Effect of Co2+ ions on structural, morphological and optical properties of ZnO nanoparticles synthesized by sol–gel auto combustion method

Effect of Co2+ ions on structural, morphological and optical properties of ZnO nanoparticles synthesized by sol–gel auto combustion method

Materials Science in Semiconductor Processing 41 (2016) 441–449 Contents lists available at ScienceDirect Materials Science in Semiconductor Process...

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Materials Science in Semiconductor Processing 41 (2016) 441–449

Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Effect of Co2 þ ions on structural, morphological and optical properties of ZnO nanoparticles synthesized by sol–gel auto combustion method Shankar D. Birajdar a, V.R. Bhagwat b, A.B. Shinde c, K.M. Jadhav a,n a

Department of Physics, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Maharashtra, India Institute of Shipbuilding Technology, Bogda-Vasco, Goa, India c Department of Physics, Abasaheb Garware College, Pune, Maharashtra, India b

art ic l e i nf o

a b s t r a c t

Article history: Received 2 August 2015 Received in revised form 24 September 2015 Accepted 2 October 2015

Undoped and Co2 þ doped ZnO nanoparticles have been successfully synthesized by sol–gel auto combustion method. The ratio of metal nitrates to citric acid was taken at 1:1.11. The synthesized nanoparticles were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), Fourier transform infrared and Uv–visible spectroscopy techniques. The X-ray powder diffraction analysis revealed the formation of single phase having hexagonal wurtzite structure. The lattice constant ‘a’ increases while ‘c’ decreases as Co2 þ concentration ‘x’ increases. The average crystallite size obtained from XRD data is in the range of 19–15 nm. The X-ray density, atomic packing factor, strain, surface area to volume ratio, etc was obtained using XRD data. SEM analysis showed that the prepared nanoparticles are in nano regime, nearly spherical and loosely agglomerates. EDAX analysis showed that composition obtained is near stoichiometries. In order to understand functional group and vibrational frequency band position of synthesized nanoparticles FTIR technique was used. FTIR analysis results observed that vibrational frequency band position of Zn–O shifted to higher frequency band with Co2 þ ion increasing host semiconductor nanoparticles. Uv–visible absorption spectra showed that absorption edge shifted to higher wavelength with increasing Co2 þ concentration while corresponding energy band gap of semiconductor nanoparticles decreases with increasing Co2 þ concentration. & 2015 Elsevier Ltd. All rights reserved.

Keywords: XRD SEM EDAX UV–Visible ZnO semiconductor Nanoparticles

1. Introduction The study of transition metal ion doped in metal oxide is of great importance from the both applied and fundamental research point of view [1–5]. In order to fabricate spintronic devices and optoelectronic devices at room temperature and above room temperature lot of considerable attention has been given by researchers and scientists on transition metal ions doped in ZnO semiconductor nanoparticles [6–11]. Among the metal oxide, ZnO is called as group II–IV semiconductor because zinc and oxygen belong to IInd and IVth groups of the periodic table, respectively. ZnO is a n-type semiconductor and has a wurtzite structure, which is stable at room temperature. The wide band gap energy (Eg ¼3.37 eV) [12] and high excitonic binding energy (60 meV) [13] at room temperature of ZnO semiconductor nanoparticles makes them useful for technological application. n

Corresponding author. E-mail address: [email protected] (K.M. Jadhav).

http://dx.doi.org/10.1016/j.mssp.2015.10.002 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

ZnO is one of the most applied semiconductor material due to high solubility limit and has potential application in various fields such as solar cell panel display, light emitting diode, biomedical application, antibacterial activity, sensors, super capacitors [14– 19]. The properties of host material ZnO can be controlled or modify by introducing transition metal ions into their lattice [20]. There are two important techniques to control and modify the properties of host material such as size of nanoparticles, energy band gap and surface area to volume ratio. A first technique is doping and another technique is a synthesis method of nanomaterial. Doping is an effective method to enhance and control the structural, optical, electrical and magnetic properties of host material [21]. Synthesis method also plays an important role in the preparation of nanoparticles [22]. Different dopant elements such as transition metals Mn, Fe, Ni, and rare earth elements Eu, Er and Tb have successfully been incorporated into the semiconductor nanocrystals ZnO [23–28]. The transition metal ions and rare earth elements have successfully doped in ZnO nanoparticles were extensively studied as they are useful in spintronic device and optoelectronic device

