Influence of different solvents on the structural, optical, impedance and dielectric properties of ZnO nanoflakes

Chinese Journal of Physics 57 (2019) 28–46

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Influence of different solvents on the structural, optical, impedance and dielectric properties of ZnO nanoflakes Sukriti Khera, Prakash Chand

T

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Department of Physics, National Institute of Technology, Kurukshetra 136119, India

A R T IC LE I N F O

ABS TRA CT

Keywords: ZnO nanoflakes Solvents FTIR Impedance Spectroscopy Dielectric properties

In this work, we have synthesized the ZnO nanoflakes using three different solvents, i.e., isopropyl alcohol (IPA), DI (de-ionized) water and ethanol via co-precipitation technique. XRD analysis revealed the hexagonal wurtzite crystal structure for all synthesized samples. The crystallite size is least for ZnO nanoflakes synthesized using ethanol and highest for IPA by applying Scherrer's formula as well as W–H plot. The optical properties were analyzed using UV–Visible spectroscopy and revealed that the maximum optical band gap has been obtained using ethanol as a solvent while the least band gap is observed for IPA. The expansion of light absorption region from UV to visible region can lead to its application in various fields such as optical LEDs, photocatalysis, laser diodes, etc. FTIR spectrum verifies the presence of vibrational modes of the ZneO bond, stretching and bending bonds of OeH bonds and O=C=O stretching bond in the prepared samples. The plot for frequency-dependent dielectric loss exhibits the change in dielectric properties. The Nyquist diagrams reveal the semi-circular arc depicting the variation between real and imaginary parts of impedance. The results show that the electrical conductivity increases with frequency and temperature. The activation energy is found using Arrhenius equation with the plot of conductivity versus temperature and the values of activation energy for ZnO nanoflakes synthesized using IPA, DI water, and ethanol is found to be 0.23 eV, 0.29 eV and 0.41 eV, respectively.

1. Introduction Nanosized transition metal-based oxides have acquired immense attention in recent years because of their ability to enhance chemical, physical and electrical properties. The use of metal oxides like ZnO, SnO2, TiO2, CuO, etc. in nanotechnology has faced a never-ending ocean of sand. Among all metal oxides, a wide variety of nanostructures are possible for ZnO such as nanoparticles, nanorods, nanobelts, nanocages, nanoflowers, nanoflakes, nanowires, nanospheres, nanocombs, nanorings, nanohelixes/nanosprings, etc. [1–3]. ZnO is inexpensive, handy, non-toxic, biodegradable, non-volatile and easy to synthesize. It is widely used in short wavelength optoelectronic devices, gas sensors, drugs, cosmetics, as an electrode in dye-sensitized solar cells, photocatalyst, in the textile industry and biomedical applications [4–12]. Because of wide and direct energy band gap of ZnO at room temperature, it is more appropriate for short wavelength optoelectronic applications. Further, it has high values of excitonic energy (60 meV), which is higher than the thermal energy value (26 meV) at room temperature, which can make certain proficient excitonic emission at room temperature. It exists in three crystal structures, i.e., rock salt, zinc-blende, and wurtzite. The thermodynamically stable structure is wurtzite while rock salt structure can be obtained at high pressures (∼9 GPa) from wurtzite. This is possible because, at higher

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Corresponding author. E-mail addresses: [email protected] (S. Khera), [email protected] (P. Chand).

https://doi.org/10.1016/j.cjph.2018.12.015 Received 20 August 2018; Received in revised form 6 December 2018; Accepted 17 December 2018 Available online 04 January 2019 0577-9073/ © 2019 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.

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pressures, the inter-ionic Coulomb interaction dominates the covalent nature due to the reduction of the lattice parameters. The crystal structure of ZnO comprises alternating planes of tetrahedrally co-ordinated Zn2+ and O2−. In this, each O2− is enclosed by four Zn2+ at the corners of a tetrahedron and vice-versa [1,2]. For the synthesis of ZnO nanomaterials, various methods can be used such as sol–gel [13,14], hydrothermal [15,16], combustion [17,18], electrochemical deposition [19], co-precipitation [20–22], solidstate method [23,24], sonochemical [25], chemical vapor deposition [26] and pulsed laser ablation [27]. We have synthesized ZnO nanoflakes using co-precipitation method because the obtained solution is homogeneous and particle size can be easily controlled by using this method. This process can be performed at lower temperatures. This is also a simple and cost-effective method. To prepare on a large scale, the large surface area of the nanostructures and thorough removal of salt impurities are required which can be accomplished by this method [28]. We have made use of different solvents in the process of synthesis to study the physical and electrical properties. The solvents play a vital role in tuning the structural, morphology and optical properties of the nanostructures. Khoza et al. reported that HDA-capped ZnO nanoparticles show multiple morphologies ranging from stars, rods to spheres [29]. Pimentel et al. examined that ZnO nanoparticles synthesized using 2-ethoxyethanol has better photocatalytic degradation efficiency than the water and ethylene glycol. He also found that change in morphology leads to shifting in peaks in UV–Visible absorption spectra [30]. From the literature survey, so far the structural, morphological and optical properties have been investigated intensely by many pioneers but, there is a need of a comprehensive study to confirm the effect of solvents on complex impedance and dielectric properties of ZnO nanoflakes. Therefore, in order to precisely understand the effect of different solvents such as IPA, DI water and ethanol, we present an efficient endeavor to examine the structural, optical, complex impedance and dielectric properties of ZnO nanoflakes synthesized via using co-precipitation technique. The current paper is aimed at exploring the effect of different solvents on various properties of ZnO nanoflakes while keeping in mind the potential use of ZnO nanoflakes in optoelectronics and devices having high-frequency applications.

