Sonochemically prepared PbWO4 tetragonal-bipyramidal microcrystals and their photoluminescence properties

Accepted Manuscript Sonochemically prepared PbWO4 tetragonal-bipyramidal microcrystals and their photoluminescence properties S. Kannan, K. Mohanraj, G. Sivakumar PII: DOI: Reference:

S1386-1425(14)01635-7 http://dx.doi.org/10.1016/j.saa.2014.11.017 SAA 12953

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

20 August 2014 31 October 2014 5 November 2014

Please cite this article as: S. Kannan, K. Mohanraj, G. Sivakumar, Sonochemically prepared PbWO4 tetragonalbipyramidal microcrystals and their photoluminescence properties, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.11.017

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Sonochemically prepared PbWO4 tetragonal-bipyramidal microcrystals and their photoluminescence properties S. Kannan1, K. Mohanraj1,* and G. Sivakumar2 1

Department of Physics, Manonmaniam Sundaranar University, Tirunelveli-627 012, Tamil

Nadu, India 2

CISL, Department of Physics, Annamalai University, Annamalai Nagar-608 002, Tamil

Nadu, India Telephone No. : +91-9788083079, Fax: 91-462-2334363 / 2322973 *

Corresponding author: [email protected]

Abstract Lead tungstate (PbWO4) microcrystals were synthesized for the first time, via different concentrations of PVA assisted sonochemical process. The concentration of PVA act as a structure directing agent and played an important role in the morphological control of resulting PbWO4 microcrystals. The product PbWO4 composing of Pb, W and O and W-O stretching vibration band of WO4 tetrahedrons were confirmed through XRD, FTIR, FESEM and EDS. The TG/DTA curves showed that the particles are crystallized at room temperature itself and the thermal stability of the product is really good. The optical properties of the product shows extradinarily high room temperature photoluminescence intensity compared to without PVA assisted product. Keywords: PbWO4, Photoluminescence, ultrasonication, PVA.

1

1. Introduction Lead compounds are one type of important solid material. Recently, lead tungstate (PbWO4) has received extensive interest due to their technical importance. It is widely used as an inorganic scintillators in high energy physics compared to other well known scintillators PbWO4 is well known for its characteristics properties such as high energy density (8.28 cm-3), short decay time (10 ns), small Molière radius (2.19 cm), low light yield (300 photons/ MeV), short radiation length (0.9 cm), emission maximum of ~410 nm, high irradiation damage resistance (107 rad) as well as low cost [1-4]. At ambient temperature, it is tetragonal (Scheelite or stolzite) structure and rare, it is found monoclinic (respite or Wolframite) structure which transforms irreversibly to the scheelite at ~ 400 ºC. [4-7]. Note that the tetragonal structure is of technological importance due to its interesting excitonic luminescence, thermoluminescence and stimulated Raman scattering behavior [8]. Expensive methods such as Czochralski [9], Bridgman [10], solid state reaction [11] and flux method for crystal growth [12] have been developed for synthesis of PbWO4. However, these methods produce inhomogeneous, coarse with different sizes, non-uniform and containing impurities and also require expensive equipments, high temperature with more processing time. In order to minimize these drawbacks, much effort has been made to synthesis PbWO4 with various morphologies, and several methods such as sonochemical [13, 14, 15], precipitation [16], complex polymerization [17], microwave [18] hydrothermal [19], solvothermal [20], microwave assisted hydrothermal [21] and polyol process [6, 13] and they have explored the scintillating property of PbWO4 by luminescence studies such as thermoluminescence (TL), photoluminescence (PL) and radio luminescence (RL). The luminescence property depends on the size and shape of the product and hence various surfactants have been used [7, 16-18, 22, 23]. However, the influence of surfactants and pH have altered the shape and size of the products [19]. Kaowphong et al. (2010) have studied the influence of pH on crystallinity and luminescence property of sonochemically prepared PbWO4 particles in aqueous medium. To the best of our knowledge, no report is found for synthesis of PbWO4 with the assistance of PVA by sonochemical method. Polyvinyl alcohol (PVA) is a non-ionic surfactant, water soluble, biodegradable molecule and also important to prepare individual nanoparticles and nanocomposites [26, 27]. It can be used as a stabilizer due to their optical 2

clarity which enables the formation of nanoparticles and significantly control the particle size and to form monodisperse size distribution. Hence, objective of the work is aimed to synthesis PbWO4 crystals for different molar concentrations of PVA by sonochemical method. 2.