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application [3,29]. Recently some reports suggested that the Co2 þ doped ZnO nanoparticles play an excellent role in spintronic device application since saturation magnetization and coercivity increase with Co2 þ doped ZnO compared to undoped ZnO [30,31]. Recently, Knut et al. have reported that the magnetic and electronic characterization of highly co-doped ZnO nanoparticles at room temperature by varying sintering temperature under solution based alkoxide chemistry. It was found that high solubility limit of codoped ZnO nanoparticles 35% [32].The structural, micro structural and optical properties of Co doped ZnO nano-crystals synthesized by solution combustion method have been reported in the literature [33,34]. The properties of ZnO doped with transition metal ions like cobalt ions (Co2 þ ) has been highly investigated because of the high solubility limit in ZnO matrix and variable oxidation state, a ferromagnetic state in elemental form at room temperature. Co2 þ doped ZnO nanostructures synthesis from physical routes have been widely studied as in thin films, nanorods and nanowires, nanoflower etc structures [35,36]. Even though reports are available on structural and optical properties of Co2 þ doped ZnO nanoparticles, still it is interesting to investigate the structural, morphological and optical properties both undoped and doped ZnO nanoparticles as they are not well addressed literature. The control of chemical composition, purity, morphology, and particle size is very important to obtain suitable metal-ions doped ZnO powders for their desired applications. A number of synthesis methods have been devoted to the fabrication of transition metal doped ZnO nanoparticles and thin film, such as auto combustion method [37], ball milling method [38], co-precipitation method [39], hydrothermal process [40], spray pyrolysis [41]. Among these synthesis methods, we have adopted sol–gel auto combustion synthesis method for preparation of Co2 þ doped ZnO nanocrystalline material. In the literature, the synthesis of ZnO/TiO2 nanopowders obtained by sol–gel auto combustion method is reported [42,43]. The sol–gel auto combustion method is widely used by many researchers and has advantages over the other methods. The advantages of this method are (i) fast heating rate and short reaction time (ii) the reagents are mixed at the molecular level (iii) high product purity and crystalline (iv) fine particle size and narrow particle size distribution (v) it is easy to control stoichiometry (vi) dopants can be easily introduced into the final product (vii) simple equipment and preparation process (viii) low processing time (ix) low external energy consumption (process initiates at low temperatures) and multiple steps are not involved [44]. Solution combustion synthesis is an effective technique for the preparation of nanoscale material. The sol gel auto combustion method is unique to obtain high porosity and high surface area to volume ratio fine particles. The method is best on the principle that once the reaction is started with low temperature an exothermic reaction occurs that becomes self-sustaining for a certain time interval. The purpose of the present work is to investigate the structural, morphological and optical properties of Co2 þ doped ZnO nanoparticles with a view to optimize the band gap for optoelectronic devices. The band gap can be tuned according to the desired application by controlling the crystallite size, doping concentration, method of preparation, etc. In this study, we have used sol–gel auto combustion method to achieve the lower nanocrystalline size thereby optimizing the band gap.

2. Experimental methods 2.1. Synthesis All of the chemical reagents used in our experiments were of analytic grade and utilized as received without further purification.

