CHAPTER 4. DIELECTRIC, FERROELECTRIC, AC CONDUCTIVITY AND NON-LINEAR OPTICAL STUDIES OF ELECTROSPUN ZnO / BaO COMPOSITE NANOFIBERS

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1 103 CHAPTER 4 DIELECTRIC, FERROELECTRIC, AC CONDUCTIVITY AND NON-LINEAR OPTICAL STUDIES OF ELECTROSPUN ZnO / BaO COMPOSITE NANOFIBERS 4.1 INTRODUCTION Nanocomposite oxides exhibit a wide variety of material properties, such as ferroelectricity, piezoelectricity, pyroelectricity, magnetism, superconductivity and opto-electronic properties (Martin et al 2010, Uchino 2000, Pratibha Singh et al 2010). Ferroelectric random-access memories (FRAM)(Furukawa et al 2010), switches, capacitors (Sang Mo Yang et al 2011), sensors, actuators and transducers are based on ferroelectric ceramic materials (Qing-Ming Wang and Cross 1999, Uchino 1998). ZnO based nanocomposite materials have been extensively studied due to their splendid properties and potential applications such as coatings, sensors, integrated components for telecommunication, solar cells, etc (Manoj K. Gupta and Binay Kumar 2011, Manoj K. Gupta et al 2010, Zou et al 2011). Barium oxide is a eco-friendly material effectively used as a catalyst to store NO x (Sanyo M. Mathew et al 2007) and zinc oxide is a semi conducting optical material. ZnO is also studied due to its non-toxicity, good electrical, optical and piezoelectric behavior ( Islam and Podder 2009). Nanocomposites are synthesized in various forms such as nanopowders, nano films, nanoribbons, nanotubes, nanofibers, etc. Due to their 1-dimensional nature and flexibility, nanofibers are employed as

2 104 connecting wires in electronic components and optical waveguides (Zheng-Ming Huang et al 2003). Zinc oxide is a semiconducting piezoelectric material that has been used in surface acoustic wave (SAW) devices, chemical sensors and ultrasonic transducers (Wang et al 2003). It is also used in microelectromechanical systems (MEMS) as sensors and actuators (Shibata et al 2002). Quasi-one-dimensional ZnO nanobelts have been used in the production of gas sensors (Comini et al 2002, Pan et al 2001), field effect transistors (FET) (Arnold et al 2003), nanocantilevers (Huges and Wang 2003) and resonators (Bai et al 2003). It is also used in communications as surface acoustic wave (SAW) sensors (Kim et al 1999). Min-Hua Zhao et al (2004) studied the piezoelectric response of ZnO nanobelts. It has been proved by Wang et al (2003) that spontaneous polarization can be induced in zinc oxide by doping with some elements having smaller ionic radii, such as lithium, to substitute the zinc ions, it is expected that the center of the positive charge in an unit cell will not overlap with that of the negative charge in the same unit cell. Ferroelectric and dielectric properties of Li-doped ZnO thin films prepared by pulsed laser deposition were studied by Wang et al (2003). Ferroelectric properties of substituted barium titanate ceramics have been studied by Parveen Kumar et al (2009). Barium titanate / polyvinylidene fluoride composite fibers were electrospun and their ferroelectric properties were investigated (Avinash Baji et al 2011) by piezoresponse force microscopy. Barium titanate films were synthesized by sol gel method and their ferroelectric property and structure were studied by Shun Hua Xiao et al (2011). Barium titanate nanofibers were synthesized by electrospinning (Junhan Yuh et al 2005).