Experimental procedure In typical synthesis, 1M Lead nitrate Pb(NO3)2 (Himedia, 99%) and 1M Sodium

tungstate dihydrate Na2WO4.2H2O (Merck, 98%) were prepared using 10 mL of distilled water (DW) individually. Then the two solutions were mixed together and stirred well for 15 min to obtain a homogeneous solution. The pH of the solution was adjusted (pH 4) by adding 1M HNO3 and 1M NH4OH solution. The white color resulting mixture was placed in a sonicator (ENERTECH, 33±3 kHz, 60 W) at room temperature for 30 min. The prepared aspowder was collected by centrification, washed several times with DW and absolute ethanol and dried in air atmosphere for 24 h. The different concentration (0, 0.07 mM, 0.11 mM, 0.14 mM and 0.21 mM) of surfactant PVA (Mw = 1,60,000, partially hydrolyzed) assisted samples prepared at the pH 4 similarly following the above procedure. Powder X-ray diffraction (XRD) patterns were recorded on X’Pert PRO diffractometer from monochromatized Cu Kα (λ =1.5406 Å). Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectra were recorded on a Perkin Elmer, Spectrum Two (Model: C92107) spectrometer with a resolution of 4 cm-1. Measurements carried on a Field Emission Scanning Electron Microscope FESEM, FEI-QUANTA-FEG 250 coupled with Energy Dispersive Spectroscopy (EDS) to investigate the morphology and surface roughness of samples. Thermographs (TG/DTA) were recorded using NETZSCH STA 449F3 TG/DTA technique at the heating rate of 20 ºC/min in N2 atmosphere to investigate the thermal stability of the product. Optical spectra were recorded using JASCO V-670 UV-Visible spectrophotometer and LS 45, Photoluminescence spectrometer to analyze the absorption and emission characteristics of the product. 3. Results and discussion Figure 1 (a-e) shows the XRD patterns of as synthesized PbWO4 particles obtained for different PVA concentrations (0, 0.07 mM, 0.11 mM, 0.14 mM and 0.21 mM). Figure 1(a) shows the without PVA assisted PbWO4 product and the remaining four patterns (Figure 1(be)) for the PVA assisted PbWO4 with same pH 4. The XDR patterns of both without and 3

