Structural and optical properties of zinc magnesium oxide

Materials Chemistry and Physics 203 (2018) 133e140

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Structural and optical properties of zinc magnesium oxide nanoparticles synthesized by chemical co-precipitation L. Srinivasa Rao a, *, T. Venkatappa Rao b, Sd. Naheed c, P. Venkateswara Rao d a Center for Nanoscience and Technology, Department of Humanities and Sciences (Physics), VNR Vignana Jyothi Institute of Engineering and Technology, Bachupally, Nizampet (S.O), Hyderabad, Telangana, PIN 500 090, India b Department of Physics, National Institute of Technology Warangal (NITW), Warangal, Telangana, PIN 506004, India c DK Junior College for Girls, Nellore, Andhra Pradesh, PIN 524004, India d Department of Physics, The University of the West Indies, Mona Campus, Kingston 7, Jamaica

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Micro strain and dislocation density of ZnMgO nanoparticles were correlated in terms of crystal parameters and size. The broadening of optical band gap of ZnMgO nanoparticles is explained in the light of Moss-Bustein effect. FTIR spectra have shown two strong absorption peaks at 567 cm1 and 667 cm1 due to stretching vibrations ZneO bond in ZnO. Formation of various point defects in ZnMgO nanocrystals has been discussed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2016 Received in revised form 21 July 2017 Accepted 19 September 2017 Available online 20 September 2017

By chemical co-precipitation method a series of ZnMgO nanoparticles were synthesized with the particular composition (1-x)ZnO-xMgO (where 0 x 2, all are in mol%). XRD spectra recorded at room temperature of these samples have shown the typical peaks of the ZnO hexagonal wurtzite structure, such as (1 0 0), (0 0 2), (1 0 1) etc. With the increasing concentration of MgO, the blue shift is attained in the absorption band of ZnO. Due to the Moss-Bustein effect the optical band gap has been widened by shifting towards higher frequency side as the concentration of MgO added progressively in the ZnO. The IR spectra revealed structural modifications in the stretching vibrations of ZnO units as a function of MgO. © 2017 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles XRD Optical band gap IR spectra

1. Introduction * Corresponding author. E-mail addresses: [email protected], [email protected] (L.S. Rao). https://doi.org/10.1016/j.matchemphys.2017.09.048 0254-0584/© 2017 Elsevier B.V. All rights reserved.

Comparing with other oxide semiconductors, zinc oxide has distinguishable thermal properties of both higher melting point

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and conductivity with a lower coefficient of linear thermal expansion [1e3]; optical properties such as high optical absorption capability in the UV region (below 365.5 nm) [4], high quantum efficiency of luminescence [5]; electrical properties like good electrical conductivity (both electronic and ionic) [6]; magnetic properties (ferromagnetism) at room temperatures when doped with some transition metal ions [7]; and chemically amphoteric [8] etc. Further, ZnO has an exciton binding energy 60 meV with the wide band gap [9]. All these features endorse ZnO based nanocomposites as the most important versatile materials, because of their variety of potential applications such as UV emitters [10], blue LEDs [11], laser diodes [12], piezoelectric devices [13], chemical sensors [14], spintronics [15] etc. To prepare the ZnO nanoparticles, different physical and chemical synthesis methods are available; amongst chemical coprecipitation is the simplest and significant method [16]. In coprecipitation method, a small portion of precipitated ion mixed in a solution with the supplementary ingredient ions, results the precipitation. ZnO nanomaterials allow the modification of chemical and physical properties by the addition of the transition metal oxide dopant in lattices of zinc oxide. For this purpose, we have chosen magnesium oxide as the dopant, which acts as a modifier oxide and enter into ZnO lattices. Thus, we may expect that oxygens of the modifier oxide disrupt the local symmetry of ZnO structure while the divalent magnesium ions (Mg2þ ions) take the interstitial positions in the vicinity of Zn2þ ions in the precipitation. Such structural modifications yield the change in the optical band gap of zinc oxide nanoparticles. As magnesium is an alkaline earth metal, it is devoid of localized d levels, the complications with optical properties arising from the electronic structures much straighten out, and the alloy consequently provides a better scope so as to enhance the band gap of wurtzite ZnO [17]. It is noteworthy that the ZnO doped with a transition metal oxide have recently received much attention by materials scientists owing to significant applications in the band gap engineering [18,19]. In this way, variable concentrations of MgO enforce the structural modifications and hence tuning of energy band gap of ZnO nano-crystalline particles. Thus, the study on structural and optical characterization of the synthesized ZnMgO nanoparticles is quite important for their practical applications. 2. Experimental 2.1. Sample preparation By chemical co-precipitation method [20] a series of Zn(1-x)MgxO (where 0 x 2, all are in mol%) nanoparticles were prepared with the particular composition. The labels of the samples are as follows: ZM0: Zn1.000Mg0.000O ZM4: Zn0.996Mg0.004O ZM8: Zn0.992Mg0.008O ZM12: Zn0.988 Mg0.012O ZM16: Zn0.984Mg0.016O ZM20: Zn0.980Mg0.020O All the chemical ingredients were weighed in stoichiometric proportions as per the above formulation. All steps of synthesis were carried out at room temperature. First source material (Zn(NO3)2.6H2O) is liquefied in a purified distilled water and 30 min continuous stirring is done to achieve complete dissolution. Later, the dopant material (Mg(NO3)2.6H2O) is dissolved and mixed to the source solution. The mixed solution is further stirred for 2 h. Triethanol-amine (TEA) is used as a capping agent. After the