Zinc nitrate hexahydrate [Zn (NO3)2.6H2O], Cobalt nitrate hexahydrate [Co (NO3)2.6H2O] and Citric acid monohydrate [C6H8O7.H2O] were used. Double distilled water was used as a solvent. Nanocrystalline Zn1 xCoxO (x¼0.00, 0.06 and 0.12) powder samples were synthesized by a sol–gel auto combustion method using citric acid as a fuel. The fuel ratio was taken according to stoichiometries proportion of metal nitrate to oxidizer ratio (1:1.11). In a typical synthesis of Zn1 xCoxO samples, the appropriate proportion of Zn(NO3)2.6H2O, Co(NO3)2.6H2O and C2H8O7.H2O were completely dissolved in a minimum amount of double distilled water to get the aqueous solution. The aqueous solution was then stirred for about 1 h in order to mix the solution uniformly. The mixed precursor solution was evaporated on a hot plate at 120 °C under constant stirring and was concentrated by heating until the excess free water gets evaporated with the formation of a gel. The ‘gel’ was subsequently swelling into foam like and underwent strong self-propagating combustion reaction to give a fine powder. The resulting powders of undoped pure ZnO were white in color. Cobalt doped ZnO powders were moss green and became darken with increasing Co2 þ concentration. All the samples were ground for a half an hour and were prepared as best nanopowders. The nano powders were subjected to sintering at 600 °C for 4 h and used for further characterization studies. The steps involving in synthesis of Zn1 xCoxO nanoparticles have been shown in Fig. 1(a). The scheme of sol gel auto combustion synthesis method of pure and Co2 þ doped ZnO nanoparticles are shown in Fig. 1(b). 2.2. Characterizations The crystalline nature and the phase purity of Zn1  xCoxO (x ¼0.00, 0.06, 0.12) nanoparticles were examined through X-ray powder diffraction analysis (XRD) technique using (Model: Xpert PRO MPD) with Cu-Kα radiations (λ ¼1.5405 Å) operated at a voltage of 45 kV and current of 40 mA. The average crystallite size of all the samples was estimated using the Debye–Scherrer's equation, and the lattice parameters were obtained using the XRD data. The surface morphology of the prepared nanoparticles was studied by scanning electron microscopy (ZEISS Ultra FE-SEM) technique. The stoichiometries proportion of the constituent ions was examined through the energy dispersive X-ray analysis (EDAX) technique. UV–visible spectra of Co2 þ doped ZnO nanopowders were performed in the range 250–800 nm using a Lambda 25-Perkin Elmer UV–Vis Spectrophotometer. The Fourier transform infrared spectroscopy (FT-IR) spectra of all the samples were recorded in the range of 400–4500 cm  1. An FT-IR spectrum is considered to be the fingerprint of a given specimen with absorption peaks corresponding to the frequencies of interatomic vibrations in the molecular system. The FT-IR spectra are generally used to investigate the chemical and structural changes that take place during the combustion process and to reveal the mechanism of self-propagating combustion.

3. Results and discussion 3.1. X-ray diffraction The X-ray diffraction patterns of Zn1  xCoxO (x¼ 0.00, 0.06 and 0.12) samples sintered at 600 °C in the air are shown in Fig. 2(a), which clearly reveals that all the observed diffraction peaks in the XRD pattern of Zn1  xCoxO can be indexed to the ZnO wurtzite structure without forming secondary phase. From Fig. 2(b) a careful analysis of the XRD peaks indicates that there is a significant shifting and broadening in (101) peaks position toward higher 2θ value relative to that of pure ZnO with increasing of Co2 þ content, which indicates that the lattice parameters of ZnO

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Fig. 1. (a) Various stages involved in the synthesis of Zn1  xCoxO (x ¼0.00, 0.06, 0.12 mol) nanoparticles. (b) Scheme of sol gel auto combustion synthesis method of Zn1  xCox O (x¼ 0.00, 0.06 and 0.12 mol) nanoparticles.

changes very slightly with Co2 þ doping, because the ionic radii of Co2 þ (0.058 nm) is closed to the ionic radii of Zn2 þ (0.060 nm). The variation of peak intensity as a function of Co2 þ concentration is shown in Fig. 2(c). From Fig. 2(c) it is found that the diffraction peak intensities decreases with increase of Co2 þ content in ZnO matrix, which indicates that the dopant Co2 þ ions are substituted in the inner lattice of Zn2 þ ions. Fig. 3(a) shows that variation of full width half maxima (FWHM) and the average crystallite size with Co2 þ content and co-relation between them. Average crystallite size of nanoparticles was calculated from the Debye– Scherrer's formula [45].

D=

0.90λ β cos θ

(1)

where, λ is the X-ray wavelength, β is the full width half maximum of the most intense peak and θ is the Bragg's angle position. The increase of the full width at half maximum of the diffraction peak reveals that the crystallite size decreases with increasing Co2 þ content. The average crystallite size of pure ZnO nanoparticles was found to be as 19 nm, and it is found to be decreased with increase cobalt concentration and represented in Table 1. Similar results have been reported in the literature [46]. The reduction in the average crystallite size is mainly due to the distortion in the host ZnO lattice by foreign impurity introducing that decrease the nucleation and growth rate of ZnO nanoparticles [47]. The substitution of Co2 þ in an interstitial position would affect the concentration of the interstitial Zn, oxygen and Zn vacancies. The observation of small changes of 2θ values in diffraction peaks and peak broadening is due to the increase of micro strain in nanoparticles and decrease size of the nanoparticles. The structural change obtained from the diffraction peaks illustrates the incorporation of Co2 þ ions into the ZnO lattice, which indicates that the crystal structure has no obvious change by the Co2 þ doping

because after doping of Co2 þ in ZnO show only single phase. The micro strain can be calculated using the formula.