with PVA assisted samples shows similar crystalline peaks at 2θ = 27.40˚, 29.61˚, 32.76˚, 44.77˚, 47.05˚, 51.24˚, 55.32˚, 56.59˚, 61.50˚, 68.75˚, 71.47˚, 72.09˚, 76.58˚ and 78.27˚. These reflections peaks corresponds to the planes of (1 1 2), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (2 2 0), (1 1 6), (3 1 2), (2 2 4), (0 0 8), (4 0 0), (2 0 8), (3 1 6), (3 3 2), (4 0 4) and (4 2 0) respectively which can be indexed to the tetragonal PbWO4 with space group of I41/a (JCPDS card: 19-0708). No peaks for PVA (at 19.33˚ and 40.43˚) and other impurity phases are detected in the XRD patterns [29, 30]. The strong and sharp peaks indicate the better crystallinity of the PbWO4. However, the intensity of the crystalline peaks decreased in 0.14 mM of PVA (Figure 1d) in comparison to 0.07 mM and 0.11 mM (Figure 1 (b and c), whereas it again increased in intensity in 0.21 mM PVA (Figure 1e) concentration is attributed to microstructural changes. The Lattice parameters and cell volumes were calculated using “Unit cell” software and their results are reported in Table 1. From table 1, it can be seen that the lattice parameters (a and c) are compressed to increase the PVA concentrations except 0.21 mM PVA. The increase of PVA concentrations promotes a rise in particle size. This behaviour can be associated with the aggregates production and nuclei formation, promoting the expansion of cell volume. Crystallite size of as-prepared particles were calculated using Scherrer formula D= 0.9λ / βCosθ, where D is crystallite size, λ is wavelength of incident radiation, β is full width half maximum of the major peak after the subtraction of instrumental broadening, and θ is diffraction angle. The calculated values are reported in table 1. Figure 2 (a-e) shows the ATR-FTIR spectra of the PbWO4 prepared for 0, 0.07 mM, 0.11 mM, 0.14 mM and 0.21 mM concentrations of PVA respectively. In the vibrational spectra (Figure 2(a-e)) a strong broad band observed at 744 cm-1 is assigned to W-O stretching vibration of WO 24− tetrahedra [15]. In high PVA concentration (Figure 2e) assisted PbWO4, two humps observed around 3364 cm-1 and 1645 cm-1 is due to OH stretching and OH bending respectively. Whereas these humps are not observed in the lower PVA concentration assisted PbWO4 samples. In addition, the following bands in pristine PVA (Figure 3a) at 2952 cm-1, 1737 cm-1, 1094 cm-1 and 1249 cm-1 due to asymmetric stretching vibration of CH2, C=O stretching vibration and CH vibration respectively[32, 33] which are not observed in low concentration of PVA assisted PbWO4 samples (Figure 2 (b-d)). It is attributed to more degradation of PVA in aqueous medium, which is acquired by the cleavage of the chemical chains by cavitation. Whereas in high PVA concentration, the components of 4

PVA is appeared due to low degradation by the overlapping of chemical chains reduces the efficiency of the cavitation [34]. The presence of PbWO4 analyzed by XRD and W-O stretching analyzed by FTIR is in good accordance. The morphologies of the as-prepared were examined by FESEM. Figure 4 (a & b) shows that the products consists of a large quantity of facets-like particles. The average size is about 126 nm and clearly visible the individual PbWO4 particles in higher magnification image. The observed morphological feature is similar to that of the early report by Kaowphong et al. (2010). It is interesting to note that tetragonal bipyramid particles are found in lower concentrations in PVA assisted samples (in Figure 4 (c-f)). The particle shape changes obviously visible with increases the PVA concentrations. In the case of higher PVA concentration, the tetragonal bipyramid like particles (Figure 4 (g & h)) exhibit agglomeration form and the average particle size are given in Table 1. It is obviously evidenced that the addition of PVA influenced to change the shape and reduce the size of the particles. Besides, the images are shown less homogenous feature may be due to low ultrasonic power [14]. In mixture aqueous solution, the dissolved WO 24− ions reacts with Pb2+ ions to from, white color precipitates as an indication of PbWO4 formation. It can be understood from the equations (1) and (2). Pb2+(aq.) + NO 32− (aq.) + Na2+(aq.) + WO 24− (aq.)

Pb(NO3)2 (aq.) + Na2WO4.2H2O (aq.)

+ 2H2O Pb2+ (aq.) + WO 24−

PbWO4 (s)

(aq.)

-----------(1)

-----------(2)

The EDS spectra of the products are shown in Figure 5 (a & b). The atomic weight percentage of Pb, W and O are found to be 12.50 %, 10.65 % and 76.85% respectively. The report is evidence by the stoichiometric PbWO4 product. The formation of tetragonal bipyramidal PbWO4 particles is schematically illustrated in Figure 6. From the top view of the FESEM images, four fold symmetry is identified which is due to tungsten atom in the oxidation state +6. It is surrounded by four oxygen atoms forming tetrahedron. Then, the remains were occupied by two lead atoms of the square mesh are placed on the centre of the squares of the preceding layer [6]. Faraji et al. (2012) were used hydrazine hydrate as reducing agent along with PVA for controlling the particle size of Ag 5