addition of capping agent stirring is continued for 1 more hour. Subsequently, 4 ml ammonia solution is added to get the base nature and the solution is stirred for next two hours so that the precipitation is formed. The precipitated solution is centrifuged at 10000 rpm and cleaned with pure distilled water until the solution is free from the organic capping and other byproducts, which were formed during the reaction process. The collected wet powder sample was calcinated at 500 C for an hour to remove any nitrates present in the powder. Finally, ZnMgO powder samples were obtained. 2.2. Characterization techniques XRD patterns of the prepared powders were recorded using Xray diffraction spectrometer (inel, XRG 3000) of Cu Ka1 calibrated at a wavelength of 1.540598 Å. SEM photographs were recorded on Carl Zeiss AG - ULTRA 55 Scanning Electron Microscope. Optical absorbance spectra in the range 250e500 nm were recorded on Evolution 600 Ultravioletevisible Spectrophotometer at a comfortable ambient temperature (27 C). IR spectra at 4000400 cm1 by making pellets of KBr matrices were recorded on Brukar FT-IR spectrometer with the resolution 1 cm1. 3. Results and discussion 3.1. X-ray diffraction studies Fig. 1 shows the XRD spectra of ZnMgO nanoparticles. The XRD patterns have shown the well defined diffraction peaks of the ZnO such as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4), (2 0 2). These reflection planes indicate that the prepared samples are of hexagonal wurtzite ZnO structure and they are free from the impurity phases, particularly Zn, Mg and MgO. Also, these reflections of different XRD phases have been well suited with a Gaussian distribution. The peaks observed in the XRD patterns indicate that the synthesized samples possess single phased [21,22]. The pertinent data on XRD spectra is presented in Table 1. Notably, the XRD patterns exhibited orientation of (1 0 0), (0 0 2), (1 0 1) planes. As the concentration of MgO increases in the ZnMgO system, peak positions of the planes moved towards the low angle side, which indicates that, the crystallographic positions of Zn2þ ions have been occupied by Mg2þ ions in the hexagonal wurtzite zinc oxide structure. Further, no peaks related to the magnesium oxide are detected in all the samples. Hence the peaks observed in Fig. 1 are related to the wurtzite structure fitting to the C46v space group [23]. In view of Fig. 1, with the addition of MgO, the peak intensity of (1 0 1) plane increases when compared to the other planes. This can be understood as follows: without altering the crystal structure, Mg2þ ions have occupied the Zn2þ positions and the magnesium ions would affect the concentration of the interstitial zinc ions and zinc vacancies as well as oxygen vacancies. A very small shift in the peak position of the diffraction peaks to the lower angles and the broadening of diffraction peaks are noticed with the increase of MgO concentrations. We can observe diffraction shift easily by using a trend line (dotted red color line) to the peak of the plane (1 0 1). Due to incorporation of Mg2þ ions into the zinc oxide lattice, the growth of micro-strain in the lattice causes the small shift in diffraction peaks; whereas the effect of the micro-strain or size of the unit cell or both may produce the line broadening of the peaks. The similar results were reported by the previous studies on ZnO nanocrystalline materials doped with Cu, Eu, Ni etc [16,21,24]. By using the following equation, the lattice constants (‘a’ and ‘c’) were calculated [25]:


135

Fig. 1. X-ray diffraction spectra of Zn(1-x) MgxO nanopaticles.

