Influence of Co-doping on the structural, optical and morphological properties of Zn0.96Mn0.04O nanoparticles by sol–gel method

Influence of Co-doping on the structural, optical and morphological properties of Zn0.96Mn0.04O nanoparticles by sol–gel method

Optical Materials 36 (2014) 797–803 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat In...

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Optical Materials 36 (2014) 797–803

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Influence of Co-doping on the structural, optical and morphological properties of Zn0.96Mn0.04O nanoparticles by sol–gel method S. Sivaselvan, S. Muthukumaran ⇑, M. Ashokkumar PG and Research Department of Physics, H.H. The Rajah’s College (Autonomous), Pudukkottai 622 001, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 17 September 2013 Received in revised form 29 November 2013 Accepted 30 November 2013 Available online 17 December 2013 Keywords: Zn0.96xMn0.04CoxO nanoparticles Microstructure Optical property Energy gap

a b s t r a c t Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles were successfully synthesized by sol–gel method. The structural and optical properties of the samples were investigated by X-ray diffraction and UV–visible photo-spectrometer. The X-ray diffraction results and c/a ratio revealed that the hexagonal wurtzite structure of ZnO is not affected by Mn/Co doping. The decreased average crystal size from 21.9 nm (Co = 0%) to 16.8 nm (Co = 6%) was due to the increase of tensile stress. The energy dispersive X-ray spectra showed the excellent oxide formation and the presence of Mn and Co in ZnO. The observed green band around 492 nm for Co = 2% and 482 nm for Co = 4% were originated from oxygen vacancies and intrinsic defects. The high optical transmittance (>70%) in visible region indicated the good optical quality crystal. The optical energy gap might be tuned by changing the Co concentration. The Fourier transform infrared spectroscopy analysis showed the stretching vibrations and the oxide formations. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide is attracting lot of attention of researchers, scientists and technologists owing to its interesting properties like wide band gap of 3.37 eV at room temperature and high exciton binding energy of 60 MeV which makes the exciton state stable even at room temperature [1,2]. Due to its excellent physical and chemical properties, it is widely used in a number of applications like photocatalysis, gas sensors, varistors and low voltage phosphor material [3,4]. It is well known that the addition of impurities into ZnO induce dramatic changes in the optical and structural properties, which is crucial to its practical applications. Doping ZnO with magnetic ions such as Fe, Co, Mn induces magnetic properties due to their possible applications in the field of spintronics [5]. Among these different metallic doping elements Mn and Co are important because (i) they are prominent luminescence activators, which can modify the luminescence of ZnO crystals by creating localized impurity levels and (ii) change the microstructure and optical properties of ZnO system. Mn-doped ZnO will be a multifunctional material with coexisting magnetic, semiconducting, and optical properties [6]. Recent neutron scattering study of Zn1xCoxO (x = 0, 0.05, 0.10, 0.20, 0.30) system by Lee et al. [7] have shown that samples are monophasic only up to x = 0.05 and the presence of impurity phases like Co3O4, CoO and Co metal clusters were reported at higher Co concentrations. The formation ZnMnO3 and ⇑ Corresponding author. Tel.: +91 04322 221558; fax: +91 04322 230490. E-mail address: [email protected] (S. Muthukumaran). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.11.031