nanoparticles. But in the present work PVA alone controlled the size of the particles. Figure 7 shows TG/DTA curves of the pure PVA. It can be seen from the TG curve of PVA that a small weight loss 7.4% from ambient to 240 ºC attributed to the evaporation of moisture, condensation of hydroxyl groups and melting of PVA. A major weight loss is observed in the range of 240-400 ºC due to degradation of PVA. In DTA curve, three endothermic humps are observed at 80 ºC, 100 ºC and 186 ºC due to the glass transition, removal of water and melting of PVA respectively. A broad peak observed 280-350 ºC is assigned to combustion of the carbonaceous mass remaining from the PVA [29, 32, 33]. Figure 8 shows the TG and DTA curves of PbWO4 as prepared (0.014 mM PVA). The TG curve shows no significant weight loss from ambient to 900 ºC. In DTA curve, a broad endothermic peak is observed 200-650 ºC attributed to PVA on the surface of PbWO4. TG and DTA curves showed that the particles are crystallized at room temperature itself and the thermal stability of the nanostructures are really good. Absorption spectra of PbWO4 are shown in Figure 9 (a-d). The absorption spectra of both PVA assisted samples showed an absorption edge in 280 nm. The absorption is attributed to a charge transfer transition in which an oxygen 2p electron goes into one of the empty tungsten 5d orbital. The excitation from O2p to Wt2g in the WO 24− group absorb UV radiation in PbWO4. In the excited state of the WO 24− groups, the hole and the electron remain together as an exciton because of their strong interaction. It is also observed that the absorption decreases with increase the PVA concentration. The direct band gap (Eg) of the synthesized PbWO4 product calculated by extrapolating the linear portion of the plot relating (αhν)2 against (hν) to (αhν)2 =0. Figure 10 shows the plots of (αhν)2 vs hν . The Eg values of 4.65 eV for without PVA and it is slightly increased (4.66-4.68 eV) in PVA assisted samples (Table 1) in comparison to 0% PVA, which are higher than reported values of 4.13 eV [6, 17, 19, 21]. The higher Eg values are due to the reduction in particle size, disorder and density of lattice defects [4]. Figure 11 (a-d) shows the room temperature PL emission spectra of the as prepared PbWO4 particles with an excitation wavelength 262 nm. It can be seen that the as prepared PbWO4 particles exhibit blue and green luminescence around 388 nm, 478 nm and 522 nm respectively [16]. The blue component is ascribed to the regular lattice of which the emitting level comprises both lead and tungstate contribution, while the green peak originate from the 6

defect centers associated with oxygen [1, 24]. PL peaks used to find trapping of charge carriers from the valence band to the conduction band. The intensity of the peaks is related to morphology, size and crystallinity of the particles [6, 18]. In the system containing PVA, the blue emission characteristics peak (478 nm) not observed, while prominent peak found at 388 nm decrease with increase the PVA concentration and the trend is in good accordance with the XRD analysis. The blue emissions were not shifted compared to without PVA and to the results reported previously in the literature [6].

4. Conclusions

PbWO4 mirocrystals were synthesized for different concentrations of PVA assisted sonochemical process. XRD patterns indicate pure tetragonal structure of PbWO4 and the result is good accordance with the JCPDS file (card no.: 19-0708) and early reports. The definite existence of tetragonal PbWO4 is confirmed by FTIR vibration. Morphology of the samples indicates that the size and shape of the PbWO4 are varied by the concentration of PVA. The TG/DTA curves showed that thermal stability of the product is really good. The band gap energy is found to be 4.67 eV which is more than the previous literature values (4 eV). The PL emission spectra showed that the absence of the blue emission at 478 nm and decrease of the green emission at 522 nm in as-prepared particles are ascribed to the radioactive recombination of a self–trapped exciton localized on a regular WO4 complex anion.

Acknowledgment

S. K acknowledged his thanks to University Grants Commission (UGC), New Delhi for providing financial support through Basic Scientific Research (BSR) fellowship.