ε=

β cos θ 4

(2)

The lattice parameters for hexagonal wurtzite structured pure and Co2 þ doped ZnO nanoparticles are calculated from the following equation.

⎡ λ2 ⎛ 4 (h2 + hk + k 2) 1 l2 ⎞ ⎤ = ⎢ ⎜ + 2 ⎟⎥ 2 2 d a c ⎠ ⎥⎦ ⎣⎢ 4 ⎝ 3

(3)

where, a and c are the lattice constants k, and l are the Miller indices and dhkl is the interplanar spacing. The interplanar spacing can be calculated from Bragg’s law. From the observed d-spacing values; lattice parameters ‘a’ and ‘c’ were calculated and are mentioned in the Table 2. The variation of lattice parameter ‘a’ and ‘c’ with Co2 þ content have been shown in Fig. 3(b). Interestingly, the lattice parameter ‘a’ an increase while ‘c’ decreases as the volume of unit cell increases with increase Co2 þ ion concentration. It may be due to small difference of ionic radii between Co2 þ high-spin in tetrahedral coordination and Zn2 þ in tetrahedral coordination [48,49]. It is expected that the substitution of the Co2 þ in place of Zn2 þ should in fact lead to decrease in the lattice parameters due to smaller ionic radius of divalent Co2 þ in tetrahedral coordination. But although decreasing nature of ‘c’ parameter can be explained with the view of small differences of ionic radii in tetrahedral site while linear increase of ‘a’ lattice parameter can not be explained. The decreasing nature of lattice parameter ‘c’ it is also not due to the entry of Co2 þ into the octahedral coordination because some researcher reported that from FTIR studies Co2 þ did not enter into octahedral coordination [50]. The small increase of lattice parameter ‘a’ and decrease of lattice parameter ‘c’ while increasing of the unit cell volume with an

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V=

3 2 ac 2

(4)

It was found that the volume of the unit cell increases with increasing of Co2 þ doping level, which is displayed in Table 2. The X-ray density was determined using the equation.

ρx =

nM NA V

(5)

where, n is number of atoms per unit cell, M is the molecular weight, NA is the Avogadro's number and V is the volume of the unit cell. The X-ray density decreases with increasing Co2 þ concentration of Zn1  xCoxO nanoparticles. It may be due to the increasing volume of unit cell as well as decreasing molecular weight of the samples. The values of X-ray density are tabulated in Table 2. The X-ray density decreases and volume of unit cell increases with increasing Co2 þ concentration respectively. It means that Co2 þ ions go to the Zn2 þ site in ZnO structure [58]. The values of atomic packing factor are listed Table 2. It is found that the atomic packing factor (APF) increases with the increasing Co2 þ content, it may be due to the decreases of voids in the samples. The atomic packing factor of bulk ZnO nanomaterials is about 74%. But in our result the atomic packing factor is 75% in hexagonal wurtzite structure. It means that APF in nanocrystals are slightly larger than that of bulk nanomaterials. It may be due to the size effect in the nanocrystalline samples. The atomic packing factor increases with increasing Co2 þ concentration; it suggests that homogeneous substitution of Co2 þ ions in the Zn site of ZnO hexagonal wurtzite structure. The Zn–O bond lengths which are obtained from lattice parameters ‘a’ and positional parameters ‘u’ in the wurtzite structure have been tabulated in Table 2. It has been observed that with the increase in Co2 þ concentration, then ‘u’ parameter and bond length (l) increases this may be due to the effect of replacement of Co2 þ ions in ZnO. It has been also observed, there is a strong correlation between the c/a ratio and positional parameter ‘u’ which is shown in Fig. 3(c). The c/a ratio decreases with increasing ‘u’ in such a way that those four tetrahedral distances remain nearly constant through a distortion of the tetrahedral angles due to the long-range polar interaction. In our case c/a ratio decreases in the Co2 þ ions doped ZnO nanoparticles relative to the pure ZnO nanoparticles. The values of c/a and u are given in Table 2. 3.2. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) analysis