lower emissivity at lower Mn concentrations were observed by Zhang et al. in Mn doped Zn0.97Co0.03O nanopowders synthesized by solid-state reaction [8]. Among the different physical and chemical methods [9,10], sol–gel is one of the most important methods to prepare the nanoparticles. Sol–gel method is a simple and easily reproducible method [11]. Highly c-axis oriented and a red-shift more than 500 MeV were observed in a (Mn, Co) co-doped ZnO film by Gu et al. [12]. The switching action between ferromagnetic and paramagnetic states in ZnO:Co, by hydrogenation and heating was studied by Singhal [13]. The presence of mixed magnetic phases (PM + AFM + FM) in (Mn, Co) doped ZnO nanoparticles due to the low saturation magnetization was noticed by Sharma et al. [14]. Many research works have been carried out on Co doped and Mn doped ZnO system separately. Even though limited literatures are available on Mn, Co co-doped ZnO [12–14], most of the works are on magnetic studies and the comprehensive study of the structural and optical properties of Mn, Co co-doped ZnO nanoparticles is still scanty. The aim of the present work is to evaluate the effect of Co concentrations on the microstructure and the optical properties of Zn0.96Mn0.04O nanoparticles extensively. 2. Experimental details 2.1. Preparation of Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles In the present investigation, Zn0.96-xMn0.04CoxO (0 6 x 6 0.08) nanoparticles have been synthesized using sol–gel method by the following procedure: The high purity chemicals (>99% purity) such

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as Zinc acetate dihydrate (Zn(CH3CO2)22H2O), Manganese (II) acetate tetrahydrate (Mn(CH3CO2)24H2O), Cobalt (II) acetate tetrahydrate (Co(CH3CO2)24H2O), N,N dimethyl-formamide (DMF) are used for the sample preparation. Initially, appropriate amounts of Zinc and Manganese acetates were dissolved in N,N dimethyl-formamide (DMF) and kept in magnetic stirrer for ½ h under constant stirring. Again, the appropriate amount of Cobalt acetate was added into the solution and stirred for another ½ h to prepare the homogeneous and clear solution. The homogeneous solution was stirred at 60 °C for 1 h. Then, the resulting sols were evaporated in hot air furnace and dried by micro-oven for 2 h. The dried precursors were collected and ground in an agate mortar. The same procedure was repeated for other samples preparation. Finally, the collected nanopowders were annealed at 500 °C under air atmosphere for 4 h followed by furnace cooling. 2.2. Characterization techniques The crystal structure of Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles was determined by powder X-ray diffraction. XRD patterns were recorded on a RigaKu C/max-2500 diffractometer using Cu Ka radiation (k = 1.5408 Å) operated at 40 kV and 30 mA in the wide angle region from 30° to 70°. The surface morphology of the samples was studied using a scanning electron microscope (SEM, Philip XL 30). The topological features and the composition of Zn, O, Mn and Co were determined by energy dispersive X-ray spectrometer on K and L lines. The UV–Visible optical absorption and transmittance spectra of Mn-doped and Mn, Co co-doped ZnO nanoparticles have been carried out with a view to explore their optical properties. The spectral absorption spectra were recorded using UV visible spectrophotometer (Model: Lambda 35, Make: Perkin Elmer) in the wavelength ranges from 300 to 900 nm using cm1 quartz cuvettes at room temperature. Halogen and deuterium lamp are used as sources for visible and UV radiations, respectively at room temperature. The presence of chemical bonding in Mn doped and Mn, Co co-doped ZnO nanoparticles was studied by FTIR spectrometer (Model: Perkin Elmer, Make: Spectrum RX I) in the range of 400–4000 cm1. The sample used for this measurement is in the form of pellets prepared by mixing the nanopowder with KBr in the concentration range of 1% per weight.

Fig. 1. Powder X-ray diffraction pattern of Zn0.96xMn0.04CoxO, 0 6 x 6 0.08 nanoparticles at room temperature.

hexagonal wurtzite structure of ZnO. The observed single phase attributed the incorporation of Mn ion into the Zn lattice site rather than interstitial ones. All the available reflections of the present XRD phases have been fitted with Gaussian distribution. Fig. 2 shows the variation of peak intensity of Zn0.96xMn0.04CoxO nanoparticles corresponding to (1 0 1) plane for different Co concentrations from 0 to 8% between 35.5° and 37.5°. As the Co concentration increases, the intensity of (1 0 1) peak increases and has maximum at Co = 2% (8488 counts) whereas the sample without Co (Zn0.96Mn0.04O) has 5258 counts. Further increase of Co concentrations decreases the peak intensity continuously as shown in inset of Fig. 2. It is noticed that the peak position (2h) is shifted to higher 2h side (D2h  36.16  36.57  0.41°) when 2% of Co is introduced into the Zn–Mn–O lattice. Further increase of Co shifts the peak position to lower 2h side (D2h  36.57  36.14  0.43°) from Co = 2–8%. The initial substitution of Co = 2% increases the peak intensity and shift the peak position towards the higher 2h side. The above increase of intensity