2. Experimental 2.1. Chemicals For the synthesis of ZnO nanoflakes via different solvents, the chemicals used were zinc acetate dihydrate [(Zn(CH3COO)2.2H2O), 99.5%, Loba Chemie, Mumbai, India], sodium hydroxide pellets [(NaOH), 97%, Loba Chemie, Mumbai, India], isopropyl alcohol (IPA)[(C3H8O), 99.5%, Loba Chemie, Mumbai, India], ethanol [(C2H5OH), 99.9%, Changshu Yangyuan Chemical, China], Ag paste [73%, PELCO] and de-ionized water [DI, Jairavik, New Delhi, India]. The chemicals used in the current work for the synthesis of ZnO nanoflakes were of analytical grade. They did not require any further purification for carrying out the synthesis.

2.2. Synthesis The ZnO nanoflakes are synthesized using different solvents by co-precipitation method. During a typical procedure for the synthesis of ZnO nanoflakes using isopropyl alcohol (IPA) as a solvent, Zn(CH3COO)2.2H2O and NaOH are used as precursors. An aqueous solution is made by dissolving 4.38 g of Zn(CH3COO)2.2H2O in 100 mL of isopropyl alcohol (IPA) and 80 mL of 1 M NaOH is added dropwise with continuous stirring with the help of magnetic stirrer at 60 °C for 2 h. Then, the solution is set aside overnight to allow the precipitates to settle down. The obtained precipitates were filtered and washed thoroughly with 5 mL each of DI water and ethanol (at least 4–5 times) and dried at 80 °C in an oven. The obtained nanoflakes are grinded using pestle mortar to get a fine powder. Similarly, the similar procedure was taken up for the synthesis ZnO nanoflakes synthesized via using ethanol and de-ionized (DI) water as solvents. The synthesis method is also described by a flowchart in Fig. 1. 2.3. Characterization The prepared samples were illustrated by X-ray Diffractometer (XRD; Rigaku Japan). It was observed in the 2θ range 20°–80° with a scanning rate of 2° per minute for the identification of phase and structure with a Cu-Kα radiation source (λ = 1.542 Å). The morphology of the synthesized samples was observed by Field Emission Scanning Electron Microscope (FESEM; Carl Zeiss Supra 55 VP). The Photoluminescence spectra were observed using RF-5301PC Shimadzu Spectro fluorophotometer (Japan) at room temperature. The luminescent properties of the prepared samples were investigated using PL spectroscopy using a Xe lamp as an irradiation source at a wavelength of λex = 350 nm. The Fourier Transform Infrared (FTIR) spectra (Alpha Bruker) were measured in the range of 400–4000 cm−1 for the powdered samples at room temperature. For FTIR measurement, we prepared a 15 mm pellet using KBr pellet press via using the ratio of sample to KBr powder as 1:100. The UV–Visible spectrum of the ZnO nanoflakes was measured via M550 Camspec UV–Visible spectrophotometer in the range of 190–1100 nm at room temperature. In this, we have dispersed synthesized ZnO nanoparticles in ethanol using an ultra-sonicator at 50 °C for 20 min and the resulting solution was then placed in quartz cuvette having 1 cm path length. The complex impedance measurements were carried out using a silver-coated pellet, with an average radius of 5 mm and the average area of 78.5 mm2, kept inside a furnace, i.e., connected by the 2-electrode system with SP240 Potentiostat (Biologic). The KBr pellet press is used to make 10 mm diameter pellets for the impedance measurements. The Nyquist plots were recorded using the EC-Lab software in the frequency range 5 MHz–100 Hz at the desired temperatures. 29

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Fig. 1. Flowchart for the synthesis of ZnO nanoflakes.