Fig. 2. (a) Powder X-ray diffraction pattern of Zn1  xCoxO (x ¼ 0.00, 0.06, 0.12 mol) nanoparticles at room temperature. (b) Magnified and high resolution XRD pattern of Zn1  xCoxO (x¼ 0.00, 0.06, 0.12 mol) nanoparticles between 35.0° and 37° along (101) plane. (c) The variation of peakintensity along (101) plane as a function of Co concentrations.

increase of Co2 þ concentration can be attributed to the small distortion of Zn2 þ tetrahedral site. Similar type results have been observed in other literature [51–55]. Some researcher also reported that, If Co2 þ ions were in an octahedral site in wurtzite structure, it would be increased in the unit cell parameters because octahedral Co2 þ has an ionic radius between 0.065 nm (low spin) and 0.074 nm (high spin) [48,49]. Similar result also has been observed also other reports [56,57]. Volume of the unit cell was calculated using the equation.

The surface morphology and chemical composition of the Co2 þ doped ZnO nanoparticles were analyzed by using SEM images and EDAX spectra. The SEM images of ZnO and Co2 þ doped ZnO nanoparticles are shown in Fig. 4(a–c), where it is observed that grains are nearly spherical. The small particles are loosely agglomerated and bound to the spherical shape due to the doping of Co2 þ concentration. The average grain size obtained through linear intercept method found to decrease with increase in Co2 þ concentration. The specific surface area determined from grain size. Found to increase with increase in Co2 þ concentration, which is attributed to decrease of grain size. The grain size, specific surface area and specific surface area to volume ratio are tabulated in Table 3, which shows that specific surface area to volume ratio increases with increasing Co2 þ concentration in ZnO nanoparticles. It may be due to the decreasing mean grain size of the nanoparticles. The obtained value of specific surface area to volume ratio is small; it means that the nanoparticles are nearly spherical in shape. The surface area to volume ratio and average grain size verses Co 2 þ concentration has been shown in Fig. 3(d).

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Fig. 3. (a) The variation of full width half maxima (FWHM) and average crystallite size of Zn1  xCoxO (x¼ 0.00, 0.06, 0.12 mol) nanoparticles. (b) Lattice parameter ‘a’ and ‘c’ with increase Co Concentration. (c) Relation between c/a ratio and positional parameter ‘u’ with Co concentration. (d) Surface area to volume ratio and average grain size from SEM with Co concentration. Table 1 Peak position (2θ), full width half maxima (FWHM), interplanar spacing (d) value, average crystallite size (D), micro-strain (ε), absorption edge (λmax) and energy band gap (Eg) of Zn1  xCoxO (x ¼0.00, 0.06, 0.12 mol) nanoparticles. 2θ (degree) FWHM β d-VaD (nm) ε (  10  3) λmax Conc. Eg (eV) (degree) lue (Å) ‘x’ (nm) (mole) 0.00 0.06 0.12

36.40 36.41 36.44

0.3284 0.3470 0.4132

2.4660 2.4680 2.4631

19.0 18.2 15.4

1.18 1.19 1.46

377 384 390

3.344 3.253 3.168

To check the chemical compositions of the synthesized ZnO nanoparticles with different Co concentrations was measured by EDAX spectra and are shown in Fig. 4. (d–f). In pure ZnO nanoparticles only Zn and O ions are detected, as shown in Fig. 4(d). From Fig. 4(e and f) EDAX spectra indicated that the presence of Zn, O and Co as well as with small amount of Carbon (Ca) peaks. The small impurity peak of carbon appears in EDAX spectra, it may be due to the carbon tape pasted on copper grids while measuring of EDAX spectra on the instrument. From EDAX result suggest that

the Co2 þ ions are substituting the Zn2 þ ions in the ZnO lattice. The relative atomic compositions also for each sample are summarized in Table 3. 3.3. UV–Vis absorption spectra and optical band gap UV–Vis absorption spectroscopy is one of the important and very useful techniques to explore the optical properties of semiconductor nanoparticles. The absorbance of nanoparticles is expected to depend on several factors such as size of nanoparticles, energy band gap and oxygen deficiency and structure of nanoparticles, surface roughness and impurity centers. The UV–visible absorption spectra, as a function of wavelength have been shown in Fig. 5(a), which attributes that strong UV absorption is characteristic of all measured samples, which attains a plateau above 377 nm, which differs from other reported results [59], and the absorption edges are found to shift towards higher wavelengths with increase Co2 þ content. The optical band gap of the nanopowders was determined by using the Tauc relation [60] as given below.