3. Results and discussion 3.1. X-ray diffraction (XRD) – structural studies The typical powder X-ray diffraction (XRD) patterns of Mn, Co co-doped ZnO nanoparticles with different Co concentrations from 0% to 8% are shown in Fig. 1. The major diffraction peaks with high intensities in the XRD pattern of Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles clearly shows the crystalline nature. The pronounced diffraction peaks of Zn0.96Mn0.04O nanoparticles clearly show the characteristic peaks at the diffraction angles 31.67° (1 0 0), 34.28° (0 0 2), 36.16° (1 0 1), 47.36° (1 0 2), 56.54° (1 1 0), 62.76° (1 0 3), 66.28° (2 0 0), 67.96° (1 1 2) and 69.24° (2 0 1). The observed sharp diffraction peaks indicate the crystal structure of Zn0.97Mn0.03O nanoparticles is high purity hexagonal wurtzite structure which is in close agreement with the pure ZnO (JCPDS 36-1451, a = b = 3.249 Å, c = 5.206 Å). The broadened diffraction peaks confirm the nano-sized particles formation. All peaks are matched with the hexagonal ZnO structure having space group P63mc with preferred orientation along (1 0 1) plane in all the samples. It is noticed from Fig. 1 that no additional peaks associated with Mn and Co metal, other oxides or any zinc, manganese or cobalt phases are observed which ascribed the substitution of Mn cannot disturb the

Fig. 2. The shift of X-ray diffraction peaks of Zn0.96xMn0.04CoxO, 0 6 x 6 0.08 nanoparticles along (1 0 1) plane from 35.5 to 37.5°. Inset shows the variation peak intensity as a function of Co concentrations.

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and shift of peak position at Co = 2% is due to the proper substitution of Co in Zn–Mn–O lattice without Mn/Co related secondary/ defect phases. The change in peak intensity is also due to the micro-strain [15] of the sample. The average crystal size of the nanoparticles is calculated after appropriate background correction from X-ray line broadening of the diffraction peaks of (1 0 1) plane using Debye Scherrer’s formula [10],

Average crystal sizeðDÞ ¼

0:9 k b cos h

(l) and volume of Zn0.96xMn0.04CoxO nanoparticles for different Co concentrations from 0 to 8%. The initial decrease in d-value, cell parameters, bond length (l) and volume (V) of Zn0.94Mn0.04Co0.02O nanoparticles compared with Zn0.96Mn0.04O nanoparticle is due to the substitution of smaller ionic radii Co2+ (0.58 Å) into the position of larger ionic radii Zn2+ (0.60 Å) in Zn–Cu–O lattice [19]. The further increase of Co concentrations after Co = 2%, increase the lattice constant and hence the volume per unit cell also increases. At higher Co concentrations (Co P 0.02), the secondary phase of Co emerged which indicates that the Co ions and Mn ions would not only substitute the Zn place but also exist as interstitial ions or enter into vacancies. The distortion produced around doping impurity is also responsible for the increase in d-value, cell parameters, bond length and volume. However, the noticed remarkable shift in peak position towards the lower 2h side, increase in d-value and cell parameters beyond Co = 2% are due the poor the crystal quality. Bond length and volume of the Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles are calculated from cell parameters by using Eqs. (3) and (4) and tabulated in Table 1. The Zn–O bond length has been calculated using the relationship [20],