3. Results and discussion 3.1. X-ray diffraction studies X-ray diffraction (XRD) is performed for material characterization as it gives significant information about the phase, the nature of the sample, average crystallite size, lattice parameters, lattice strain, crystal orientation, dislocation density, etc. Fig. 2 shows the powder XRD patterns for the ZnO nanoflakes prepared using isopropyl alcohol (IPA), de-ionized water (DI) and ethanol as solvents. The figure confirms that all the peaks match up to the characteristic peaks of wurtzite phase with the hexagonal unit cell of JCPDS card no. 80-0075 belonging to the P63mc space group with lattice parameters as a = b = 3.253 Å and c = 5.209 Å. This indicates that

Fig. 2. Room temperature XRD patterns of ZnO nanoflakes synthesized using different solvents (a) isopropyl alcohol (b) DI water (c) ethanol. 30

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Table 1 Comparison of crystallite size (D), lattice constants, unit cell volume (V), X-ray density (dx), specific surface area (S), atomic packing fraction (APF) and bond length. Solvent

IPA DI Ethanol

Crystallite Size (D) (nm)

∼27 ∼24 ∼21

Lattice Constants

a = b (Å)

c (Å)

3.254 3.252 3.254

5.212 5.205 5.210

Volume of the unit cell (V) (Å)3

X-Ray Density (*104) (kg/m3)

Atomic Packing Fraction

Specific Surface Area (S) (m2/g)

Positional Parameter (u) (Å)

Bond-length (L) (Å)

47.50 47.76 47.67

0.56 0.57 0.58

0.7549 0.7555 0.7552

48.27 55.79 66.17

0.3799 0.3800 0.3801

1.980 1.979 1.978

the synthesized samples have no contamination and possess polycrystalline phase. The ZnO nanoflakes prepared with isopropyl alcohol (IPA), de-ionized water (DI) and ethanol as solvents reveals the highest intensities of the crystal growth orientation on the (101) plane. The higher XRD peak intensities of ZnO nanoflakes synthesized via using the isopropyl alcohol (IPA) solvent are due to the better crystallinity. However, the isopropyl alcohol (IPA) solvent shows the highest intensity at 36.14° of the crystal growth orientation on the (101) plane. Further, the average crystallite size (D) was determined along this plane. The average crystallite size of ZnO nanoflakes is estimated via using Scherrer's formula from the broadening of XRD peaks [31]:

D=

kλ βhkl cosθ

(1)

where, λ = wavelength of the incident X-rays radiation (1.5418 Å), k is Scherrer's constant, also known as shape factor ∼0.89, βhkl = full width at half maximum (FWHM) of the peak corresponding to (101), i.e., peak having maximum intensity, θ = Bragg's angle in radians, corresponding to 2θ having maximum intensity and D is the average crystallite size. The average crystallite size determined using Eq. (1) is presented in Table 1. The result shows that the ZnO nanoflakes synthesized using isopropyl alcohol (IPA) produce the largest crystallite size (27 nm). However, the ZnO nanoflakes synthesized using ethanol as solvent produce the smallest crystallite size (21 nm). From Table 1, it is observed that the average crystallite size (∼27–21 nm) is found to be decreased for ZnO nanoflakes synthesized via different solvents. The variation in crystallite size may be endorsed with a change in polarity of the solvent as the hydrolysis rate will vary in solvents [30]. The investigation clearly revealed that the selection of the solvent medium is a key factor for obtaining high-quality ZnO nanoflakes and can significantly control the crystallite size. The lattice parameters (a = b, c), unit cell volume (V), lattice strain, positional parameter (u), bond length (L), specific surface area (S), Xray density (dx) and atomic packing fraction (APF) play a vital role in enhancement of the structural properties of the metal oxide nanoflakes. The variation in different parameters like lattice constants (a = b, c), unit cell volume (V), lattice strain, positional parameter (u), bond length (L), specific surface area (S), X-ray density (dx) and atomic packing fraction (APF) are calculated for all the ZnO samples prepared using isopropyl alcohol (IPA), de-ionized water (DI) and ethanol as solvents and presented in Table 1. For the hexagonal structure, the lattice parameters (a = b, c) can be calculated by using the following Eqs. (2) and (3) [32]:

a=b=

c=

λ 3 sinθ100

(2)

λ sinθ002

(3)

The volume of the unit cell is found using the formula [32]:

V=

3 2 ac 2

(4) 3

The volume of the unit cell of ZnO synthesized using IPA, DI and ethanol are 47.50, 47.76 and 47.67 Å . The lattice constants has the maximum value of a = b = 3.254 Å and c = 5.212 Å for ZnO prepared using IPA as solvent. The lattice strain induced in the powder samples because of crystal imperfection can be estimated via using the following relation [32]:

ɛ=

βhkl 4tanθ

(5)

It is observed that there is an increase in the value of strain with an increase in ionic radii of the precursor. The lattice parameters can be used to find positional parameter (u) and ZneO bond length (L) according to the following relations [32]:

u=

L=

a2 + 0.25 3c 2

(6)

a2 + (0.5 − u)2c 2 3

(7)

The calculated value of ZneO bond length is 1.980, 1.979 and 1.978 for nanoflakes synthesized using IPA, DI and ethanol respectively with minimum value resulting in the presence of more structural defects [33]. The calculated values of positional 31

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parameter and bond-length for ZnO samples synthesized via isopropyl alcohol (IPA), de-ionized water (DI) and ethanol solvents are in agreement with the reported values [32,34]. The X-ray density (dx) is calculated by using the following relation [32]:

dx =

nM NV

(8)

where M = molecular weight of the sample, n = co-ordination number of the unit cell, i.e., number of atoms per unit cell, N = Avogadro's number and V = volume of the unit cell. The values of X-ray density is found to be 5.6 × 103, 5.7 × 103 and 5.8 × 103 kg/m3 for IPA, DI and ethanol solvent respectively. It increases with decrease in average crystallite size. The specific surface area (S) can be found by the formula as given below: (9)

S = 6000/ Ddx 2

where, dx is the X-ray density and D is the crystallite size. The specific surface area comes out to be highest (66.17 m /g) for ZnO nanoflakes with ethanol as solvent. This could contribute an efficient photocatalyst for achieving higher degradation rates in practical applications. The atomic packing fraction (APF) is calculated using the formula [32]:

2π a 3 3 c

APF =

(10)

The variation in atomic packing fraction (APF) is shown in Table 1. The packing fraction value comes out to be approximately 75% but it is 74% for bulk ZnO nanomaterials. Hence, APF in nanoflakes is greater than bulk due to the size effects [35]. 3.2. Williamson–Hall (W–H) plot The W–H analysis is employed to investigate the crystal distortions and imperfections originating from strain-induced and sizeinduced broadening [36]. The broadening of diffraction peaks follows tanθ dependency with strain while the crystallite size varies as 1/cosθ Eqs. (1) and (5). This distinction was due to the fact that the reflection broadening leads to microstructural changes that cause alteration in crystallite size and microstrain [37,38]. The separation of size and strain broadening are analyzed using W–H plot depending upon the two θ positions. The broadening of the peak is obtained by taking the sum of Eqs. (1) and (5):

βhkl = βs + βD βhkl = Taking

(14)

kλ + 4ɛtanθ Dcosθ

1 cosθ

(15)

as common on RHS in Eq. (15):

βhkl cosθ =

kλ + 4ɛsinθ D

(16)

Now, we plot a graph using Eq. (16) for uniform deformation model (UDM). In UDM, the strain is isotropic, i.e., it remains same for the all crystallographic directions. Consequently, the properties of nanoflakes do not depend on the direction along which they are deliberated [36]. The Eq. (16) is the equation for a straight line of the form: y = mx + c . A graph is plotted with 4sinθ along the x-axis and βhkl cosθ along the y-axis for ZnO nanoflakes synthesized via different solvents and depicted in Fig. 3(a–c). From the linear curve fitting for this data, we can extract strain and average crystallite size using slope

Fig. 3. W–H plot for ZnO nanoflakes synthesized using different solvents (a) isopropyl alcohol (b) DI water (c) ethanol. 32

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Table 2 Variation of the crystallite size and the strain calculated via Scherrer's formula and W–H plot. Solvent

IPA DI Ethanol

Crystallite Size (D) (nm)

Strain

Scherrer's formula

UDM (W–H plot)

Scherrer's Formula

UDM (W–H plot)