Table 1 The pertinent data on XRD spectra of ZnMgO nanoparticles. Sample Bragg's angle at (1 0 1) planes (2q )

FWHM Inter planar ( ) spacing (d101) nm

Grain size (D) nm

Lattice parameters a ¼ b (A )

Lattice parameters Ratio c (A ) c/a

micro strain (ε) (x 103)

dislocation density (r) x 1015 lines/m2

ZM0 ZM4 ZM8 ZM12 ZM16 ZM20

0.2781 0.3091 0.3399 0.3499 0.3708 0.3808

52.5513 47.2639 42.9811 41.7527 39.3958 38.3339

3.2764 3.2795 3.2827 3.2922 3.3049 3.3081

5.1874 5.1906 5.1907 5.1997 5.2122 5.2167

1.1517 1.2805 1.4081 1.4495 1.5363 1.5788

3.6210 4.4765 5.4131 5.7362 6.4431 6.8051

1

4 ¼ 2 3 d

36.8487 36.7256 36.7251 36.7253 36.6944 36.4472

h2 þ hk þ k2 l2 þ a2 c2

2.4373 2.4451 2.4452 2.4452 2.4471 2.4632

(1)

where d ¼ interplanar distance and (h k l) ¼ miller indices. From Fig. 1, it was detected that the full width at half maxima (FWHM) of peak (1 0 1) in XRD and the lattice constants (a, b and c) were also varied by the dopant MgO. Thus, ‘a’ and ‘c’, the lattice constants are identified greater than that of pure ZnO values (a ¼ b ¼ 3.253 A , c ¼ 5.213 A calculated from ICSD (086254) using POWD.12þþ) [26]. The dopant Mg2þ ion (with smaller ionic radius than Zn2þ ion sites in their tetrahedral coordinates) attributes the variation in the lattice parameters of ZnMgO crystals. The average grain size, D, of the samples for observed intense peak position of (1 0 1) planes is estimated using the DebyeeScherrer's equation [27].

D¼

0:9 l Bcosq

(2)

where, l ¼ wavelength of X-rays (1.540598 Å), q ¼ Bragg's angle and B ¼ full width at half maxima. With the increasing concentration of MgO, the average grain dimensions of ZnMgO nanopowders decreases from 52.55 nm to 38.33 nm (as shown in Table 1). The decrease in the grain dimension is due to the modifications in the host zinc oxide lattice by incorporation of the dopant ions (magnesium ions), which is used

1.5832 1.5827 1.5812 1.5794 1.5771 1.5770

to decrease the growth rate by declining the nucleation process [28]. The SEM photographs ZM4 and ZM12 samples are shown in Fig. 2, which confirm morphology of ZnMgO nanoparticles. These images show that the prepared particles have the grain size less than 100 nm, which is constituent with the grain size obtained from the XRD spectra. The SEM images exhibit homogeneous substitution of Mg2þ ions in regular zinc ion sites, which reveals that magnesium ions decrease the grain size of the ZnO nanoparticles gradually as MgO content increases in the composition of the crystal system. The micro-strain (ε) in crystals and the dislocation density (r) have been obtained using the relations [29]:

ε¼

r¼

Bcosq 4 n D2hkl

(3)

(4)

where, eq. (4) is known as Williamson and Smallman's formula, Dhkl is grain size and n ¼ 1 minimum dislocation density. In chemical co-precipitation method, the development of micro-strains in the crystals is mainly due to the alteration of ZnO lattices by incorporation of MgO during the chemical reactions at different temperatures. As the concentration of MgO increases, the micro-strain of the ZnMgO nanoparticles also increases (as shown

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nanoparticles are well agreed with the literature of ZnO nanoparticles mixed with the other dopants like Ni, Mo, CdS, CdO etc [22,25,30,33,34]. The value of Eg of zinc oxide lattice is found to be increased with the increase of MgO content. Thus, MgO modifies the absorption band and the optical band gap of the host zinc oxide lattice. The blue shift indicates increase of the optical band gap energy with the increasing concentration of MgO. This can be understood with the help of the MosseBurstein effect [35] as follows: the dopant ions (Mg2þ ions) disrupt the zinc oxide lattice, which leads to the generation of oxygen vacancies and hence growth of electron density in the conduction band of ZnO [36]. Therefore, the Fermilevel is used to shift gradually towards the conduction band; at the same time the absorption edge shifts towards shorter wavelengths (or higher frequencies) side. Thus, the optical band gap is widened because of the blue shift [37]. Lattice parameters affect the band gap of semiconductors too; with a decrease of c/a ratio the increased trend of the band gap values were observed. As there is a variation of electronegativities of zinc ions and magnesium ions, the substitution of Zn2þ (with ionic radius 0.60 Å) by Mg2þ (with ionic radius 0.57 Å) results in the c-axis compression and a consequently band gap is widening as a function of MgO concentration [38].