7

References

[1] M. Nikl, phys. stat. sol. (a) 178 (2000) 595-620. [2] P. Adzic, N. Almedia, D. Andelin, I. Anicin, Z. Antunovic, R. Arcidiacono, M.W. Arenton, E. Auffray, S. Argiro, A. Askew, S. Baccaro, S. Biffioni et al., Journal of Instrumentation 5 (2010) P03010. DOI: 10.1088/1748-0221/5/03/P03010. [3] M. Nikl, Meas. Sci. Technol. 17 (2006) R37-R54. [4] J.H. Ryu, J.–W. Yoon, K.B. Shim, N. Koshizaki, Appl. Phys. A 84 (2006) 181-185. [5] M. Itoh, M. Fujita, Physical Review 62 (2000) 12825-12829. [6] T. George, S. Joseph, A.T. Sunny, S. Mathew, J. Nanopart. Res. 10 (2008) 567-575. [7] X. Wang, Y. Ding, Z.L. Wang, C. Hu, J. Appl. Phys. 109 (2011) 124309-5. [8] X.–L. Hu, Y.–J. Zhu, Langmuir 20 (2004) 1521-1523. [9] K. Nitsch, M. Nikl, S. Ganschow, P. Reiche, R. Uecker, J. Crystal Growth 165 (1996) 163-165. [10] K. Tanji, M. Ishii, Y. Usuki, M. Kobayashi, K. Hara, H. Takano, N. Senguttuvan, J. Crystal Growth 204 (1999) 505-511. [11] G. Blasse, L.H. Brixner, Chem. Phys. Lett. 173 (1990) 409-411. [12] N. Senguttuvan, P. Mohan, S.M. Babu, C. Subramanian, J. Crystal Growth 183 (1998) 391-397. [13] H. Tang, Q. Wu, X. Yang, B. Yang, C. Li, Cryst. Res. Technol. 45 (2010) 1094-1098. [14] J. Geng, Y. Lv, D. Lu, J.J. Zhu, Nanotechnology 17 (2006) 2614-2620. [15] A. Phuruangrat, T. Thongtem, S. Thongtem, J. Crystal Growth 311 (2009) 40764081. [16] X. Yang, J. Huang, J. Am. Ceram. Soc. 95 (2012) 3334-3338. [17] L.S. Cavalcante, J.C. Sczancoski, V.C. Albarici, J.M.E. Matos, J.A. Varela, E. Longo, Mater. Sci. Eng. B 150 (2008) 18-25. [18] A. Phuruangrat, T. Thongtem, S. Thongtem, Current Applied Physics 10 (2010) 342345. [19] F. Lei, B. Yan, H.H. Chen, Q. Zhang, J.T. Zhao, Crystal Grow. Des. 9 (2009) 37303736. [20] X. He, M. Cao, Nanotechnology 17 (2006) 3139-3143. [21] P. Kwolek, T. Tokarski, T. Lokcik, K. Szacilowski, Archives of Metallurgy and Materials 58 (2013) 217-222. [22] G. Wang, C. Hao, Y. Zhang, Mater. Lett. 62 (2008) 3163-3166. 8

[23] J. Yang, C. Lu, H. Su, J. Ma, H. Cheng, L. Qi, Nanotechnology19 (2008) 35608 (7pp). [24] C. Yu, F. Cao, X. Li, G. Li, Y. Xie, J.C. Yu, Q. Shu, Q. Fan, J. Chen, Chemical Engineering Journal 219 (2013) 86-95. [25] Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 1389-1395. [26] R.S. Patil, M.R. Kokate, C.L. Jambhale, S.M. Pawar, S.H. Han, S.S. Kolekar, Adv. Nat. Sci. Nanosci. Nanotechnol. 3 (2012) 15013 (7pp). [27] N. Faraji, W.M.M. Yunus, A. Kharazmi, E. Saion, M. Shahmiri, N. Tamchek, J. Europ. Opt. Rap. Public 7 (2012) 12040-7. [28] C.T.S. Turk, Z.S. Bayindir, U. Badilli, J. Fac. Pharm. Ankara 38 (2009), 257-268. [29] S. Kaowphong, T. Thongtem, S. Thongtem, Russian J. Inorg. Chem. 55 (2010) 577582. [30] Z. Guo, D. Zhang, S. Wei, Z. Wang, A.B. Karki, Y. Li, P. Bernazzani, D.P. Young, J.A. Gomes, D.L. Cocke, T. Ho, J. Nanopart. Res. 12 (2010) 2415-2426. [31] R. Ricciardi, F. Auriemma, C.D. Rosa, F. Lauprêtre, Macromolecules 37 (2004) 1921-1927. [32] G. Attia, M.F.H Abd El-kader, Int. J. Electrochem. Sci. 8 (2013) 5672-5687. [33] F.H. Abd El-kader, S.A. Gaafar, M.S. Rizk, N.A. Kamal, J. Appl. Polymer Science 72 (1999) 1395-1406. [34] S.P. Vijayalakshmi, G. Madras, Appl. Polymer Science 100 (2006) 4888-4892.