ð1Þ

where k is the wavelength of X-ray used (1.5406 Å), b is the angular peak width at half maximum in radian along (1 0 1) plane and h is Bragg’s diffraction angle. Fig. 3 shows the variation of average crystal size and full width at half maximum (FWHM) as a function of Co concentrations from 0% to 8%. The initial substation of Co increases the FWHM up to Co = 6% than gradually decreased whereas as the inverse trend the average crystal size is decreased up to Co = 6% and then increased. The decrease of average crystal size from Co = 0% (21.9 nm) to 6% (16.8 nm) is due to the increase of compressive stress which is 1.583  103 at Co = 0% and increased to 2.068  103 for Co = 6%. The increase of tensile stress is produced by lattice distortion around Co atom inside the Zn–Mn–O lattice. The existing strong Co–O covalent bond [16] at higher Co concentrations (Co = 8%) makes the interaction of Co with O is stronger than Zn with O which reduces the oxygen defects in the lattice and hence promotes the crystallinity of nanoparticles [17]. It is supported by EDX results discussed latter. The micro-strain (e) can be calculated using the formula [18],

Micro-strain ðeÞ ¼

b cos h 4

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 ! u a2 1 t Bond length ðlÞ ¼  u c2 3 2

ð3Þ

2

a where u ¼ 3c 2 þ 0:25 is the potential parameter of the hexagonal structure. The volume of unit cell of hexagonal system has been zcalculated from the equation [21],

Volume ðVÞ ¼ 0:866  a2  c

ð2Þ

Table 1 shows the variation of peak position (2h), full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and c, c/a ratio, bond length, micro-strain (e), stress (r), bond length

ð4Þ

The stress (r) in the ZnO plans can be determined using the following expression [22],

r ¼ 233  109 ððC bulk  CÞ=C bulk Þ

ð5Þ

where C is the lattice constant of ZnO plans calculated from X-ray diffraction data, Cbulk is the strain-free lattice parameter of ZnO (5.2061 Å). The stress is decreased from 0.9962 GPa (Co = 0%) to 0.0958 GPa (Co = 1%) when Co is introduced in Zn–Cu–O. The negative sign of stress indicates that the stress is tensile and the unit cells are under the state of compression. The observed higher stress at Co = 2% (2.0650 GPa) is due to lattice distortion produced around Co atom inside the Zn–Cu–O lattice. After Co = 2%, the tensile stress is starts to relieve and the lattices are relaxed. The observed constant c/a ratio revealing that there is no change in hexagonal wurtzite structure by Co doping. The observed shift in XRD peak position, change in peak intensity, d-value, cell parameters, bond length, volume and stress confirms the substitution of Co into Zn–Mn–O lattice.

Fig. 3. The variation of average crystal size (D) and full width at half maximum (FWHM, b) value of Zn0.96xMn0.04CoxO, 0 6 x 6 0.08 nanoparticles as a function of Co concentrations.

Table 1 The variation of peak position (2h), full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and c, c/a ratio, bond length, micro-strain (e), stress (r), bond length (l) and volume of Zn0.96xMn0.04CoxO nanoparticles for different Co concentrations from 0% to 8%. Samples

Zn0.96Mn0.04O Zn0.94Mn0.04Co0.02O Zn0.92Mn0.04Co0.04O Zn0.90Mn0.04Co0.06O Zn0.88Mn0.04Co0.08O

Peak position, 2h (°)

36.16 36.57 36.42 36.29 36.14

FWHM, b (radians)

0.0066 0.008 0.0082 0.0087 0.0072

d-Value

Cell parameters

0

0

(Å A)

(Å A)

2.4817 2.4551 2.4649 2.4732 2.4834

a=b

c

3.2595 3.2186 3.2329 3.2435 3.2605

5.2199 5.1600 5.1858 5.2031 5.2239

c/a ratio

Micro-strain, e (103)

Stress, r (GPa)

Bond length, l 0

0

(Å A)

(Å A)