Fig. 2. SEM photographs of ZnMgO nanoparticles (a) ZM4 sample and (b) ZM12 sample.

in Table 1), since the atoms are switching from the equilibrium position to a non-equilibrium position due to the change in the microstructure, size and shape of the particles. This viewpoint is supported by the observed rise in dislocation density of ZnO particles by means of gradual increase of addition of MgO [24,30]. Therefore, the change (i.e., increase) of strain results the increase of lattice constants and decrease of the size of the particle; so that the broadening and a notable shift towards low angle side in x-ray diffraction peaks have been observed [16,23,31]. 3.2. Optical absorption and band gap studies The optical absorbance spectra of ZnMgO nanoparticles recorded at room temperature is shown in Fig. 3. The absorbance spectrum of host ZnO has exhibited a broad absorption band with a peak position about 380 nm. A gradual blue shift is observed in the absorption band with the increasing concentration of MgO. The absorption band for ZM20 sample is shifted to 366 nm (as shown in Table 2). The reason for this is the size dependence of energy bands of ZnO nanoparticles due to the effect of quantum confinement [32]. As grain dimension of these particles decreases then the band gap becomes broad and ultimately, the blue shift in optical absorbance takes place as observed. By using the Tauc's plot (Fig. 4) the optical band gap of the samples is calculated [33]:

ahn ¼ B (hn Eg)n

(5)

where, Eg ¼ optical band gap energy, hn ¼ incident photon energy; B is a constant and n values depend up on the mode of inter band transition. The values of n are as follows: 1/2 for direct allowed, 3/2 for direct forbidden, 2 for indirect allowed and 3 for indirect forbidden transitions. In Fig. 4, the extrapolation of the linear portion of the absorption edge intercepts with energy axis, gives optical band gap, Eg for the direct allowed transition [25]. The absorption band and the optical band gap values obtained for ZnMgO

3.3. FT-IR spectra The FT-IR spectra of the ZnMgO powders have been shown in Fig. 5, which were noted in between 4000 cm1 and 400 cm1 wavenumber. The spectra exhibited a strong absorption peak at around 3450 cm1 for corresponding to the stretching vibration of eOH bond of H2O in MgeZneO lattice, because of the moisture in the solution and the atmosphere [16]. Another, notable band at 1694 cm1 is also noted at to the first overtone of a vital stretching mode of eOH. These vibrations could be assigned due to the existence of bound H2O on the surface of the ZnMgO nanocrystals [39]. The absorption peaks are noticed around 2290 cm1and 2330 cm1 due to the existence of CO2 molecule in air [16,40]. Because of the asymmetrical and symmetrical stretching of the zinc carboxylate, two peaks are noted between 1513 cm1 and 1378 cm1 respectively [40, 41]. Further two peaks are observed at 2810 cm1 and 3000 cm1 owed to asymmetric and symmetric stretching vibration of alkane (C-H) groups respectively [40,42]. The size of the ZnMgO nanoparticles depends on the content of the carboxylate (COO-) and hydroxyl (-OH) groups in the samples. More clearly, particle size decreases with increasing the content of COO- and-OH groups in the crystal samples. The COO- and -OH groups may be assigned due to reactive carbon containing plasma species during synthesis and the hygroscopic nature of ZnO respectively. As a result, FTIR determined the impurities mostly near at ZnO surfaces [43]. For the host material ZnO, the absorption peaks are attributed due to the Zn-O stretching modes in between 400 and 700 cm1. The strong absorption peaks at 567 cm1 and 667 cm1 are attributed due to stretching vibrations ZneO bond in ZnO [44,45]. The band at 567 cm1 is mainly simulated by oxygen defects and/or impurities in ZnO [6]. The predicted structure of ZnMgO nanocrystalline solids with various defects is shown in Fig. 6. In the process of synthesis, two important types of point defects viz., Schottky defects and Frenkel defects are possible to be present in ZnMgO nano composite, which can be explained by using KrozerVink notation [46]. Interstitial sites occupied by zinc atoms (Znxi ), together with an equal concentration of vacancies (VxZn ) come from the Frenkel reaction as follows:


137

Fig. 3. Absorbance spectra of ZnMgO nanoparticles.

Table 2 The pertinent data on absorption band and optical band gap of ZnMgO nanoparticles. Sample

Band position (nm)

Energy band gap (Eg) eV

ZM0 ZM4 ZM8 ZM12 ZM16 ZM20

380 378 375 373 370 366

3.37 3.41 3.46 3.51 3.56 3.61

Znxi 4Zni þ e0

(7)

0 Zni 4Zn i þe

(8)

Thus, using equations (7) and (8), one by one in eq. (6), we can obtain 00

ZnxZn þ Vxi 4Zn i þ VZn

ZnxZn þ Vxi 4Znxi þ VxZn

(6)

Ionization reactions for Zn ionic states is shown below:

(9)

In view of (9), thus, a vacancy in ZnO crystal is created by moving the Zn2þ ion to an empty interstitial site (void space) of the crystal. Then there creates a pair of vacancy defect and an interstitial defect, known as Frenkel defect. In ionic crystals since cations are generally smaller than anions, so that cations get transferred into interstitial sites. In this case, the bonding in ZnO is largely ionic with the

Fig. 4. Tauc plots of ZnMgO particles.

