Effect of Mn Doping Concentration on Structural, Morphological and Optical Studies of ZnO Nano-particles

We report the crystal structure, morphology and optical properties of the Zn1-xMnxO nanoparticles with nominal composition (x = 0.00, 0.05, 0.10, 0.15 & 0.20) were synthesized by sol-gel route. The crystal structure analysis was carried out by X-ray diffraction technique (XRD), morphology and particle size was investigated using Transition electron microscopy (TEM), Optical characterization was performed using UV-VIS technique. The XRD studies show that samples have wurtzite (hexagonal) crystal structure. In addition to this XRD results also indicate no extra impurity or secondary phases are observed. The lattice parameters a and c of Zn1-xMnxO nanoparticles increase with increasing Mn content which indicates that Mn ions go to Zn ions in the ZnO lattice. The atomic packing fraction (APF) increase of Mn doped ZnO nanoparticles with increasing Mn content. TEM microphotographs show that the pure and 10% Mn doped ZnO nanoparticles are in spherical shape and their average particle size of pure and 10% Mn doped ZnO nanoparticles are found in the range of 20-60 nm. The optical energy band gap decreases with increasing Mn concentration, it may be owing to Mn clustering.


Introduction
Zinc oxide (ZnO) has attracted much attention in the last few decades due to its wide variety of application such as sensors, field-emission transistors, ultraviolet photo-detectors, biomedical systems such as ultrasensitive DNA sequence detectors [1][2][3][4][5]. ZnO is a semiconductor with a wide band gap i.e. 3.37 eV. The stable structure of ZnO is mostly in wurtzite structure, in which four atoms of oxygen in tetrahedral co-ordination surround each atom of zinc. Zinc oxide is also used for spintronics applications with magnetic ions i.e. Co, Ni, Mn, V and Fe doping. For this reason, Mn doping has valuable spin off in electrical, magnetic and optical properties of ZnO. Thus Mn doped ZnO has become a reasonable choice for diluted magnetic semiconductor (DMS). Diluted magnetic semiconductor (DMS) materials have many unique applications such as magneto-optical, magneto-electrical and magneto-transport properties etc [6].
Different methods have been used to synthesis of nanopowders of Mn doped ZnO like sol-gel [7], co-precipitation [8], combustion [9], solid state sintering methods [10], hydrothermal synthesis [11], thermal evaporation of Zn [12], spray pyrolysis [13] and low temperature wet-chemical method [14] these methods have occurred worldwide in nanotechnology. In this study we focused on the structural, morphological and optical properties of Mn doped ZnO nanoparticles prepared by sol-gel technique. Effect of Mn doping on the lattice parameters, crystallite size, volume of unit cell, atomic packing fraction (APF), X-ray density and energy band gap were investigated qualitatively and quantitatively in this paper.

Experimental Details
The samples of Mn substituted ZnO with nominal compositions Zn 1-x Mn x O (x = 0.00, 0.05, 0.10, 0.15 & 0.20) were synthesized by sol-gel route using high purity (99.99% AR grade) acetates of Zn and Mn i.e. Zn (CH 3 CO 2 ) 2. 2H 2 O and Mn (CH 3 CO 2 ) 2. 4H 2 O. In this procedure, to prepare pure ZnO, the appropriate amount of Zinc acetates (Zn (CH 3 CO 2 ) 2. 2H 2 O) were dissolved in N-dimethylformamide (DMF), stirred for 3 hours using hot plate with a magnetic stirrer. The temperature was maintained at 30-40 0 C for complete dissolution. It was then raised and held at 70-80 0 C for 2 hours until all fluid was evaporated, then gel is obtained. These gels were heated at 150 0 C for 3 hrs in the furnace for dehydration. These precursors were ground in an agate mortar by pestle for 30 minutes to obtain fine powder. These powders placed in a Al 2 O 3 crucible for sintering in furnace at 450 0 C for 8 hrs followed by furnace cooling at room temperature. To get the powdered sample of pure ZnO.
For preparation of Mn doped ZnO, the appropriate amounts of two precursors of Zinc acetate (ZnCH 3 CO 2 ) 2. 2H 2 O) and Manganese acetate (Mn (CH 3 CO 2 ) 2. 4H 2 O) were thoroughly dissolved in N-dimethylformamide (DMF), stirred for 3 hours using hot plate with a magnetic stirrer. The temperature was maintained at 30-40 0 C for complete dissolution. It was then raised and held at 70-80 0 C for 2 hours until all fluid was evaporated then gel is obtained. These gels were heated at 150 0 C for 3 hrs in the furnace for dehydration. These precursors were ground in an agate mortar by pestle for 30 minutes to obtain fine powder. These powders placed in a Al 2 O 3 crucible for sintering in furnace at 450 0 C for 8 hrs followed by furnace cooling at room temperature. This time, dark yellowish colored powders were obtained which indicate that only single phase is formed for getting the powdered samples of Zn 1-x Mn x O system. The crystal structure of all the samples studied using X-ray diffractometer (Model: PW-3710) with Cuk α (λ = 1.5406 A o ) radiation. It was used for recording X-ray diffraction pattern operating at 40 kV and 40 mA.
To study the sample morphology, the powders were ultrasonically mixed with ethanol and suspended on a Cu mesh, which was the sample holder of a transmission electron microscope (TEM) operated at 200 kV (TEM-Model CM 200, SUPERTWIN). Absorption spectra of the samples in the UV-visible spectral region were recorded using UV-VIS-NIR (JASCO V 570) spectrophotometer for the determination of energy band gap of Mn doped ZnO nanocrystals.