∼27 ∼24 ∼21

∼30 ∼29 ∼18

0.0048 0.0054 0.0064

0.0045 0.0048 0.0075

and y-intercept in Fig. 3. The strain observed in the samples synthesized with isopropyl alcohol (IPA), de-ionized water (DI) and ethanol solvents can be explained by the uniform deformation model (UDM) which states that the diffraction line broadening is because of reduction in crystallite size and strain (??) effect in W–H plot. The variation in crystallite size and strain calculated via Scherrer's formula and W–H plot have been depicted in Table 2. In both the cases, the average crystallite size is maximum for IPA solvent. This shows that the values obtained from W–H plot and Scherrer's formula are in good agreement with each other. 3.3. FESEM studies Field Emission Scanning Electron Microscopy (FESEM) is done to identify the surface morphology of the synthesized ZnO nanoflakes. Fig. 4(a–c) represents the FESEM images to show the effect of the solvents on the ZnO nanoflakes. FESEM images revealed the nanoflakes-type morphology for all the samples synthesized using different solvents. However, there is agglomeration taking place in ZnO nanoflakes which may be due to the higher surface area to volume ratio. The higher rate of crystallization leads to large bulk particles with severe agglomeration. The bigger crystallite size observed for ZnO nanoflakes synthesized using isopropyl alcohol (IPA) solvent can be endorsed to the agglomeration of particles. 3.4. Photoluminescence studies The photoluminescence spectroscopy is performed to probe into the electronic structure of the samples. It is performed to detect the electronic band gap, impurity levels and defects, quality of material and recombination mechanisms. Fig. 5 displays the emission spectrum depicting the photoluminescence of ZnO NPs synthesized using IPA, DI water, and ethanol as solvents. The PL spectra reveal two emission bands, one in the UV region and another in the visible region. The quality of the crystal can be analyzed using the luminescence in the UV region and the structural defects can be confirmed using the luminescence in the visible region [39]. The visible emission is due to the deep level defects present in ZnO such as zinc or oxygen interstitials and singly or doubly ionized oxygen vacancies. In UV range, we have got emission centered at around 389 nm because of the recombination of free excitons during an exciton–exciton collision evolution which is endorsed to a near band-edge (NBE) transition of ZnO [39,40]. In visible region, a broad blue emission band is observed between 400–500 nm centered at 470 nm. The blue emission centered at 470 nm is occurred due to the switching of electrons from shallow donor levels (Zni) to shallow acceptor levels (Vzn) [41]. The PL emission in yellow/orange region occurred due to oxygen interstitials (∼570–620 nm). The weak green emission band above 500 nm occurs due to the occurrence of singly ionized oxygen vacancies and zinc interstitials (Zn2+) in deep level transitions [42]. The green emission band disappears due to decrease in oxygen vacancies [14]. The low emission in the green region and strong intensity of the UV band should be ascribed to the high purity with good crystallinity of the prepared ZnO nanoparticles [42]. It can also be confirmed from the highest average crystallite size of ZnO nanoflakes for IPA solvent as shown in the Table 1. The intensity for ethanol is found to be highest due to the dominance of recombination of free excitons over surface and bulk-related defects [31]. 3.5. UV–Visible studies The optical properties play a major role in the analysis of the band gap of the semiconducting nanostructures. The inset of Fig. 6 shows the absorption spectra for ZnO nanoflakes synthesized via co-precipitation method using different solvents. It can be concluded that ZnO NPs are transparent for the visible range of the electromagnetic spectrum as there is no peak in that region. The strong absorption peaks are observed at 368, 353 and 349 nm for DI water, IPA and ethanol respectively. The absorption of UV light in this region is endorsed to the intrinsic band gap incorporation of ZnO nanoflakes due to the electronic transitions from the valence band to the conduction band (O2p to Zn3d) [22]. The presence of a single absorption peak ensures the purity of the sample. The optical band gap is calculated using Tauc equation [43]:

αhν = B (hν − Eg )n

(17)

where, α= optical absorption coefficient, B = constant that depends upon the effective mass of charge carriers present in conduction and valence band, hν= energy of the incident photon in eV, Eg= energy band gap in eV and the value of n depends upon the type of electronic transition. It is 1/2 for allowed direct and 2 for indirect electronic transitions. The optical absorption coefficient α can be found out using [44]: 33

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Fig. 4. (a–c) FESEM images of ZnO nanoflakes synthesized using different solvents (a) isopropyl alcohol(IPA) (b) DI water (c) ethanol.

α=

4πA λ

(18)

where A = absorbance of the sample, λ=wavelength of incident photon = 1.54 Å. Fig. 6 depicts the variation of the parameter; (αhν)2, with photon energy (hν) of ZnO nanoflakes prepared using various solvents. The value of optical energy band gap is determined by the extrapolation of the tangent to the obtained curve in the energy axis. 34

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Fig. 5. Room temperature photoluminescence (PL) spectra at an excitation wavelength of 350 nm of ZnO nanoflakes synthesized using different solvents.