138


Fig. 5. FTIR spectra of ZnMgO nanoparticles.

Frenkel defect

Schottky defect

the crystal. These excess zinc ions occupy the interstitial sites together with an equal concentration of vacancies [47]. The presence of such ions in interstitial positions may distort (expand) the lattice and hence increase unit cell dimensions (as shown in Table 1) with increasing the concentration of MgO [48]. In this way we can conclude that concentration of the Frenkel defects can be gradually enhanced as a function of MgO content in the ZnMgO crystal composition. Vacant Zn sites (VxZn ), together with an equal concentration of vacant oxygen sites (VxO ) can be created from the Schottky reaction as follows:

04VxZn þ VxO

(10)

And, ionization reactions for oxygen and zinc ionic states are:

VxO 4VO þ e0

(11)

0 VO 4V O þe

(12)

VxZn 4VZn þ h

(13)

VZn 4VZn þ h

(14)

0

0

Impurity where Zn-

00

Thus, using equations (11)e(14), one by one in eq. (10), we can obtain

O-

00

04VZn þ V O

Fig. 6. Schematic of defects in ZnMgO nanocomposite.

(15)

More precisely, A

2þ

A

2

corresponding radii of 0.74 for Zn ion and 1.40 for O ion. Hence, Zn2þ ion easily occupies the interstitial site, simultaneously by creating the vacancy at its actual site. Generally, the Frenkel defects are created while growing the crystal, when the molecular formula does not confirm to the stoichiometric ratio. Here, the molecular formula of Zinc oxide is ZnO. There is one oxygen ion for every zinc ion in the crystal. This is called stoichiometric ratio of ZnO. During the growth of the crystal suppose the molecular formula becomes ZnyO where y > 1, then there are excess zinc ions in

00

x ZnxZn þ OxO 4VZn þ V O þ ZnZn

surface

þ OxO

surface

(16)

Thus, a vacancy in ZnO crystal is created by moving the oxygen ion to the crystal surface. Then the charge neutrality maybe achieved by creating another vacancy at usual Zn ion site. Such pair of vacant sites is known as Schottky defect [49]. In other words, formation of the Schottky defect can be considered as the removal of pair of interior anion and cation from the crystal and the replacement of these two ions on the crystal surface. Therefore, it is natural


to expect that decreasing the size of a material will increase the vacancy (Schottky defects) concentration due to an increasing surface area to volume ratio [50]. Thus, in the present study, since we have seen (in Table 1) that the grain size (D) of the ZnMgO nanoparticles decreases and dislocation density (r) increases with increasing the concentration of MgO. Hence the concentration of Schottky defects can be gradually enhanced as a function of MgO content in the ZnMgO crystal composition. The concentration of the both Zn interstitials (Frenkel defects) and oxygen vacancies (Schottky defects) are known to be the predominant ionic defect types in pure and MgO doped ZnO crystal. However, which defect dominates in native, undoped ZnO is still a matter of great controversy [51]. At high concentrations of MgO, the spectra of two samples ZM16 and ZM20 have shown a weaker shoulder at 605 cm1 and 614 cm1 respectively. The increased rate of oxygen defects may be responsible to ascribe this shoulder [52]. The band at 667 cm1 is unaffected by MgO doping, but the band at 567 cm1 shifted slightly towards the high frequency side. This may be due to the incorporation of Mg2þ in to the zinc oxide crystal lattice. Earlier studies on Raman spectra and FTIR spectra of ZnO nanoparticles doped with metal oxides have given a similar description [16,37,38]. Undoped ZnO sample exhibited a notable band at 908 cm1. Moreover, a vibration mode around 845 cm1 is also observed in all ZnMgO samples; it should be an intrinsic mode of ZnO induced by Mg2þsubstitution. Obviously, the change in the micro structures by the dopant MgO into zinc oxide lattice can be obtained [16]. Furthermore, as we discussed in the sections 3.1 and 3.2, the XRD and band gap studies have already supported this viewpoint. 4. Conclusions The study on structural and optical properties of zinc magnesium oxide nanoparticles synthesized by chemical co-precipitation portrays the conclusions as follows: 1. The broadening and a small shift towards low angle side in XRD peaks shows that a gradual rise in dislocation density and hence, increase of micro strain and lattice constants with subsequent reduction in the particle size of ZnO nanoparticles with the addition of MgO. 2. The increase of the optical band gap energy and hence blue shift in the absorption with MgO-doping is mainly due to the MosseBurstein effect. 3. The strong absorption peaks at 567 cm1 and 667 cm1 in FTIR spectra are attributed to stretching vibrations ZneO bond in ZnO. Moreover, an intrinsic vibration mode of ZnO induced by Mg2þsubstitution around 845 cm1 is also observed in all ZnMgO samples, which determines the change in the micro structural features by the addition of MgO into the zinc oxide lattice. Acknowledgement One of the authors (Dr. L. Srinivasa Rao) wishes to thank management of VNR Vignana Jyothi Institute of Engineering and Technology for their encouragement for this research work. References [1] A.N. Fouda, E.A. Eid, Influence of ZnO nano-particles addition on thermal analysis, microstructure evolution and tensile behavior of Sne5.0 wt% Sbe0.5 wt% Cu lead-free solder alloy, Mater. Sci. Eng. A 632 (2015) 82e87. [2] Wenying Zhou, Zijun Wang, Lina Dong, Xuezhen Sui, Qingguo Chen, Dielectric properties and thermal conductivity of PVDF reinforced with three types of Zn particles, Compos. Part A Appl. Sci. Manuf. 79 (2015) 183e191.