Table 2 Lattice parameters (a and c), c/a ratio, volume of the unit cell (V), X-ray density (dx), atomic packing fraction (APF), positional parameter (u) and bond-length of Zn1  xCoxO (x¼ 0.00, 0.06, 0.12 mol) nanoparticles. Conc. ‘x’ (mole)

0.00 0.06 0.12

Lattice parameters a (Å)

c (Å)

c/a

3.2435 3.2452 3.2471

5.2084 5.2073 5.2062

1.6058 1.6046 1.6033

V (Å)3

dx (gm/cm3)

APF (%)

u

Zn–O (l) (Å)

47.45 47.49 47.53

5.6969 5.6647 5.6325

75.302 75.358 75.418

0.3793 0.3795 0.3797

1.9750 1.9760 1.9766

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Fig. 4. (a–c) and (d–f).SEM image and corresponding EDAX spectra of n1  xCoxO (x¼ 0.00, 0.06, 0.12 mol) nanoparticles.

Table 3 Elemental percentage from EDAX graph and average grain size, specific surface area and surface area to volume ratio of Zn1  xCoxO (x¼ 0.00, 0.06, 0.12 mol) nanoparticles. Conc. ‘x’ (mole)

Elements 2þ

Zn (at%)

Co (at%)

O (at%)

0.00 0.06 0.12

50 39.91 30.90

– 9.77 18.65

50 50 50



αhv = B (hv − Eg )n

2

S/V ratio

Ca (at%)

Specific Average grain Size surface using SEM area (cm 2 images /gm) (nm)

– 0.32 0.45

49.40 39.34 33.61

0.4493 0.5668 0.6652

21.32 26.92 31.62

(6)

where,α is the absorption coefficient (α ¼ 2.303), B is a constant, h

is the planks constant, ν is the photon energy and Eg is the optical energy band gap and the value of n¼ 1/2, 1, 3/2 and 2 is depending on the nature of the transition responsible for absorption and n¼ 1/2 for direct band gap semiconductor. The optical energy band gap (Eg) is estimated from the intercept of the linear region in the curve of a plot of (αhν)2 on the Y-axis verses photon energy (hν) on the X-axis as shown in Fig. 5(b).The absorption edge and corresponding energy band gap are tabulated in Table 1 for all the samples. The variation of crystallite size and the optical energy band gap with the changes in doping concentration of Co2 þ ions have been shown in the Fig. 5(c). We observed in our result the energy band gap of pure and Co substituted ZnO nanoparticles decreases with increasing Co2 þ concentration. It may be due to spd spin exchange interaction between the s-p band electrons and localized d electrons of transition metal ions, it has been theoretically explained using the second-order perturbation theory [61– 63].The s-d and p-d exchange interaction gives rise to a negative

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Fig. 5. (a) UV–visible absorption spectra of Zn1  xCoxO (x¼ 0.0, 0.06, 0.12 mol) nanoparticles. (b) The plot of (αhν) 2 verses photon energy (hν) of Zn1  xCoxO (x ¼0.0, 0.06, 0.12 mol) nanoparticles (c) relation between energy band gap and crystallite size versus of Zn1  xCoxO (x ¼0.00, 0.06, 0.12 mol) nanoparticles.

and a positive correction to the conduction band and the valance band energies respectively, and lead to narrowing of the band gap [64]. Moreover, the narrowing of band gap is due to many-body effects on the conduction band and valence band [65]. The manybody effects that can shrink the band gap originate from the electron interaction and impurity scattering. It has been attributed to the merging of an impurity band into the conduction band, thereby shrinking the band gap [66]. In our result, it is found that band gap 3.344 eV for undoped ZnO and it starts decreasing in 0.06 and 0.12 of Co2 þ doped ZnO samples and found to be 3.253 and 3.168 eV respectively. The optical energy band gap goes on decreasing with increasing Co2 þ concentration it may be due to change the lattice parameter, reduction in average crystallite size and variation of surface area to volume ratio. Our experimental result suggests that a red shift of the band gap. It indicates that the red shift of the band gap confirm the substitution of Co2 þ ions in ZnO nanoparticles. Similar result has been also reported [67]. 3.4. Fourier transforms infrared (FTIR) studies Fourier transform infrared absorption measurement technique was employed to conform the wurtzite structure formation and also used to obtain information about the chemical bonding in a material. It is used to identify the elemental constituents of a material. The characteristic peaks exhibited by FTIR spectra of pure undoped and Co2 þ doped ZnO nanoparticles are shown in Fig. 6. FTIR spectra for all the samples are assigned at room temperature