3 105 In the recent past, several nanofibers have been synthesized by the electrospinning process (Xiuli Zhou et al 2011, Siddheswaran et al 2006, Gui-Ping et al 2010). In the present work, electrospun nanocomposite fibers of ZnO / BaO were synthesized by calcining the Poly (Vinyl Alcohol) (PVA) / zinc acetate / barium acetate composite sol derived fibers at 1123 K for 8 h. The pure inorganic species obtained at this temperature had an average diameter of 100 nm which was revealed by Scanning Electron Microscope (SEM) images. Powder X-ray Diffraction (XRD) was used to characterize the sample and Energy Dispersive X-ray analysis (EDAX) confirmed the elements present in ZnO / BaO. AFM and UV characterization were also performed. Non-linear optical, dielectric and ferroelectric properties have been studied. 4.2 EXPERIMENTAL Synthesis of PVA / Zinc Acetate / Barium Acetate Composite Sol For the preparation of the sol, high molecular weight poly (vinyl alcohol) (M.W = 1,25,000 g/mol) with high purity purchased from Merck, barium acetate extra pure and zinc acetate dihydrate purchased from Aldrich (99% purity) were used. Deionised water was used as solvent. Zinc acetate / PVA solution was obtained by dissolving 2.5 g of zinc acetate in 20 ml of deionised water and then added to 60 g of aqueous PVA solution (about 10 wt %) with constant stirring at 323 K for 1 h. 20 ml of deionised water was added to 2.5 g of barium acetate and then this solution was slowly added into the previously prepared zinc acetate / PVA solution. 8 ml of ethanol and 1 ml of acetic acid were added to this solution while stirring because acetic acid controls precipitation and acts as hydrolysis

4 106 /condensation catalyst. Stirring the above prepared solution for 6 h keeping in a water bath maintained at 323 K resulted in a viscous, clear sol solution of PVA / zinc acetate / barium acetate. The obtained sol was used in the electrospinning process. Viscosity of the solution was measured as 1.23 Pa.s and the conductivity was found to be 18.7 ms/cm Deposition of Nanofibers Electrospinning apparatus and the electrospinning processes are explained elsewhere (Siddheswaran et al 2006). Deposition of the narrow nanofibers can be obtained under optimized conditions only. Several attempts have been made to form fibers with required dimensions and properties. The prepared sol was loaded into a 2 ml syringe with 25 gauge opening and the positive of the high voltage supply was connected to the needle and negative was connected to the collector screen. The temperature of the sol was maintained constantly at room temperature. The applied voltage was maintained at 18 kv since the ejection of the sol was uniform at this voltage Effect of Working Distance on the Fiber Morphology The effect of the external parameter ie working distance (distance between the electrodes) on the morphology of nanofibers was investigated. ImageJ software was used to reckon the fiber diameter. The working distance was experimented between 5 to 20 cm. Whenever the working distance was between 5 to 10 cm, excessive sol and beads were found in the SEM images as shown in Figure 4.1. This may be due to ejection of more sol and the evaporation of the solvent could not be performed completely at shorter distances.

5 107 Figure 4.1 Fibers deposited at working distances between 5 to 10 cm At working distances beyond 16 cm, the sol could not be pulled from the spinneret because of longer distance and the applied voltage was not sufficient to form the fibers and scattered droplets were only formed as shown in Figure 4.2. At distances less than 5 cm, spilling of the solution takes place as shown in Figure 4.3. Figure 4.2 Scattered droplets deposited at working distances beyond 16 cm

6 108 Figure 4.3 Spilt solution deposited at working distances less than 5 cm Working distances between 11 to 15 cm produced fibers without beads. This range was found to be the optimum range for the deposition of the fibers with the sol. In this range, when the working distance was shortened, the fiber diameter increased. This is due to the fact that the morphology of the fiber structure transformed quickly before the complete evaporation of solvent. The diameter of composite fibers obtained at various working distances are shown in Figure 4.4. Similar results have been obtained by researchers (Lin-Jer Chen et al 2011). From the observations it is found that at an optimum distance of 15 cm, the narrow nanofibers were formed with diameter of 150 nm as shown in Figure 4.5.

7 109 Figure 4.4 Variation of fiber diameter with working distance The optimum values of the electrospinning parameters maintained for the preparation of PVA / zinc acetate / barium acetate sol are given in Table 4.1. The sol with the following optimized parameters was used to form uniform fibers in the electrospinning process. Conductivity of the sol was measured by EQUIP-TRONICS model EQ-660A digital conductivity meter and viscosity of the sol was measured by Digital Viscometer.