139

[3] M.F. Malek, M.H. Mamat, M.Z. Musa, Z. Khusaimi, M.Z. Sahdan, A.B. Suriani, A. Ishak, I. Saurdi, S.A. Rahman, M. Rusop, Thermal annealing-induced formation of ZnO nanoparticles: minimum strain and stress ameliorate preferred c-axis orientation and crystal-growth properties, J. Alloys Compd. 610 (2014) 575e588. [4] Peter Podbrs cek, Goran Dra zi c, Alojz An zlovar, Zorica Crnjak Orel, The preparation of zinc silicate/ZnO particles and their use as an efficient UV absorber, Mater. Res. Bull. 46 (11) (2011) 2105e2111. [5] Ishant Chauhan, Sandeep Nigam, V. Sudarsan, R.K. Vatsa, Hetero-junction assisted improved luminescence of Eu3þ doped ZnOeSnO2 nanocomposite, J. Lumin. 176 (2016) 124e129. [6] M. Boujnah, M. Boumdyan, S. Naji, A. Benyoussef, A. El Kenz, M. Loulidi, High efficiency of transmittance and electrical conductivity of V doped ZnO used in solar cells applications, J. Alloys Compd. 671 (2016) 560e565. [7] Hao Gu, Wen Zhang, Yongbing Xu, Mi Ya, Effect of oxygen deficiency on room temperature ferromagnetism in Codoped ZnO, Appl. Phys. Lett. 100 (2012) 202401, https://doi.org/10.1063/1.4717741. C. Oliveira, Davy Keyson, Guilherme [8] Max Rocha Quirino, Mateus Jose rdio Lucena, Jo~ Leoca ao Bosco L. Oliveira, Lucianna Gama, Synthesis of zinc oxide by microwave hydrothermal method for application to transesterification of soybean oil (biodiesel), Mater. Chem. Phys. 185 (2017) 24e30. [9] Marian Zamfirescu, Alexey Kavokin, Bernard Gil, Guillaume Malpuech, Mikhail Kaliteevski, ZnO as a material mostly adapted for the realization of room-temperature polariton lasers, Phys. Rev. B 65 (2002), 161205(R). [10] S. Tüzemen, Emre Gür, Principal issues in producing new ultraviolet light emitters based on transparent semiconductor zinc oxide, Opt. Mater. 30 (2) (2007) 292e310. [11] Leta T. Jule, Francis B. Dejene, Kittessa T. Roro, Zelalem N. Urgessa, Johannes R. Botha, Rapid synthesis of blue emitting ZnO nanoparticles for fluorescent applications, Phys. B Condens. Matter 497 (2016) 71e77. [12] J.D. McNamara, N.M. Albarakati, M.A. Reshchikov, Abrupt and tunable quenching of photoluminescence in ZnO, J. Luminescence 178 (2016) 301e306. [13] Guofeng Hu, Wenxi Guo, Ruomeng Yu, Xiaonian Yang, Ranran Zhou, Caofeng Pan, Zhong Lin Wang, Enhanced performances of flexible ZnO/ perovskite solar cells by piezo-phototronic effect, Nano Energy 23 (2016) 27e33. [14] Vardan Galstyan, Elisabetta Comini, Camilla Baratto, Guido Faglia, Giorgio Sberveglieri, Nanostructured ZnO chemical gas sensors, Ceram. Int. 41 (10 B) (2015) 14239e14244. [15] Liqiang Zhang, Yinzhu Zhang, Zhizhen Ye, Jianguo Lu, Bin Lu, The effects of group-I elements co-doping with Mn in ZnO dilute magnetic semiconductor, J. Appl. Phys. 111 (2012) 123524, https://doi.org/10.1063/1.4729530. [16] S. Muthukumaran, R. Gopalakrishnan, Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method, Opt. Mater. 34 (2012) 1946e1953. [17] Rajkrishna Dutta, Nibir Mandal, Mg doping in wurtzite ZnO coupled with native point defects: a mechanism for enhanced n-type conductivity and photoluminescence, Appl. Phys. Lett. 101 (2012) 042106, https://doi.org/ 10.1063/1.4738990. [18] Jae-chul Lee, Ji-eun Lee, Ju-won Lee, Jae-choon Lee, N.G. Subramaniam, Taewon Kang, Rajeev Ahuja, Se concentration dependent band gap engineering in ZnO1-xSex thin film for optoelectronic applications, J. Alloys Compd. 585 (2014) 94e97. [19] Anitha, Usha, Jithin, Christy, Varughese, Characterization, thermal effect on optical band gap energy and photoluminescence in wurtzite ZnO: Er nanocrystallites, Mater. Today Proc. 3 (2) (2016) 145e151. [20] H. Yang, X. Song, X. Zhang, W. Ao, G. Qiu, Synthesis of vanadium-doped SnO2 nanoparticles by chemical co-precipitation method, Mater. Lett. 57 (2003) 3124e3127. [21] Hayoung Yoon, Jun Hua Wu, Ji Hyun Min, Ji Sung Lee, Jae-Seon Ju, Young Keun Kim, Magnetic and optical properties of monosized Eu-doped ZnO nanocrystals from nanoemulsion, J. Appl. Phys. 111 (2012) 07B523, https:// doi.org/10.1063/1.3676422. [22] Hao Wang, Y. Chen, H.B. Wang, C. Zhang, F.J. Yang, J.X. Duan, C.P. Yang, Y.M. Xu, M.J. Zhou, Q. Li, High resolution transmission electron microscopy and Raman scattering studies of room temperature ferromagnetic Ni-doped ZnO nanocrystals, Appl. Phys. Lett. 90 (2007) 052505, https://doi.org/ 10.1063/1.2435606. [23] Suresh Kumar, Pankaj Sharma, Vineet Sharma, CdS nanofilms, Synthesis and the role of annealing on structural and optical properties, J. Appl. Phys. 111 (2012) 043519, https://doi.org/10.1063/1.3688042. [24] S. Fabbiyola, V. Sailaja, L. John Kennedy, M. Bououdina, J. Judith Vijaya, Optical and magnetic properties of Ni-doped ZnO nanoparticles, J. Alloys Compd. 694 (2017) 522e531. [25] R. Swapna, M.C. Santhosh Kumar, The role of substrate temperature on the properties of nanocrystalline Mo doped ZnO thin films by spray pyrolysis, Ceram. Int. 38 (2012) 3875e3883. [26] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Band gap engineering based on MgxZn1xO and CdyZn1yO, Appl. Phys. Lett. 78 (2001) 1237. https://doi.org/10.1063/1. 1350632. [27] K. Venkateswarlu, A. Chandra Bose, N. Rameshbabu, X-ray peak broadening studies of nanocrystalline hydroxyapatite by WilliamsoneHall analysis, Phys. B 405 (2010) 4256e4261.

