Analysis on Spectroscopic and Dielectric Study of PbS/PVA Polymer Nanocomposite via Facile

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Analysis on Spectroscopic and Dielectric Study of PbS/PVA Polymer Nanocomposite via Facile Hydrothermal Process 1 S. Sharon Tamil Selvi 1, J. Mary Linet1,a 1 – Department of Physics, Loyola College, Chennai, India a – linet.mary@gmail.com DOI 10.2412/mmse.11.28.806 provided by Seo4U.link

Keywords: lead sulphide, polyvinyl alcohol, hydrothermal technique, electrical property.

ABSTRACT. Lead Sulfide (PbS) nanoparticles have received much attention owing to their attractive nonlinear optical properties. The present study reports the synthesis of PbS/ Polyvinyl alcohol (PVA) nanocomposite through hydrothermal technique and characterized byX-ray diffraction (XRD), High Resolution Transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), UV-Visible spectroscopy (UV-Vis) and Dielectric analysis. XRD spectra revealed the formation of cubic phase of PbS nanoparticles in PVA polymer matrix with average crystallite size was found to be 28 nm. HRTEM analysis confirmed the formation of cubic particles with the average diameter of 30 ± 2.45 nm.

FTIR spectra confirmed the presence of organic molecules on the PbS nanoparticles. The UV-vis absorption spectra of the PbS/PVA nanocomposite exhibit a significant blue shift from bulk PbS. The electrical property of the material was studied briefly using dielectric measurements and it reveals that the dielectric constant of PbS in the PVA matrix is maximum at lower frequency and decreases with increase in frequency. The higher value of dielectric loss at lower frequency and the decrease of dielectric loss with frequency are due to the free charge motion within the material. AC conductivity of PbS in polymer matrix increases with increase in frequency.

Introduction. Nanoscience is concerned with the study of the unique properties of matter at its nano level and it utilizes to craft novel structures, devices and systems. The usage of nanoparticles as polymer fillers relates to the well-built contemporary interest in progress and application of novel materials [1]. Polymer nanocomposites are diverse and versatile functional materials in which nanoscale inorganic particles are dispersed in an organic polymer matrix to enhanced optical, mechanical, magnetic, and optoelectronic properties [2]. Prologue of stabilizers persuade on the chemical properties and physical properties of semiconductor materials. Capping agents with strong binding molecule form dense layer on the particle surface that stabilizes nanoparticles better, while weak binding molecule consequence fast particle growth leading to large nanoparticles size and aggregation [3]. Hence, the choice of a pertinent capping agent and its concentration becomes the requirement for particle size regime, stabilization against aggregation and high quantum yield during synthesis of nanoparticles [4]. The semiconductor materials have attracted fabulous curiosity owing to its size and shape dependent optical and electronic properties. Among them, Lead sulphide (PbS) is an significant IV–VI semiconductor owe to its narrow band gap (0.41 eV) and large Bohr excitonic radius (18 nm), which leads to potential applications in electroluminescence devices, infrared (IR) detectors, solar absorbers, Pb2+ ion selective sensor and photography [5]. Besides, it has extensive applications in optical devices such as optical switch due to its non-linear optical properties.

Properties of PVA (polyvinyl alcohol) like the transparency over the whole visible spectrum, good adhesion to hydrophilic surfaces, formation of oxygen resistant films and water soluble makes a good choice for the fabrication of optical devices and colloidal stabilizer [6]. For the materialization of PVA/PbS nanoparticles facile hydrothermal method was utilized and it is well known that the hydrothermal technique is an environment affable method for preparation of materials since reactions 1

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

are carried out in a sealed container. Hence, the present work aims at the hydrothermal synthesis of PbS nanoparticles in PVA matrix. To emphasize the structural, optical morphological and dielectric properties of PbS–PVA nanoparticles, the characterizations like XRD, UV, FTIR, HRTEM and dielectric studies have been studied and discussed in detail.