1.601 1.603 1.604 1.604 1.602

1.583 1.907 1.955 2.068 1.719

0.6194 2.0650 0.9076 0.1334 0.7966

1.9834 1.9592 1.9682 1.9747 1.9843

48.0263 46.2925 46.9363 47.4022 48.0940

Volume, V

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3.2. Scanning electron microscope – microstructure The surface morphology of Zn0.96-xMn0.04CoxO (x = 0, 0.04 and 0.08) nanoparticles are shown in Fig. 4a–f. Fig. 4a shows the surface morphology of Zn0.96Mn0.04O nanoparticles. It contains almost tightly packed spheroid-like structure with grain size around 15–30 nm which are uniformly distributed through nano-clusters structure. The magnified and high resolution surface image of Zn0.96Mn0.04O nanoparticles is shown in Fig. 4b. Fig. 4c and d gives the surface morphology of Zn0.92Mn0.04Co0.04O nanoparticles with different magnifications. The substitution of Co atom inside the Zn–Mn–O lattice induces the distortion which is responsible for the reduction in size. In addition to the reduced size the grains are agglomerated each other with size around 10–25 nm. Fig. 4e and f shows the surface morphology of Zn0.88Mn0.04Co0.08O nanoparticles with different magnifications. Tensile stress is completely relieved and the lattices are relaxed which depress the lattice depression and create the better crystallization than Zn0.92Mn0.04Co0.04O nanoparticles. The grains are tightly packed with uniform spheroid-like structure and agglomerated with each other having size around 20–30 nm. The presence of agglomerated particles suggested that the growth process starts with the colloidal nanoclusters. The nano-clusters aggregate into larger secondary spherical particles in order to minimize their surface energy. Then these secondary spherical particles further collide and merge with each

other to form multimers. A good correlation is found to exist between XRD and SEM studies. 3.3. Energy dispersive X-ray (EDX) spectra – compositional analysis The typical EDX spectra of Zn0.96xMn0.04CoxO (x = 0, 0.04 and 0.08) nanoparticles are shown in Fig. 5a–c. The quantitative atomic percentage of the compositional elements such as Zn, Mn, Co and O present in Zn0.96Mn0.04O nanoparticles under different Co concentrations are present in Table 2. The EDX analysis confirms the presence of Mn and Co in Zn–O lattice and purity of the system. The atomic percentage of Mn/Zn ratio is derived to be 4.14% for Zn0.96Mn0.04O. The atomic percentage of Zn is increased from 39.89% to 41.85% where as O percentage is decreased 58.46% to 54.56% when Co concentration is increased from 0% to 4%. The higher percentage of O in Zn0.96Mn0.04O sample support the secondary/defect related O phases. The atomic percentage of Co/Zn + Mn ratio is derived to be 3.98% and 7.59% for Zn0.92Mn0.04Co0.04O and Zn0.88Mn0.04Co0.08O nanoparticles, respectively. The proportional increase of Co concentrations and the decrease of Zn concentrations in Zn0.96xMn0.04CoxO (x = 0.04 and 0.08) with increasing Co concentrations confirm the incorporation of Co into Zn–Mn–O lattice in which Zn is substituted by Co. The calculated weight and atomic percentage are nearly equal to their nominal stoichiometry within the experimental error.

Fig. 4. Scanning electron microscope (SEM) images of (a), (b) Zn0.96Mn0.04O, (c), (d) Zn0.92Mn0.04Cu0.04O and (e), (f) Zn0.88Mn0.04Cu0.08O nanoparticles with different magnifications at room temperature.