9

Figure 1. XRD patterns of PbWO4 particles prepared with PVA in the molar concentrations of a) 0 mM, b) 0.07 mM, c) 0.11 mM, d) 0.14 mM and e) 0.21 mM.

Figure 2. ATR-FTIR spectra of PbWO4 particles prepared with PVA in the molar concentrations of a) 0 mM, b) 0.07 mM, c) 0.11 mM, d) 0.14 mM and e) 0.21 mM.

10

Figure 3. ATR-FTIR spectra of (a) pristine PVA, as prepared PbWO4 (b) without PVA and (c) with 0.14 mM PVA concentration.

11

Figure 4. FESEM images of PbWO4 microcrystals prepared with PVA in the molar concentrations of a ) 0 mM, b) 0.07 mM, c) 0.14 mM and d) 0.21 mM. 12

Figure 5. EDS spectra of PbWO4 particles prepared with PVA in the molar concentrations of a) 0 mM and b) 0.14 mM.

Figure 6. Schematic illustration for the formations of PbWO4 polygonal and tetragonalbipyramidal microcrystals.

13

Figure 7. TG/DTA curves of pure PVA.

Figure 8. TG/DTA curves of PbWO4 microcrystals prepared with PVA in the molar concentration of 0.14 mM.

14

Figure 9. Absorption spectra of PbWO4 particles prepared with PVA in the molar concentrations of a) 0 mM, b) 0.07 mM, c) 0.14 mM and d) 0.21 mM.

Figure 10. Plots of (αhν)2 to (hν) for as-prepared PbWO4 microcrystals with PVA in the molar concentrations of a) 0 mM, b) 0.07 mM, c) 0.14 mM and d) 0.21 mM. 15

Figure 11. PL emission spectra of synthesized PbWO4 microcrystals using PVA in the molar concentrations of a) 0 mM, b) 0.07 mM, c) 0.14 mM and d) 0.21 mM.

16

Table 1. Lattice parameters, cell volume, particle size and band gap of synthesized PbWO4 particles. Concentration of precursor solution (M)

PVA

Pb(NO3)2

Na2WO4

(mM)

-

-

-

pH

-

Lattice

Cell

Crystallite

parameters (Å)

volume

size

size

(Å3)

(nm)

(nm)

359.45

-

-

a= 5.4619,

Particle Band gap (eV)

-

c=12.049

1

1

-

12

Reference

JCPDS No: 19-0807

a=5.4602(4),

359.06

79.37

124.9

4.65

This work

359.19

47.63

106.6

4.68

This work

358.73

95.32

-

-

This work

357.57

79.37

108.4

4.68

This work

361.77

68.05

125.8

4.66

This work

c=12.0431(4)

1

1

0.07

4

a=5.4605(2), c=12.0462(3)

1

1

0.11

4

a=5.4580(1), c=12.0421(1)

1

1

0.14

4

a=5.4483(8), c=12.0454(5)

1

1

0.21

4

a=5.4912(8), c=11.9973(4)