Materials and synthesis. All reagents such as Lead (II) acetate trihydrate ((Pb(CH3COO))2* 3H2O), Thiourea CH4N2S pure (Merk), (C6H15NO3) and PVA (polyvinyl alcohol) were used as received.

Deionized water was used as solvent.

Synthesis of PbS nanoparticles using PVA as capping agent. Initially, 2g of PVA dissolved in 10 ml of deionized water and stirred by a magnetic stirrer at 70 °C for 2 hours. (Pb(CH 3COO))2* 3H2O (0.2 M) was taken together with 30 ml of deionized water, and 0.2M of CH4N2S was also taken and mixed with another 30 ml of deionized water separately. The above two solutions were then mixed and followed by the as prepared PVA solution. The final solution was later transferred to the Teflonlined autoclave, placed inside a furnace at a temperature of 180˚C for 10 h and then gradually cooled to room temperature. The precipitate was then filtered and washed repeatedly with water and ethanol to remove any non-reacted chemicals or impurities. The final products were then dried in air at 100 ˚C for 4h and collected for characterization.

Results and discussion. X-Ray diffraction studies:The crystallographic phases of the PbS nanoparticles within the matrices of capping group PVA was shown in Figure 1. The diffractogram exhibits several peaks pertaining to the (111), (200), (220), (311), (222), (400) and (331) planes of the cubic phase of PbS according the JCPDS NO 78-1901 [7]. The lattice parameters thereby calculated was found to the a=b=c 5.931 Å corresponding to the reported values of PbS. The average crystallite size of the PVA lead sulphide particle was estimated by the Scherrer equation [8].

D= kλ/βcosθ

(1)

where λ – is the wavelength of the Cu kα X-rays (1.5405 Å), β – is the full width at half maximum (FWHM) of the observed peak.

20

(331)

(222)

30

(400)

40

(311)

(111)

50

PVA/PbS

(PVA)

INTENSITY (COUNTS)

60

(220)

(200)

The crystallite size was found to be 28 nm for the samples prepared with capped PVA.

10 0 10

20

30

40

50

60

70

2 THETA (DEGREE)

Fig. 1. XRD pattern of PVA capped PbS nanoparticles.

Morphological characterization. Fig. 2 (a and b) shows the HRTEM micrographs of the cubic PbS/PVA nanocomposite, which suggests that the hydrothermal synthesis is a potential method and MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

good uniformity of the particles obtained. The statistical information of the mean particle size of nanoparticles was determined to be 30 ± 2.45 nm that agrees well with the crystallite size calculated from XRD. Fig. 2 (c) shows the selected area diffraction pattern of as synthesized PVA capped PbS NPs. The presence of dark and light spots in the diffraction pattern indicates the formation of small nanoparticles. Polymer matrix played an important role in the formation of cubic NPs.

a)

b)

c)

Fig. 2. HRTEM images of PVA capped PbS NPs. (a) 50 nm, (b) 20 nm, (c) SAED pattern.

UV-vis absorption spectroscopy. The visible spectra of the PbS nanoparticles capped with PVA is shown in Fig. 3. From the UV-vis absorption spectra of the samples were recorded in the wavelength range of 200 to 800 nm. An optical band gap is obtained by the following equation with the help of absorption spectra [9]. (αhυ)1/n = A(hυ - Eg)

(2)

To determine the energy band gap, (αhυ)2 vs (hυ) was plotted. Where ‘α’ is the absorption coefficient, hυ is the photon energy, A is a constant, Eg is the band gap and ‘n’ is ½ for the direct transition. Thus, a plot of (αhυ)2 vs (hυ) is a straight line whose intercept on the energy axis gives the energy gap, such a representation is known as the Tauc Method. The band gap energy of PVA capped PbS nanoparticles was found to be 4.17 eV. According to the above equation, the energy gap of PVA capped PbS nanoparticles was shown in Fig. 3 (b). PbS nanoparticles exhibit larger blue-shift due to the size confinement below of bulk excitonic Bohr radius (18 nm). It is significant shifted towards blue from its bulk counterparts (0.41 eV) PbS [5]. In the present work, it is clearly seen that the PVA matrix have played a vital role in the quantum confinement.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