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the increase of Co concentrations which may be due to the created charge carries by Co introduced into Zn–Mn–O lattice. The magnified absorption spectra from 330 to 350 nm (Fig. 6b) show the clear transition of absorption in UV region. Fig. 6c gives the absorption changes in the visible region from 350 to 900 nm. In visible region, the absorption is increased from Co = 0% and reaches maximum for Co = 4% and then decreased. The typical room temperature transmittance spectra of Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles are shown in Fig. 7. The transmission spectra of the Zn0.96xCu0.04CoxO nanoparticles show just opposite trend of the optical absorption spectra. It is noticed from Fig. 7 that the magnitude of average transmittance is almost >70% in the visible region which reflects all the samples have good optical quality with low scattering or absorption losses. There is no visible band near the green region for Zn0.96Mn0.04O whereas it is induced when Co is introduced into Zn–Mn–O lattice. The absorption corresponding to green region is minimum at Co = 0% which starts to increase for Co = 2% and reached maximum for Co = 4% then again decreased. The observed green band around 492 nm for Co = 2% and 482 nm for Co = 4% are the oxygen vacancies and intrinsic defects [24]. The presence of oxygen vacancies and intrinsic defects such as Zn interstitials in Zn0.92Mn0.04Co0.04O are supported by EDX results given in Table 2. The optical band gap of the films is determined by using the Tauc relation between the optical absorption coefficient ‘a’ and the incident photon energy ‘hm’ [25],

ahm ¼ Aðhm  Eg Þn

Fig. 5. Energy dispersive X-ray (EDX) spectra of (a) Zn0.96Mn0.04O, (b) Zn0.92Mn0.04Cu0.04O and (c) Zn0.88Mn0.04Cu0.08O nanoparticles at room temperature.

3.4. UV–Visible spectra – optical studies The absorbance is expected to depend on several factors, such as band gap, oxygen deficiency surface roughness and impurity centers [23]. The UV–Visible optical absorption spectra of Zn0.96xMn0.04CoxO (0 6 x 6 0.08) nanoparticles have been taken out at room temperature from 310 to 900 nm and shown in Fig. 6a–c. The absorption of Zn0.96xMn0.04CoxO nanoparticles is higher than Zn0.96Mn0.04O nanoparticles. It is divided into two regions, namely (i) UV region and (ii) visible region. It is noticed from Fig. 6a that in UV region, the absorption is increased with

ð3Þ

where A is a constant, hm is the incident photon energy, Eg is optical band gap of the material and the exponent n depends upon the type of transition. The values of n for direct allowed, indirect allowed, direct forbidden are ½, 2, 3/2, respectively. In the present case, n is taken as 1/2. The energy band gap of Zn0.96xMn0.04CoxO nanoparticles are estimated by plotting (ahm)2 versus ht as shown in Fig. 8a. The extrapolation of the straight line to the energy (hm) axis gives the band gap of the material. The variation of energy gap as a function of Co concentrations from 0% to 8% is shown in Fig. 8b. The observed energy gap of Zn0.96Mn0.04O nanoparticles is decreased from 3.675 to 3.66 eV when Co concentration is increased from 0% to 2%. In this region, more charge carriers are created by Co-doping which induces the absorption and reduce the band gap. The initial narrowing of band gap is due to many-body effects on the conduction and valence bands which can shrink band gap originate from electron interaction and impurity scattering. It has been attributed by merging of an impurity band into the conduction band, causes shrinking of band gap. A small increase in energy gap from Co = 2% (3.66 eV) to Co = 4% (3.665 eV) may be due to the creation of defect related phases such as oxygen vacancies and intrinsic defects in addition to the charge carriers. The further increase of Co concentrations decreases the energy gap from Co = 4% (3.665 eV) to Co = 8% (3.645 eV). Burstein-Moss effect [26] is responsible for the present red shift in energy gap after Co = 4%.

Table 2 The quantitative analysis of the compositional elements present in Zn0.96xMn0.04CoxO nanoparticles for different Co = 0%, 4% and 8% using EDX analysis. Samples

Percentage of the elements (%) Atomic (%)

Zn0.97Mn0.04O Zn0.92Mn0.04Co0.04O Zn0.88Mn0.04Co0.08O

Zn

O

Mn

Co

39.89 41.85 41.62

58.46 54.56 53.36

1.65 1.85 1.73

– 1.74 3.29

Mn/Zn + Co ratio (%)

Co/Zn + Mn ratio (%)

4.14 4.24 3.85

– 3.98 7.59

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Fig. 7. Transmittance spectra of Zn0.96xMn0.04CoxO, 0 6 x 6 0.08 nanoparticles as a function of wavelength from 340 to 900 nm.