140


[28] P.K. Sharma, R.K. Dutta, A.C. Pandey, Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO: Cu2þ nanorods, J. Magn. Magn. Mater. 321 (24) (2009) 4001e4005. [29] S. Prabahar, M. Dhanam, CdS thin films from two different chemical bathsdstructural and optical analysis, J. Cryst. Growth 285 (1e2) (2005) 41e48. [30] Abdollah Hajalilou, Saiful Amri Mazlan, Kamyar Shameli, A comparative study of different concentrations of pure Zn powder effects on synthesis, structure, magnetic and microwave-absorbing properties in mechanically-alloyed NieZn ferrite, J. Phys. Chem. Solids 96e97 (2016) 49e59. [31] N. Tiwari, S. Doke, A. Lohar, Shailaja Mahamuni, C. Kamal, Aparna Chakrabarti, R.J. Choudhary, P. Mondal, S.N. Jha, D. Bhattacharyya, Local structure investigation of (Co, Cu) co-doped ZnO nanocrystals and its correlation with magnetic properties, J. Phys. Chem. Solids 90 (2016) 100e113. [32] Mohd Arshad, Mohd Meenhaz Ansari, Arham S. Ahmed, Puspendra Tripathi, S.S.Z. Ashraf, A.H. Naqvi, Ameer Azam, Band gap engineering and enhanced photoluminescence of Mg doped ZnO nanoparticles synthesized by wet chemical route, J. Lumin. 161 (2015) 275e280. [33] Waleed E. Mahmoud, A.A. Al-Ghamdi, Synthesis of CdZnO thin film as a potential candidate for optical switches, Opt. Laser Technol. 42 (7) (2010) 1134e1138. [34] Trilok Singh, D.K. Pandya, R. Singh, Synthesis of cadmium oxide doped ZnO nanostructures using electrochemical deposition, J. Alloys Compd. 509 (2011) 5095e5098. [35] a) E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632; b) T.S. Moss, The interpretation of the properties of indium antimonide, Proc. Phys. Soc. Sect. B 67 (10) (1954) 775e782. [36] I. Hamberg, C.G. Granqvist, K.-F. Berggren, B.E. Sernelius, L. Engstrom, Band gap widening in heavily doped oxide semiconductors used as transparent heat-reflectors, Sol. Energy Mater. 12 (1985) 479e490. [37] T.B. Iveti c, M.R. Dimitrievska, N.L. Fin cur, LjR. Ða canin, I.O. Gúth, B.F. Abramovi c, S.R. Luki c-Petrovi c, Effect of annealing temperature on structural and optical properties of Mg-doped ZnO nanoparticles and their photocatalytic efficiency in alprazolam degradation, Ceram. Int. 40 (1B) (2014) 1545e1552. [38] V. Etacheri, R. Roshan, V. Kumar, Mg-doped ZnO nanoparticles for efficient sunlight-driven photocatalysis, ACS Appl. Mater. Interfaces 4 (2012) 2717e2725. [39] K. Petcharoena, A. Sirivat, Materials, Synthesis and characterization of magnetite nanoparticles via the chemical co-precipitation method, Sci. Eng. B