80

0.5 0.4 0.3 0.2



0.1 200

PbS/PVA= 4.17 eV

70

hcmeV

Absorbance (a.u)

0.6

300

400

500

600

700

60 50 40 30 20 10 0

800

0

1

2

Wavelength (nm)

3

4

5

6

7

hev)

a)

b)

Fig. 3. (a) Absorption spectra, (b) Tauc plot of PbS/PVA nanocomposite.

Fourier transform infrared spectroscopy: The Figure 4 shows the FTIR spectrum of PVA capped PbS nanoparticles. The absorption peak at 1092 cm-1 indicates the presence of lead sulphide (PbS).

Water has been used as solvent in the synthesis process. So that the hydroxyl group (OH) is occurred predominantly (2400 -3400 cm-1) which indicates the moisture surroundings of nanoparticles. 1412 cm-1 indicates the presence of N-O symmetric stretch and C-N stretch. Br-stretching (Wave number 637.98 cm-1) indicates that mixing of KBr with lead sulphide nanoparticles for making pellet while FTIR analyses [10], [11].

Transmittence %

100 80 60

2362

637.98

40 1245

20

2920

1092 1412

3369

1609

0 500

1000

1500

2000

2500

3000

3500

4000

Wavelength cm-1

Fig. 4. Shows the FTIR spectrum of PVA capped PbS nanoparticles.

Dielectric Studies.

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7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5

8

O

50 C O

100 C O

200 C

1

2

3

4

5

6

50 °C 100 °C 150 °C 200 °C

7

O

150 C

Dielectric Loss (tan)

Dielectric constant (r)

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

6 5 4 3 2 1 0

7

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Log f

Log f

a)

b)

Fig. 5. (a) Log f versus Dielectric constant, (b) Dielectric Loss with Log f for PVA capped PbS.

0.000018

50 C

0.000016

100 C

0

AC conductivity (ac)

0

0

150 C

0.000014

0

200 C

0.000012 0.000010 0.000008 0.000006 0.000004 0.000002 0.000000

-0.000002 1

2

3

4

5

6

7

Log f

Fig. 6. Log f versus AC conductivity for PVA capped lead sulphide.

The dielectric constant (εr) of the material was calculated for different frequencies from the measured capacitance values. The plot of the dielectric constant versus log f is shown in Fig. 5 (a). It is observed that the dielectric constant has high value in the low frequency region and thereafter decreases with the applied frequency. The high value of (εr) at low frequencies may be due to the presence of all the four polarizations namely space charge, orientation and, electronic and ionic polarization and the low values at higher frequencies may be due to the loss of significance of these polarizations gradually.

The higher value of dielectric loss at lower frequency and the decrease of dielectric loss with frequency are due to the free charge motion within the material (Fig. 5 (b). The AC electrical conductivity was determined using the relation: σac = ω εo εr tan δ (ω = 2πf, f is the frequency) With the high AC resistance, it can be mentioned that the space charge polarization plays an important role in the electrical property of the sample [11].

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Summary. The PbS/PVA nanocomposites have been successfully synthesized using hydrothermal method. The structural and microscopic investigations of the sample indicate formation of clustered cubic PbS nanoparticles with cubic crystal structure. The UV analysis demonstrates that the nanostructures were blue shifted compared with the bulk PbS’s band gap which may be due to exciton confinement. The measurement of the dielectric properties of PbS NPs provides evidence of polarization effects.

Acknowledgement Authors are grateful to UGC minor research project [MRP-5666/15 (SERO/UGC)] for providing financial support to undertake this work.

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