Fig. 6. Plot of (αhν)2 versus photon energy (hν) ZnO nanoflakes synthesized using different solvents (a) isopropyl alcohol (b)ethanol (c) DI water. The inset shows the plot of absorption versus wavelength spectra of ZnO nanoflakes. Table 3 Comparison of energy band gap (Eg) and refractive index (n) calculated through different empirical relations. Solvent

IPA DI Ethanol

Energy gap (Eg) eV

3.06 3.22 3.24

Refractive Index (n) Moss

Ravindra

Reddy

Vandamme

Ghosh

2.360 2.331 2.327

2.187 2.088 2.075

2.749 2.710 2.705

2.310 2.265 2.260

2.358 2.325 2.321

The values of optical band gap are estimated by using Tauc plot and came out to be 3.06 eV, 3.22 eV, and 3.24 eV, respectively for samples synthesized using IPA, DI, and ethanol. These values are greater than that of the energy gap of the bulk, i.e., 3.30 eV [45]. There is a blue shift in the samples synthesized using IPA, DI, and ethanol indicating a decrease in crystallite size. The blue shift observed in the nanoflakes can be attributed to the quantum confinement effect in nanorange as the size of the synthesized nanoflakes 35

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Fig. 7. FTIR spectra of ZnO nanoflakes synthesized using different solvents.

Table 4 Variation in the absorption dips in the FTIR spectra of ZnO nanoflakes synthesized using different solvents. Wave numbers (cm−1)

DI

IPA

Ethanol

3421 2322 1423 1561 514

3414 2378 1415 1566 500

3384 2342 1413 1579 498

Functional groups

OeH stretching Existence of CO2 O=C=O OeH bending ZneO

is much larger than the Bohr radius of ZnO [46]. These nanoflakes can be considered for use in various optical devices like laser diode, light-emitting diode (LED) and photo-detector as they all lie in UV range. The optical band gap and the refractive index are associated with each other using various relations. The nanoflakes synthesized using IPA have absorption in visible region along with UV region while others can act as a UV blocking material. Hence, they can be used as photocatalysts for degradation of dyes into mineralized products. Refractive index is the amount of transparency to the incident photon. The first attempt to relate the two was made by Moss which is given by Moss relation as [47]:

n4Eg = 95eV

(19)

where Eg is the energy band gap in eV and n = refractive index for the synthesized sample. Then, Ravindra proposed a linear equation between refractive index and optical band gap [43,47,48]:

n = 4.084 − 0.62Eg

(20)

Herve–Vandamme proposed an empirical relation, i.e., denoted as [43,47]: 2

A ⎞ n2 = 1 + ⎛⎜ ⎟ B + Eg ⎠ ⎝

(21)

where, A is the ionization energy of Hydrogen = 13.6 eV and B is a constant = 3.47 eV. Here, B is assumed to be the difference between the UV resonance energy and the band gap energy. After inserting the value of A and B in the above equation, we get: 2

13.6 ⎞ n2 = 1 + ⎜⎛ ⎟ 3.47 + Eg ⎠ ⎝

(22) 36

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Fig. 8. (a–c) Plots of real parts of dielectric constant with frequency for ZnO nanoflakes at different temperatures synthesized using different solvents.

Reddy relation [47]:

n4 (Eg − 0.365) = 154eV

(23)

Ghosh et al. proposed an empirical relation pertaining to the suggestions given by Penn and Van Vechten regarding the band structure and quantum electric confinements [43]:

n2 − 1 =

A (B + Eg )2

(24)

where A = 25Eg + 212, B = 0.21Eg + 4.25

n2 − 1 =

25Eg + 212 (1.21Eg + 4.25)2

(25) 37

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Fig. 9. (a–c) Plots of imaginary parts of dielectric constant with frequency for ZnO nanoflakes at different temperatures synthesized using different solvents.

In the current work, we have determined the refractive indices of ZnO nanoflakes using the above mentioned empirical relations proposed by various researchers. The variation in refractive index and optical band gap (Eg) for ZnO nanoflakes is shown in Table 3. The energy band gap found using Reddy relation is quite close to the experimental results. It decreases with increase in energy band gap. 3.6. FTIR studies Fourier Transform Infrared (FTIR) spectra are analyzed to identify the molecular structures of the ZnO nanoflakes. When infrared radiation falls on the sample, the absorbed infrared radiation stimulates molecules from a lower to a higher vibrational state. The wavelengths which are wrapped up in the sample hence, give us information about the functional group attached because the wavelength is a characteristic of its molecular structure. All samples prepared using different solvents exhibit different characteristic absorption dips. Fig. 7 shows the room-temperature FTIR spectra of ZnO nanoflakes as a function of wave number and Table 4 depicts the variation in the absorption dips in the FTIR spectra of ZnO nanoflakes synthesized using different solvents. The absorption dips found in the region 500–600 cm−1 can be ascribed to Zn-O bond for different vibrational modes [49,50]. The peaks around 2300 cm−1 arise from the atmospheric CO2 on the metallic cations [51]. The presence of OeH stretching and bending bonds can be 38