Fig. 6. UV–Visible absorption spectra of Zn0.96xMn0.04CoxO, 0 6 x 6 0.08 nanoparticles as a function of wavelength from (a) 310 to 350 nm, (b) 330 to 350 nm and (c) 350 to 900 nm.

Fig. 8. (a) The (ahm)2 versus hm curves of Zn0.96xMn0.04CoxO nanoparticles with different Co concentrations from 0% to 8% for the optical energy gap calculation (b) variation of energy gap as a function of Co concentrations from 0% to 8%.

3.5. Fourier transform infra red (FTIR) studies The band positions and number of absorption peaks are depending on crystalline structure, chemical composition and also on morphology [27]. FTIR is used to identify the chemical bonding and the elemental constituents of a material. The typical FTIR spectra of Zn0.96xMn0.04CoxO nanoparticles for Co = 0%, 4 and 8% are shown in Fig. 9. The broad absorption peaks around 2900–3700 cm1 and 1119–1145 cm1 are attributed to normal polymeric O–H stretching vibration of H2O in Mn–Co–Zn–O lattice [28]. A weak absorption around 2936 cm1 is assigned to a residual organic component [29]. A sharp peak around 1581–1630  m1 is attributed to H–O–H bending vibration, which is assigned to a small amount of H2O in the ZnO nanoparticles [30]. The principal absorption peaks observed between 1383 and 1411 cm1 are

corresponding to the asymmetric and symmetric stretching of carboxyl group ([email protected]) [31]. The absorption peaks observed between 2300 and 2400 cm1 are because of the existence of CO2 molecule in air [31]. The visible band over the range of 1000–400 cm1 corresponds to metal–oxygen bonds (M–O–M) [32]. The change in intensity and FWHM corresponding to the frequency around 859–917 cm1 represents a change in density of defect states surrounding to Mn /Co ions in Zn–O lattice. FTIR spectra exhibit strong vibrations around 648–667 cm1 which are assigned to stretching frequency of Co–O bond [33]. The Co–O bond is assigned to the stretching frequency at 648 cm1 for Co = 4% which is shifted to higher frequency as 667 cm1 for Co = 8%. The observed percentage of transmittance is maximum for Co = 4%. The characteristic peak at 555 cm1 is assigned to Zn–O stretching frequency for Zn0.96Mn0.04O i.e. without

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References

Fig. 9. FTIR spectra of Zn0.96xMn0.04CoxO nanoparticles for different Co concentrations from 0% to 8% in the wave number range from 400 to 4000 cm1 at room temperature.

Co concentration which is shifted to 525 cm1 for Co = 4% and 519 cm1 for Co = 8%. The change in the characteristic frequency for Co–O and Zn–O bands reflects that Zn–O–Mn network is perturbed by the presence of Co in its environment. 4. Conclusions Zn0.96xMn0.04CoxO nanoparticles were successfully synthesized for different Co concentrations from 0% to 8% by sol–gel method. The X-ray diffraction pattern and constant c/a ratio confirmed that wurtzite hexagonal structure was not changed by Mn/Co doping. The change in peak position, average crystal size, lattice constant, stress, strain and volume confirmed the substitution of Co in Zn–Mn–O lattice. The energy dispersive X-ray spectra showed the excellent oxide formation, wherein dopant ions are present in the host crystal lattice. The additional absorption peaks around 492 and 482 nm in optical absorption and transmittance spectra proved the presence of more oxygen vacancies and intrinsic defects in Co = 2% and 4% samples. The tuning of energy gap by Co-doping and the higher transmittance % showed the way to the industrial applications especially as transparent electrode. The Fourier transform infrared spectroscopy analysis showed the stretching vibrations and the oxide formations.

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