8 110 Table 4.1 Optimum values of electrospinning parameters maintained to obtain PVA / zinc acetate / barium acetate composite and ceramic fibers Electrospinning parameters Applied voltage Distance between electrodes Size of the needle opening Conductivity of the sol Temperature of the sol Viscosity of the sol Calcining temperature Time of calcining Average diameter of fibers after calcining Optimized values 18 kv 15 cm 25 gauge 18.7 ms/cm 303 K 1.23 Pa.s 1123 K 8 h 80 nm 4.3 CHARACTERIZATION Morphology and Topography of the Fibers JEOL 840-A Scanning Electron Microscope was used to observe Scanning Electron Microscopy images to study the surface topography, morphology and cross-section of the nanofibers. Figure 4.5 shows the SEM image of the PVA / zinc acetate / barium acetate composite fibers. The average diameter of these fibers is found to be around 200 nm. Calcining at 1123 K for 8 h yielded ZnO / BaO nanocomposite fibers with average diameter less than 100 nm. The decrease in fiber diameter can be seen in the SEM image shown in Figure 4.6. The calcined fibers possess cylindrical morphology with uniform diameter and uniformly distributed on the collector plate.

9 111 Figure 4.5 SEM image of PVA / zinc acetate / barium acetate composite nanofibers Figure 4.6 SEM image of ZnO/BaO composite nanofibers AFM image was also captured to validate the cylindrical morphology of the nanofibers by using AFM (NanoSurf Easy Scan2, Switzerland). From Figure 4.7, we can observe the cylindrical morphology of

10 112 the PVA / zinc acetate / barium acetate composite fibers. The fibers are found to be uniformly coated on the collector screen. Figure 4.7 AFM image of PVA / zinc acetate / barium acetate composite nanofibers Powder X-ray Diffraction The phase composition of the prepared samples were studied from the powder X-ray diffraction (XRD) patterns of the samples observed by a Rigaku D/max-A diffractometer fitted with CuK radiation ( = Å) at a scan speed of 0.01 /s at room temperature. The intensity data was recorded by continuous scan in 2 mode from 10 to 70. Figure 4.8 shows the powder XRD patterns of the composite nanofibers calcined at 1123 K. The diffraction peaks exhibited at 31.6 (100), 34.3 (002), 36.1 (101), 47.2 (102), 56.2 (110), 62.4 (103), 65.8 (200) and 68.5 (112) were corresponding to ZnO phases and could be indexed to the hexagonal wurtzite zinc oxide (Yong Liu et al 2006). The peak corresponding to 24.8 o could be indexed to BaO phase.

11 113 Figure 4.8 Powder XRD pattern of ZnO/BaO nanocomposite EDAX Analysis EDAX spectrum was also captured from JEOL 840-A Scanning Electron Microscope. Energy Dispersive X-ray Analysis was also performed on the samples to determine the elemental composition. The EDAX pattern given in Figure 4.9 confirms the formation of ZnO and BaO phases together in the ZnO / BaO nanocomposite. The spectrum gives the composition of ZnO and BaO in equivalent ratio.

12 114 Figure 4.9 EDAX spectrum of ZnO/BaO nanocomposite Optical Absorption Studies VARIAN CARY 5E spectrophotometer was used to obtain the UV spectrum. To determine the band-gap energy (E g ), it was assumed that the fundamental absorption edge of ZnO / BaO nanocomposite is due to the direct allowed transition. The spectrum was recorded in the wavelength region of nm. The UV absorption edge for the ZnO / BaO nanocomposite was observed to be around 270 nm as shown in Figure 4.10.

13 115 Figure 4.10 UV spectrum of ZnO/BaO nanocomposite The optical absorption coefficient ( ) varies with the excitation light energy h (Amit Kumar Chawla et al 2007) and is given by the expression, n h Ah ( E ) (4.1) g near the band gap, where is absorption coefficient, is frequency in Hz, A is a constant, h is Planck s constant and E g is the direct allowed energy gap. For allowed direct transition, the constant n = 1/2. From the plot of ( ) 2 versus h shown in the Figure 4.11, band gap energy E g is evaluated by extrapolating the linear fitted regions to ( ) 2 = 0. The band gap energy of the nanocomposite was found to be 4.2 ev.