Structural Analysis
The structure parameters and phase purity of pure and different concentration of Mn substituted ZnO nanoparticles sintered at 450 0 C were estimated using X-ray diffraction (XRD). It indicates that all sintered samples have hexagonal (wurtzite) crystal structure. It also has been seen that the XRD peak broadening increases with increasing Mn content, when the doping of Mn up to 20%. There is no any extra peak of manganese metal, oxides, or any impurity phases were observed. It indicates that all samples are single phase. Sharma et al [15] reported that the presence of secondary phases like ZnMn 2

Lattice Parameters
The lattice parameters a & c of the samples were calculated using following equation.
[ ] Where a & c are the lattice parameters, d is the interplanar distance and (hkl) are the Miller indices. The values of lattice parameters are listed in table 1. The lattice parameters go on increasing with increasing Mn content of Zn 1-x Mn x O nanoparticles because of higher ionic radius of Mn 2+ ions in comparison to the Zn 2+ ions. However, the lattice parameters increase up to 20% Mn doping. But Abrishami et al [16] reported that the Mn doping for x=15% in ZnO nanometer-sized crystals the lattice constant decrease, it may be due to Mn 3+ and Mn 4+ brings. In our case there is no reduction in the lattice parameters with increasing Mn up to 20% in ZnO nanocrystals. It indicates that, no formation of Mn 3+ or Mn 4+ ions in the Zn 1-x Mn x O nanoparticles. The lattice parameters versus Mn content of Zn 1-x Mn x O nanoparticles plot is shown in figure 2.

Volume of Unit Cell and X-Ray Density
The volume of unit cell was calculated using the equation It was found that the volume of unit cell increases with increasing Mn content. It may be due to increasing the lattice parameters a and c. These values of volume of unit cell are tabulated in table 1. The X-ray density was estimated using the equation Where n is number of atoms per unit cell, M is the molecular weight, N A is the Avogadro's number and V is the volume of unit cell. The X-ray density reduces with increasing Mn content of Zn 1-x Mn x O nanoparticles. It may be due the increasing volume of unit cell as well as decreasing molecular weight of the samples. The molecular weight of Mn (58.930 amu) ions is smaller than that of Zn (65.38 amu) ions. The values of X-ray density of Zn 1-x Mn x O nanoparticles are listed in table 1. The volume of unit cell increases and X-ray density decreases with increasing Mn content respectively. It means that Mn ions go to Zn site in ZnO structure.

Atomic Packing Fraction (APF)
Atomic packing fraction (APF) was calculated using formula Where a & c are the lattice parameters. The value of APF is listed in table 1. It is found that the APF increases with increasing Mn content, it may be due the decrement of voids in the samples. The APF of bulk hexagonal ZnO materials is about 74% but in our case the APF of Zn 1-x Mn x O nanoparticles is nearly 75% in hexagonal structure. It means that APF in nanocrystals are slightly larger than that of bulk materials. It may be due to the size effect in the nanocrystalline samples. The APF increases with increasing Mn content of Zn 1-x Mn x O nanoparticles, it indicates homogenous substitution of Mn ions in the Zn site of ZnO structure.

Average Crystallite Size (D)
The average crystallite size was estimated using Scherrer's formula.
Where D is the average crystallite size in nanometers, K is a constant or shape factor and equal to 0.9, λ is the wavelength of the radiation (1.54056 A 0 with CuK α radiation), β hkl is the peak width at half maximum intensity and θ is the peak position. The average crystalline size was in the range of 33-43 nm. The average crystallite size decreases with increasing Mn content of Zn 1-x Mn x O nanoparticles. It may be due to the small grain growth of Mn doped ZnO nanoparticles as comprised with pure ZnO nanoparticles. The values of average crystallite size are listed in table 1.