17

Figure captions

Figure 1. XRD patterns of PbWO4 particles prepared with PVA in the molar concentrations of (a) 0, (b) 0.07 mM, (c) 0.11 mM, (d) 0.14 mM and (e) 0.21 mM. Figure 2. ATR-FTIR spectra of PbWO4 particles prepared with PVA in the molar concentrations of (a) 0, (b) 0.07 mM, (c) 0.11 mM, (d) 0.14 mM and (e) 0.21 mM. Figure 3. ATR-FTIR spectra of (a) pristine PVA, as prepared PbWO4 (b) without PVA and (c) with 0.14 mM PVA concentration. Figure 4.FESEM images of PbWO4 particles prepared with PVA in the molar concentrations of (a & b) 0, (c & d) 0.07 mM, (e & f) 0.14 mM and (g & h) 0.21 mM. Figure 5. EDS spectra of PbWO4 particles prepared with PVA in the molar concentrations of (a) 0 and (b) 0.14 mM. Figure 6. Schematic illustration for the formations of PbWO4 polygonal and tetragonalbipyramidal microcrystals. Figure 7. TG/DTA curves of pure PVA. Figure 8. TG/DTA curves of PbWO4 microcrystals prepared with PVA in the molar concentrations of 0.14 mM. Figure 9. Absorption spectra of PbWO4 particles prepared with PVA in the molar concentrations of (a) 0, (b) 0.07 mM, (c) 0.14 mM and (d) 0.21 mM. Figure 10. Plots of (αhν)2 to (hν) for as-prepared PbWO4 particles with PVA in the molar concentrations of (a) 0, (b) 0.07 mM, (c) 0.14 mM and (d) 0.21 mM. Figure 11. PL emission spectra of synthesized PbWO4 particles using PVA in the molar concentrations of (a) 0, (b) 0.07 mM, (c) 0.14 mM and (d) 0.21 mM.

Table caption

Table 1. Lattice parameters, cell volume, particle size and band gap of synthesized PbWO4 particles.

18

Supporting Information Sonochemically prepared PbWO4 tetragonal-bipyramidal microcrystals and their photoluminescence properties

S. Kannan1, K. Mohanraj1, * and G. Sivakumar2 1

Department of Physics, Manonmaniam Sundaranar University, Tirunelveli-627 012,

Tamilnadu, India 2

CISL, Department of Physics, Annamalai University, Annamalai Nagar-608 002,

Tamilnadu, India *

Corresponding author: [email protected]

Fig. S1. XRD pattern of pure PVA. References.

1. Ricciardi et al., Macromolecules 37 (2004) 1921-1927. 2. Guo et al., J. Nanopart. Res. 12 (2010) 2415-2426.

19

Figure 3. ATR-FTIR spectra of (a) pristine PVA, as prepared PbWO4 (b) without PVA and (c) with 0.14 mM PVA concentration. Wave numbers

- peaks assigned to

3700-3250

- Symmetric vibration of OH groups

2952, 2903

- Asymmetric, symmetric vibrations of CH2 respectively

1737, 1710, 1574

- Stretching vibration of C=O

1645

- Bending vibration of OH

1440

- C-OH groups

1369

- Stretching bands of C-H

1249

- Vibration of CH

1150

- Stretching of C-C-C

1094

- Stretching vibrations of C-O

853

- Stretching vibrations of C-C

620

- Stretching vibrations of OH

References

1. G. Attia, M.F.H. Abd El-Kadar, Int. J. Electrochem. Sci. 8 (2013) 5672-5687. 2. F.H. Abd El-Kadar et al., J. Appl. Polymer Science 72 (1999) 1395-1406. 20

FESEM images

Fig. S3 (a & b). Graphical view point for bipyramidal shape and four fold symmetry on top view.

21

Graphical abstract

Highlights 1. The article is first report on synthesis of PbWO4 microcrystals using PVA by sonochemical route. 2. Three dimensional morphology is obtained at low pH (i.e. pH=4) 3. Prominent and medium intense with narrow emission peaks are found in the PL study. 4. Phase pure, high crystalline, defect less PbWO4 particles are obtained for low

concentration of PVA. 5. The PbWO4 microcrystals were prepared in environmentally friendly with low cost

and less toxicity.