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Fig. 10. (a–c) Variation of dielectric loss (tan δ) with frequency for ZnO nanoflakes synthesized at different temperatures using different solvents (a) isopropyl alcohol (b) ethanol (c) DI water.

confirmed by the absorption dips at 3421 and 1564 cm−1 due to the H2O adsorbed on the surface of the crystallites [52]. The small dip around 1412 cm−1 can be linked with the asymmetric stretching modes of the O=C=O bond. This might be present because of the residues left during the synthesis process [53]. The curve shows the variation of transmittance of IR radiation through the samples with a change in solvents. The shift in the absorption bands can be due to the change in the morphology of the samples. This change could be due to increasing boiling points of the solvents as optical transparency increases with increase in boiling point. Ethanol has the lowest transmittance due to the lowest boiling point [43].

3.7. Complex impedance and dielectric studies Complex impedance analysis is carried out to study the electrical properties of the ZnO nanoflakes. The frequency dependent properties are analyzed using the parameters such as complex dielectric constant (ɛ*), complex impedance (Z*), and dielectric loss (tan δ). These parameters can be represented as [54,55]: 39

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Fig. 11. (a–c) Nyquist plots for ZnO nanoflakes synthesized at different temperatures using different solvents. The inset shows Nyquist plot at 473 K for ZnO nanoflakes.

ɛ* = ɛ′ + jɛ″

(26)

Z * = Z′ + jZ″

(27)

tan δ =

ɛ″ Z′ = ɛ′ Z″

(28)

Here, ɛ′ and ɛ″ are the real part and imaginary part of complex dielectric constant and, Z′ and Z″ are the real part and imaginary part of complex impedance, respectively. Here, j 2 = −1 in the above equations. The imaginary part of dielectric constant exhibits the dissipated energy due to polarisation and ionic conduction while the real part of dielectric constant exhibits the stored energy or the polarizability of the material [56]. The dielectric constant and dielectric loss can be calculated via the following formula [57]: 40

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Fig. 12. (a–c) Plots of imaginary parts of impedance with frequency for ZnO nanoflakes synthesized at different temperatures using different solvents. The inset shows the variation at 473 K for ZnO nanoflakes.

ɛ′ =

CP t Aϵ 0

(29) (30)

ɛ″ = ɛ′ tan δ

where, Cp = Capacitance in Farad in µF, t = thickness of the pellet in mm, ∈ 0 is the permittivity of free space, A = area of the pellet in mm2, and tan δ is the dielectric loss. The measurements are made using the silver-coated pellets as electrodes that create a dielectric medium by acting as a disc capacitor [58,59]. The temperature dependent study of ɛ′ and ɛ″ over a range of frequencies (100 Hz–5 MHz) is shown in Figs. 8(a–c) and 9(a–c). With the increase in frequency, the dielectric constant decreases and then, attains a constant value. Fig. 10(a–c) shows the dielectric loss is highest in case of ZnO nanoflakes prepared using ethanol as solvent. This is due to the maximum number of defects (oxygen vacancies and zinc interstitials) produced in ZnO synthesized using ethanol as solvent. This leads to the increase in dipole moment and hence, orientation polarization also increases. An exponential decrease is observed for dielectric loss at the lower side of frequencies and it turn into the stable at higher values range of frequencies. The curves depict an enhancement in the value of dielectric loss with raise in temperature. The decrease can be due to the capacitance formed at 41

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Fig. 13. (a–c) Plots of real parts of impedance with frequency for ZnO nanoflakes synthesized at different temperatures using different solvents. The inset shows the variation at 473 K for ZnO nanoflakes.

the junction of the electrode or the voids in the sample [60]. This dielectric dispersion can be illuminated on the basis of surface charge polarization. The surface charge polarization arises because of the oxygen vacancies, dangling bonds present at the grain boundaries that create dipoles in ZnO [56]. The decrease in the crystallite size can also be owing to the increase in the amount of grains per unit volume. This results in the higher amount of dipoles per unit volume due to increase in vacancies which lead to increase in polarization and increase in dielectric constant at lower frequencies [61]. There is an increase in the value of ɛ′ and ɛ″ through raise in temperature which assures the semiconducting nature of the synthesized samples. The lower values of dielectric constant and dielectric loss in higher frequency section make it an appropriate material to be used in devices having high frequency applications [62]. Fig. 11(a–c) shows the temperature-dependent Nyquist or Cole–Cole plots, under dark conditions, for ZnO nanoflakes synthesized using different solvents in a broad range of frequency from 100 Hz to 5 MHz. The complex impedance spectra indicate the formation of a single semicircular arc which confirms the presence of the grain effect and single relaxation process in the synthesized samples. There is a shift in intercept towards the origin with the rise in temperature which results in the decrease of the bulk resistance of the ZnO nanoflakes [63]. The plot of imaginary and real parts of complex impedance versus frequency is depicted in Figs. 12(a–c) and 42

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Fig. 14. (a–c) Variations of AC conductivity with frequency for ZnO nanoflakes at different temperatures synthesized using different solvents.