Surface Area to Volume Ratio
It is very important to know the surface area to volume ratio of the nanomaterials were general and in particular the selected system. Therefore, we have initiated the program to determine the surface area to volume ratio of Mn doped ZnO nanoparticles using the following equation Where N surface and N volume are the number of Zn-O pair at surface and in the volume respectively. S is the surface area, V is the volume, R 0 is the ZnO distance (0.18 nm) and R-is the average particle radius. The surface area to volume ratio versus Mn concentration plot is shown in figure 3.
The values of surface area to volume ratio are listed in table 1. Table 1 show that the surface area to volume ratio increases with increasing Mn doping ZnO samples. It may be due to the decreasing in average crystallite size of the Zn 1-x Mn x O nanoparticles. The reduction in average crystallite size with increasing Mn content which increases the surface area to volume ratio. The values of surface to volume ratio is small, it means that the particles are completely spherical in shape. Similar results are reported in literature [17]. The surface area to volume ratio is large due to the small particle and the surface effect described for (Zn, Mn)-O pairs at the surface changing the Zn 1-x Mn x O nanoparticles of average crystallite size. The surface area to volume ratio versus average crystallite size is shown in figure 4.

Surface Morphology
The particle size is determined of pure and Mn substituted ZnO nanoparticles using TEM. From XRD, the average crystallite size was calculated using Scherrer's formula. However, from TEM the particle size is directly measuring and also morphology of the particles. Figures 5 (a & c) show the TEM images of pure and 10% Mn substituted ZnO nanoparticles. The TEM images of pure and Mn substituted ZnO nanoparticles show the samples are in spherical shape with smooth surface. The particle size of pure and Mn substituted ZnO nanoparticles are estimated to about 20-60 nm in diameter. The

Optical Studies
The effect of Mn on the energy band gap of pure ZnO and substituted of Mn 2+ ions in tetrahedral sites of wurtzite structure of ZnO was confirmed using UV-Visible optical spectroscopy measured in the range of 200 -1000 nm. The UV-Vis absorption spectra of the ZnO and Mn doped ZnO nanoparticles prepared at 450 0 C are shown in figure 6. The red shift in the absorption edge was observed in the prepared samples. It may be due to the surface effect and average crystallite size or morphology. It is observed that the enhancement in red shift band edge with increasing Mn concentration. It indicates for the incorporation of Mn ions into the Zn site of the ZnO lattice. The absorption coefficient α is calculated from the absorption spectra using the equation.
Where d is the thickness of samples cell (1 cm) and A is the absorbance. The relation between the incident photon energy (hν) and the absorption coefficients (α) is given below  (αhυ) 2 versus photon energy of Zn0.95Mn0.05O nanoparticles.   Sanyal et al [18] reported that the decrement in energy band gap due to 2% doping of Mn in the ZnO. It may be due to the formation of large number of defects inside the samples. However, in our case the energy band gap decrease with increasing Mn content upto 20% in the ZnO nanoparticles. It may be due to Mn clustering, reduction in average crystallite size and enhancement of surface area to 122 Effect of Mn Doping Concentration on Structural, Morphological and Optical Studies of ZnO Nano-particles volume ratio. Wang et al [19] also reported that decreasing energy band gap with increasing Mn content upto 5% in the ZnO. It may be due to sp-d exchange interaction between transition metal ions and Zn anions.

Conclusion
The samples of Mn doped ZnO nanoparticles were synthesized successfully by sol-gel method. The crystal structure, morphology and optical properties of the Zn 1-x Mn x O nanoparticles were investigated at length. The pure and Mn doped ZnO nanoparticles have wurtzite (hexagonal) crystal structure. The lattice parameters a and c increase with increasing Mn content of Zn 1-x Mn x O nanoparticles. The volume of unit cell increases with increasing Mn content which indicates that Mn 2+ ions go to Zn 2+ ions in ZnO lattice. The X-ray density decreases with increasing Mn content. Reduction in average crystallite size with increase the Mn content and surface area to volume ratio increases with increasing Mn content i.e. size effect was observed in the samples. The APF of Zn 1-x Mn x O nanoparticles increases with Mn content. It may be due to the reduction of voids in the prepared samples. TEM results show that pure and 10% Mn doped ZnO nanoparticles are spherical in shape and the average particle size are in the range of 20 -60 nm. The optical energy band gap decreases with increasing Mn content of Zn 1-x Mn x O nanoparticles. It may be owing to Mn clustering, reduction in average crystallite size and enhancement of surface area to volume ratio.