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Shock wave induced anatase to rutile TiO2 phase transition using pressure driven shock tube
Materials Letters 219 (2018) 72–75
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
Shock wave induced anatase to rutile TiO2 phase transition using pressure driven shock tube S. Kalaiarasi, A. Sivakumar, S.A. Martin Britto Dhas, M. Jose ⇑ Department of Physics, Sacred Heart College (Autonomous), Tirupattur-635601, India
a r t i c l e
i n f o
Article history: Received 11 November 2017 Received in revised form 24 January 2018 Accepted 14 February 2018 Available online 14 February 2018 Keywords: Nanoparticles Phase-transformation FT-Raman X-ray techniques Crystal structure
a b s t r a c t Pressure driven shock tube was employed to switch TiO2 nanoparticles from anatase to rutile phase at an applied pressure of about 2.683 MPa. The crystal structure and surface morphology of the anatase-rutile phase transformation are investigated for pre and post loaded shock using X-ray diffraction, FT – Raman spectroscopy and transmission electron microscopy respectively. The results from the experimental investigations elucidated that the TiO2 nanoparticles under post shock facilitated the formation of new crystallographic phase (rutile) from anatase phase. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Interest in titanium dioxide has received tremendous interest among research community and industrialists in energy and environmental catalysis owing to its fascinating properties at nano dimensions [1]. Most importantly, TiO2 has found its utility in photocatalyst, chemical sensor, dielectric materials for capacitors, filters, ceramics, cosmetics and fiber manufacturing, etc [2]. Reports available in the literatures indicate that, interaction of materials with shock heated gas leads to formation of a new solid or stabilization of a material in new crystallographic phase. Phase transformation can be induced in materials under extreme pressure and temperature due to application of shock waves for a very short duration of time [3]. Phase transformation of Titanium dioxide nanoparticles (TiO2 NPs) using shock waves with different shock wave loading techniques were employed to modify its structure. Interesting crystallographic phase transformation from anatase TiO2 to N doped rutile TiO2 was demonstrated using shock compressed nitrogen gas at high temperature for short duration of 3.5 ms [4]. TiO2 NPs is found to transform from tetragonal rutile to the monoclinic baddeleyite structure between 20 and 30 GPa [5]. The morphology-tuned structural phase transition from anatase to baddeleyite phase under high pressure was found in the anatase TiO2 structure [6–7]. In this paper, we have demonstrated the impact of shock waves on the morphology and structural changes from anatase to rutile phase transition of TiO2 NPs. The pressure induced phase transi⇑ Corresponding author. E-mail address: [email protected] (M. Jose). https://doi.org/10.1016/j.matlet.2018.02.064 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
tions in TiO2 NPs is studied using powder XRD analysis and FT – Raman Spectroscopy and the corresponding morphological evolution is studied using transmission electron microscopy (TEM) analysis. The findings suggest that the anatase phase of TiO2 was stable upto 60 shocks of Mach number 2.7. Further, the stability of anatase phase below 90 shocks at applied pressure (2.367 Mpa) and temperature (987 K) was demonstrated. However, when the number of shocks reaches 90, the anatase TiO2 undergoes shock induced phase transitions to rutile which could be due to the lattice dynamical instabilities caused by the applied shock. 2. Synthetic procedure for material preparation Typical procedure was adopted to prepare the TiO2 NPs [8]. Aqueous titanium citratocomplex solution was prepared by dissolving titanium isopropoxide in anhydrous citric acid solution at 1 mol concentration and the mixture was stirred for 30 min. Then 25 wt% aqueous Hexamethylenetetramine and ammonium was added drop wise to adjust the pH at 6.0 and the above solution was transferred into teflon lined autoclave maintained at 200 °C for 12 h in an electric hot air oven. The product was decanted with ethanol and thereafter the resultant product was calcinated at 600 °C for further analyses. 3. Experimental description Indigenously designed semi automated table top pressure driven shock tube (PDST) capable of producing shock waves of about
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S. Kalaiarasi et al. / Materials Letters 219 (2018) 72–75
Fig. 1. Schematic diagram of pressure driven shock tube.
Fig. 2. X-Ray diffraction intensities of TiO2 NPs.
1–5 Mac numbers is shown in Fig. 1. Input pressure unit is employed to raise the pressure in driver section and at a critical pressure, the diaphragm raptures, consequently, shock wave propagates into the driven section and tends to impact the freshly prepared TiO2 NPs which is placed 1 cm apart from the open end of the driven section. For this investigation, the sample is loaded with number of shock waves viz,. 30, 60 and 90 with Mach number 2.7 at applied dynamic pressure (2.367 MPa) and temperature (987 K) and the influence of shock waves on the structural and morphology changes of the TiO2 nanoparticles was systematically analyzed. 4. Result and discussion 4.1. Powder XRD analysis and Raman spectroscopy analysis Investigations of XRD analysis (Fig. 2a) indicates that as synthesized TiO2 NPs comprises of mixed phase with a characteristic anatase phase at 2h = 25.12°(1 0 1), 37.9°(0 0 4), 47.6°(2 0 0), 53.4°(1 0 5), 54.08°(2 1 1), 62.8°(2 0 4), 68.7°(1 1 6), 70.3°(2 2 0),75.05°(2 1 5) and 82.3°(3 0 3) and a reflection of brookite peak at 2h = 43.22°(2 1 2) (JCPDS No. 86-1157, space group = I41/amd, lattice parameter, a, b = 3.7913(3) and c = 9.527(2) Å). XRD patterns obtained for the sample with shock number at 30 and 60 do not show any additional peaks, however, complete relapse of the brookite peak takes place (Fig. 2b and c). Less impact over reduction in interlayer spacing and the collapse of the atomic particles endures the formation
Fig. 3. Raman spectra of TiO2 NPs.