13(a–c). With the increase in frequency, the impedance decreases. At higher frequencies, the value of impedance becomes constant. Among the three solvents, ethanol has the highest value of impedance. Along with the increase in temperature, there is a shift in the peaks towards the higher frequency region due to relaxation in the material. 3.8. Electrical ac conductivity The dependence of temperature and frequency on ac conductivity of ZnO nanoflakes is studied for the synthesized samples. The AC conductivity can be estimated via the following equation [64]:

σAC = ∈0 ɛ′ω tan δ

(31)

where, ϵ 0 is the permittivity in free space, ɛ′ is the real part of the complex dielectric constant, ω is the angular frequency (ω = 2πf) of applied ac field and tan δ is the dielectric loss. Fig. 14 (a–c) depicts the variation of AC conductivity with frequency at various temperatures. With the increase in temperature, the conductivity becomes constant with respect to frequency. For IPA, the AC 43

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Fig. 15. Linearly fitted curve showing the variation of AC conductivity with inverse temperature (1000/T) for ZnO nanoflakes synthesized using different solvents (a) isopropyl alcohol (b) ethanol (c) DI water.

conductivity decreases while for ethanol and DI water, AC conductivity is found to be increased with increasing in temperature. The increase in ac conductivity is due to the hopping of charge carriers which consists zinc interstitials (Zni+), negative oxygen ion (O2−) and positive oxygen vacancies (VO2+) leading to the polarization of ZnO [58]. There is a gradual increase in the ac conductivity with enhance in the frequency of applied AC field owing to the improvement in the migration of electrons with frequency. The ac conductivity decreases with increase in crystallite size due to the thermally activated hopping mechanism of charge carriers. The hopping of charge carriers becomes difficult as the energy gap between them increases as we go from IPA through DI to ethanol [65]. The activation energy can be found out from σ-T relationship using Arrhenius equation that tells us about the thermally activated transport properties of the material:

E σac = σ0 exp ⎛− a ⎞ ⎝ kB T ⎠ ⎜

⎟

(31)

where, σ0 is the pre-exponential factor, kB is Boltzmann's constant in J/K, T is the absolute temperature in Kelvin and Ea is the activation energy in eV. Fig. 15 depicts the plot of ac conductivity (lnσAC) versus inverse temperature (1000/T). It has been observed that with a reduction in crystallite size, there is an enhancement in activation energy for the synthesized samples. The value of activation energy is found to be 0.23, 0.29 and 0.41 eV, respectively, for ZnO nanoflakes synthesized using IPA, DI water and ethanol solvents respectively. 4. Conclusions In conclusion, ZnO nanoflakes have been synthesized via co-precipitation method successfully using isopropyl alcohol (IPA), DI water and ethanol as solvents. XRD analysis confirms the hexagonal wurtzite crystalline structure of the synthesized ZnO nanoflakes. The crystallite size and strain obtained using Scherrer formula and W–H plot are in good agreement with each other. The nanoflakes synthesized using IPA is highly crystalline as compared to other solvents. FESEM reveals the nanoflakes type morphology for all the samples synthesized by different solvent. The photoluminescence spectra verified that there are the peaks lying in UV as well as visible regions in emission spectra obtained at an excitation wavelength of 350 nm. Thus, the photoluminescence spectra also confirm the purity and crystallinity of the sample. The optical band gap determined using Tauc's plot is found to increase with a decrease in crystallite size. The energy band gap for ZnO nanoflakes synthesized using isopropyl alcohol (IPA), DI water and ethanol as solvents are 3.06, 3.22 and 3.22 eV respectively. The refractive indices are found with optical band gap using various models. The FTIR spectra showed the presence of vibrational modes of Zn-O in the range 500–600 cm−1. The complex impedance, dielectric constant, dielectric loss and AC conductivity (σAC) are established using frequency and temperature-dependent study of ZnO nanoflakes. The complex dielectric permittivity (ϵ′ and ϵ′′) decreases with frequency and then, attains a constant value. The dielectric loss (tan δ) increases with increase in temperature and is highest for ZnO nanoflakes prepared using ethanol. The ac conductivity shows a significant variation for different solvents. From Arrhenius equation, the activation energy came out to be 0.23, 0.29 and 0.41 eV, respectively, for ZnO nanoflakes synthesized using IPA, DI water and ethanol solvents, respectively. The above properties make ZnO 44

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