1 which isattributed to the presence of hydroxyl group (O-H stretching). It is important to notice that the doping specie affects the spectra and it has been observed that the broadening at the shoulder of ZnO band at 480 cm^-1 attached to the ZnO peaks which assigned as Ni-O stretching mode, which is be due to the Ni ion present in the doped samples. Hence, it is nice supporting result of previous investigations in this paper.

Figure 4. FTIR spectra on undoped and Ni-doped ZnO

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3.2. Optical Properties

The effect of Ni substitution on wurtzite structure of ZnO was further confirmed using UV–visible optical spectroscopy measured in the range 300–800 nm. Figure 5 shows the room temperature optical absorption spectra of undoped and nickel doped ZnO nano-particles. The absorption band edge of undoped ZnO is observed at 373 nm and it gets shifted towards longer wavelength region for the 1 and 3 mol. % Ni-doped ZnO samples and goes to lower wavelength for 5 mol.%. The observed red shift in the absorption band edge with nickel doping in ZnO may be due to the sp-d exchange interactions between the band electrons and the localized d-electrons of the Ni²⁺ ions. Such a red shift in band edge with increasing nickel dopant is a clear indication for the incorporation of Ni ions into the Zn site of the ZnO matrix ^{[26, 27, 28]}.

Figure 5.Absorbance spectra of undoped and Ni-doped ZnO

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It may be noted that the band gap of pure NiO is in the range of 3.5- 4.0 eV. So the doping is not tuning of the band gap in the direction of NiO at lower doping amount. Perhaps, it is not surprising that the band gap upon doping is a consequence of the interaction between Ni and ZnO states. This band gap reduction at lower percentage of Ni could be attributed to the introduction of Ni states which lowers the valance band states. Instead, at higher % of Ni substitution there is increased the NiO sublattice like phase which may not adhered to the ZnO lattice, therefore, NiO sublatice is dominating for the higher band gap occurrence. Otherwise, the quantum confinement effect is directly showing the variation of band gap with the crystallite size.

The absorption coefficient and the energy band gap can be described by the following equation;

where is the photon energy, is the absorption coefficient, and is a constant.

A plot of versus is made to determine bandgap E_g using the linear fit process. The extrapolated absorption threshold of the undoped and Ni-doped ZnO nano-particles is shown in Figure 6.

Figure 6. Band gap spectra of undoped and Ni ZnO NPs

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It reflects the band gap energies of the corresponding samples and indicates that the bandgap decreases with 1 and 3 mol % nickel doping and further doping at 5 mol. % band gap goes to the higher value. The bandgap decreasing with increasing nickel dopant could be interpreted mainly due to the sp–d exchange interactions between the band electrons and the localized d electrons of the Ni²⁺ ions. Quantum mechanically, we can interpret the absorption falls at higher percentage of Ni which results in the bandgap broaden. It is due to the fact that as the crystallite decreasing in size to low dimension as enough, the quantum confinement effect came in existence which enhances the band gap ^[29]. Therefore, the energy bandgap linearly depends on 1/D² as shown in Figure 7. In the strong confinement regime, the confinement energy of the first excited electronic state can be approximated by the following equation ^[30];

where the undoped bandgap E_undopedis 3.01eV (this work) and the bulk exciton binding energy can be taken as 60 meV. According to Beni and Rice ^[31] the electron and hole effective masses are taken as and , respectively. Additionally, is the reduced Planck’s constant and D is crystallite size calculated from XRD measurements.

Figure 7. Energy bandgap variation with crystallite size

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