177 (2012) 421e427. [40] Shiv Kumar, Subhrajit Mukherjee, Ranjan Kr Singh, S. Chatterjee, A.K. Ghosh, Structural and optical properties of sol-gel derived nanocrystalline Fe doped ZnO, J. Appl. Phys. 110 (2011) 103508, https://doi.org/10.1063/1.3658221. [41] Xiaoyong Xu, Chunxiang Xu, Yi Lin, Tao Ding, Shengjiang Fang, Zengliang Shi, Weiwei Xia, Jingguo Hu, Surface photoluminescence and magnetism in hydrothermally grown undoped ZnO nanorod arrays, Appl. Phys. Lett. 100 (2012) 172401, https://doi.org/10.1063/1.4705412. [42] Jianming Wu, Hong Yana, Xuehu Zhang, Liqiao Wei, Xuguang Liu, Bingshe Xu, Magnesium hydroxide nanoparticles synthesized in water-in-oil microemulsions, J. Colloid Interface Sci. 324 (2008) 167e171. [43] G. Xiong, U. Pal, J.G. Serrano, K.B. Ucer, R.T. Williams, Photoluminescence and FTIR study of ZnO nanoparticles: the impurity and defect perspective, Phys. Stat. Sol. (c) 3 (10) (2006) 3577e3581, https://doi.org/10.1002/ pssc.200672164. [44] Kamellia Nejati, Zolfaghar Rezvani, Rafat Pakizevand, Synthesis of ZnO nanoparticles and investigation of the ionic template effect on their size and shape, Int. Nano Lett. 1 (2) (2011) 75e81. [45] Mehrnaz Gharagozlou, Sanaz Naghibi, Sensitization of ZnO nanoparticle by vitamin B12: investigation of microstructure, FTIR and optical properties, Mater. Res. Bull. 84 (2016) 71e78. €ger, H.J. Vink, Relations between the concentrations of imperfections [46] F.A. Kro in crystalline solids, Solid State Phys. 3 (1956) 307e435. https://doi.org/10. 1016/S0081-1947(08)60135-6. [47] M.S. Vijaya, G. Rangarajan, Materials Science, Page Number (91-92), Fourth reprint, McGraw Hill Education (India) Private Limited, New Delhi, India, 2014. ISBN 10: 0070534691/ISBN 13: 9780070534698. [48] J.D. Lee, Concise Inorganic Chemistry, fourth ed., Champman &Hall (reprinted by Blackwell Science) (Oxford University Press), 2005, ISBN 0-632-054 59-X, pp. 59e61. [49] C. Kittel, Introduction to Solid State Physics, seventh ed., Wiley, New York, 1995, ISBN 9971-51-180-0, pp. 541e542. [50] G. Guisbiers, Schottky defects in nanoparticles, J. Phys. Chem. C 115 (6) (2011) 2616e2621, https://doi.org/10.1021/jp108041q. [51] K.I. Hagemark, Defect structure of Zn-doped ZnO, J. Solid State Chem. 16 (1976) 293e299. https://doi.org/10.1016/0022-4596(76)90044-X. [52] K. Sowri Babu, A. Ramachandra Reddy, Ch Sujatha, K. Venugopal Reddy, A.N. Mallika, Synthesis and optical characterization of porous ZnO, J. Adv. Ceram. 2 (3) (2013) 260e265, https://doi.org/10.1007/s40145-013-0069-6. ISSN 2226-4108.