Fig. 6. FTIR spectra of Zn1  xCoxO for (x¼ 0.0, 0.06, 0.12 mol) nanoparticles.

and are listed in Table 4. The two absorption peaks are observed between 1659 cm  1 and 1400 cm  1 corresponding to asymmetric and symmetric stretching of the carboxyl group (C ¼O) [68]. The broad absorption peaks show the presence of O–H stretching mode of H2O in the ZnO nanocrystals around 3389 cm  1 for pure ZnO, 3403 cm  1 for 0.06 of Co2 þ and 3409 cm  1 for 0.12 of Co2 þ [69]. The absorption peaks observed between 2355 cm  1 and

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Table 4 FT-IR peaks and their assignments for Zn1  xCoxO (x¼ 0.00, 0.06, 0.12 mol) nanoparticles. Wavenumber (cm  1) Conc. ‘x’ O–H Sym(mole) Zn–O Stretching metric 1 bending (cm ) (cm  1)

Lewis acidity C¼ C (cm  1)

Brownsted acidity C ¼O (cm  1)

O–C–O (cm  1)

O–H bending (cm  1)

0.00 0.06 0.12

1194 1207 1218

1529 1536 1550

2355 2362 2376

3389 3403 3409

454 467 481

625 626 637

2376 cm  1 is because of the existence of O–C–O molecule. The presence of traces of carbon may be due to the use of citric acid as a chelating agent. The amount of combustion energy may be lower than the energy required for the formation of the final product which results in the presence of impurity of carbon atom in the final product. Similar explanation is reported by [70]. The strong absorption band around 1529 cm  1, 1536 cm  1 and 1550 cm  1 are assigned to Brownsted acidity (C ¼O band) for 0.00, 0.06 and 0.12 Co2 þ concentration, respectively. In the FTIR spectra broad band shows the presence of Lewis acidity (C ¼C band) in the molecule at 1194 cm  1, 1207 cm  1, 1218 cm  1 for 0.00, 0.06 and 0.12 Co2 þ concentration respectively. The main absorption band of Zn–O stretching vibration at 454 cm  1, 467 cm  1, 481 cm  1 for 0.00, 0.06 and 0.12 Co2 þ concentration, respectively [71,72]. The change in the peak position of the ZnO absorption band reflects that the Zn–O–Zn network is perturbed by the presence of Co2 þ in its environment. From the FTIR study it is confirmed that Co2 þ ions are substituted into the ZnO system synthesized by auto combustion techniques.

4. Conclusions Zn1  xCoxO, (x ¼0.00, 0.06–0.12) nanoparticles have been successfully synthesized by a sol–gel auto combustion method. XRD result confirms the prepared samples are in the nanoscale regime having hexagonal wurtzite structure. The variation of the lattice parameters, reduction in mean crystalline size and shifting of 2θ value to higher angles, all these variations suggest that Co2 þ ions have completely doped into the ZnO structure. SEM image reveals that the sample consists of nearly spherical shaped grains in nanometer sizes without any sign of phase segregation. The FTIR analysis confirms the presence of Zn–O bond, functional group and chemical bonding. The absorption spectra results show that the electronic structure of ZnO nanoparticles can be tuned when Co2 þ ion is doped into ZnO lattice which may give good optical properties. The energy band gap of the present system for undoped ZnO is 3.344 eV and it starts decreasing in 0.06 and 0.12 of Co2 þ -doped ZnO samples and found to be 3.253 and 3.168 eV with doping of Co2 þ concentration. Finally, our result suggests that with the small amount of metal ions Co2 þ introducing in ZnO matrix can control structural, morphological and optical properties.

Acknowledgment One of the authors is thankful to Tata Institute of Fundamental Research (TIFR), Mumbai for providing XRD, SEM and EDAX characterization data.

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