14 116 Figure 4.11 Plot of variation of ( ) 2 vs h Second Harmonic Efficiency (SHG) test Kurtz Perry powder technique was employed to test the second harmonic generation (SHG) efficiency of the sample. Nd: YAG laser with modulated radiation of wavelength 1064 nm with pulse energy of 4.9 mj/pulse, pulse width of 8 ns and repetition rate of 10 Hz was used to illuminate the sample. KDP reference sample was also given the same input. The SHG efficiency of the nanocomposite was found to be 0.55 times than that of Potassium di-hydrogen phosphate (KDP) Dielectric Studies Dielectric measurements with respect to frequency at different temperatures were taken by using HIOKI LCR HITESTER meter. Pellets of ZnO / BaO composite of thickness 0.93 mm and 6.07 mm diameter were made by applying a pressure of 4 tonne in a hand operated hydraulic press for dielectric measurements from collected non-woven mesh of fibers. Silver paste was coated on both the sides of the sample for better contact.

15 117 using the relation The dielectric constant of the nanocomposite was measured by Cd r A 0 (4.2) where C is the capacitance, d is the thickness of the sample, A is the surface area of the sample and 0 is the absolute permittivity of the free space ( F/m). Figure 4.12 Log frequency vs Dielectric constant Dielectric constant was calculated for various frequencies from 50 Hz to 5 MHz at various temperatures of 303, 313, 323, 333 and 343 K. Figure 4.12 gives the variation of dielectric constant with respect to Log frequency at temperatures of 303, 313, 323, 333 and 343 K. From the figure, it is seen clearly that the dielectric constant value is large at low frequencies

16 118 and decreases at high frequencies. This is attributed to the presence of all polarization mechanisms such as electronic, ionic, orientation and space charge polarization (Barsoum 1977). Figure 4.13 shows the variation of dielectric loss with respect to log frequency at temperatures of 303, 313, 323, 333 and 343 K. Dielectric loss also follows a similar trend. Figure 4.13 Log frequency vs Dielectric loss Ferroelectric Transition Figure 4.14 shows the variation of dielectric constant with respect to temperature. It is clearly seen that the dielectric constant increases up to 323 K and decreases gradually. This indicates that the material begins to undergo phase transition from ferroelectric to paraelectric at 323 K which is called as the Curie temperature (T c ). The peak at 323 K indicates the ferroelectric phase transition of the ZnO/BaO nanocomposite. Earlier reports

17 119 on ferroelectric properties of materials support this behavior (Mathe et al 2002, Arora et al 2004). Figure 4.14 Temperature dependence of dielectric constant A.C. Conductivity Studies The A.C. conductivity of the nanocomposite sample can be calculated by using the relation a. c 2 f 0 r tan. Where 0 is permittivity in free space, r is relative permittivity, f is the frequency and tan is the loss factor. The A.C. electrical conductivity of the ZnO / BaO nanocomposite as a function of frequency and temperature is shown in Figure Irrespective of temperature, for an increase in frequency, there is a slight increase in the electrical conductivity of the nanocomposite at the low frequency region up to 50 khz. At high frequencies ie above 0.1 MHz, there

18 120 is a sudden increase in the conductivity and it is relatively large at high temperatures. The high ac conductivity at high frequencies is the trend required for a small polaron hopping (Peter Barber et al 2009). In nanomaterial systems, conductivity is exhibited with highly localized carriers bound to the lattice with lattice strain ie with polaron conduction (Mahajan et al 2000). Figure 4.15 Log frequency vs AC conductivity 4.4 CONCLUSIONS Nanocomposite fibers of ZnO / BaO have been fabricated by electrospinning the sol of the PVA/ zinc acetate / barium acetate precursor and their successive calcinations at 1123 K for 8 h. The fibers were characterized by SEM, AFM, XRD, UV and EDAX. From the calcinations, it was found that the diameters of the fibers reduced to large extent and

19 121 crystalline phase was obtained. The non linear optical, dielectric and ferroelectric properties of the nanocomposite were studied. This ferroelectric to paraelectric transition exhibited at 323 K makes the ferroelectric material suitable for various applications. Nanofibers possessing ferroelectric properties could be used as potential candidates in nonvolatile ferroelectric random access memories, ferroelectric capacitors, RAM for computers and RFID cards.