of phase pure anatase TiO2 NPs and anatase phase remains stable upto 60 numbers of shocks. Interestingly, when the number of shocks is increased to 90, anatase phase disappears completely and rutile phase is evolved which is evident by the appearance of diffraction peaks (Fig. 2d) corresponding to the planes (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0) and (3 0 1) (JCPDS No.87-0920, space group: P42/mnm, lattice parameter a, b = 4.591(3) Å, and c = 2. 959(2) Å). The sharp and intense peaks suggest that transformed rutile TiO2 is highly crystalline. The calculated grain size of as prepared TiO2 NPs is 21 nm and shock induced anatase phase TiO2 NPs using Scherrer equation at shock 30, 60, 90, is 18 nm, 31 nm and 44 nm respectively, which clearly elucidates the impact of shock in phase transformation. The shocks induce the grain growth of the particle, as demonstrated by TEM and crystallite sizes analyses from Scherrer equation. Thus, when it reaches the critical particle size, from the thermodynamic point of view, anatase transforms to rutile. This happens because the phase stability in nanoscale is directly related to the grain sizes of the particles. Fig. 3(a–d) shows the Raman spectra of as prepared and post shock loaded TiO2 NPs. Initially, the as synthesized TiO2 NPs contains mixed phase as evidenced from the appearance of four Raman modes at 142 cm 1, 397 cm 1, 515 cm 1, 638 cm 1 corresponding to anatase phase and one peak corresponding to brookite phase at 194 cm 1 (Fig. 3a) The high intensity peak at 142 cm 1 is in good agreement with the Raman spectrum of polycrystalline anatase TiO2 which is due to the linear combination of asymmetric bending of OATiAO bonds
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S. Kalaiarasi et al. / Materials Letters 219 (2018) 72–75
Fig. 4. (a–l). TEM Micrographs of TiO2 NPs at pre and post shock condition. (a–c) As prepared, (d–f) – 30 shocks, (g–i) – 60 shocks and (j–l) – 90 shocks.
respectively [9]. Eventhough the spectra recorded for the TiO2 NPs with shock numbers at 30 and 60 (Fig. 3b and c) do not reflect any additional peaks, the brookite peak indexed for the as prepared
TiO2 NPs with vibration band at 194 cm 1 seem to disappear eventually. As the number of shock pulse is increased to 90 (Fig. 3d), the bands at 145, 445 and 620 cm 1 increasingly sharpen substantiat-
S. Kalaiarasi et al. / Materials Letters 219 (2018) 72–75
75
ing the rutile phase transformation without forming any intermediate structures which can be ascribed to the asymmetric bending of OATiAO bonds at rutile phase (1 1 0), caused due to the reverse movement of O atoms created by the accelerated shock across the OATiAO bond [10].
impact of internal energy advancing catastrophic damage which can be evidenced by the exhibited diffraction spacing from Fig. 4 (c, f, i and l).
4.2. Morphological analysis
Shock wave influenced phase transition in TiO2 NPs was experimentally demonstrated. Structural transformation from anatase and rutile phase of TiO2 was investigated using powder XRD, FT – Raman spectra and TEM analysis. The findings suggest that the anatase phase TiO2 was stable till 60 shocks of Mach number 2.7 and becomes rutile phase completely by loading of 90 shock pulses. Below 90 shocks, the anatase phase was very stable even at applied pressure (2.367 Mpa) and temperature (987 K). When the number of shocks reaches 90, it was very clear that the anatase phase of TiO2 undergoes shock induced phase transitions which is due to the lattice dynamical instabilities caused by the applied shock.
Fig. 4(a–c) shows the TEM images and the SAED patterns of as prepared TiO2 NPs without the acceleration of shock wave exhibited tetragonal like morphology and the calculated interplanar spacing is 3.51 Å (1 0 1) which corresponds to anatase phase TiO2 NPs. Lattice fringes of shock treated TiO2 NPs at shock number 30, 60 and 90 evidenced from Fig. 4(e, h and k) show the spatial relationship between the calculated d-spacing corresponding to anatase and rutile phase respectively, which are in good agreement with JCPDS card no. 86-1157 and 87-0920. Moreover, existence of rutile phase was ascertained by SAED pattern (Fig. 4l) with interplanar spacing of 3.26 Å (1 1 0) which is evidently discerned by diffraction spots of reciprocal lattice indicating the reduction of d-spacing’s due to shock further enhancing the shrinkage of unit cells. Statistical analyses of transmission micrographs depict the crystallite size of the nanoparticles. However, the as prepared samples have tetragonal subunits, up to approximately 30 nm in size and it is very clear that particulate size decreases to 26 nm when loaded with 30 shock pulse. Obviously, the crystallite size seems to increase significantly to 44 nm with increased shock numbers to 60. Nucleation sites that are closely associated at the interfaces of anatase nanoparticles undergo phase transformation to rutile at shock 90 with size of about 57 nm (Fig. 4j and k). Further, under concurrent pressure (2.367 MPa) and temperature (987 K), unstable atomic sites undergo phase change inorder to reduce the
5. Conclusions
References [1] Yu. Bai, Ivan Mora-Sero, Filippo De Angelis, Juan Bisquert, Peng Wang, Chem. Rev. 114 (2014) 10095–10130. [2] Jing Bai, Baoxue Zhou, Chem. Rev. 114 (2014) 10131–10176. [3] K. Vasu, H.S.S.R. Matte, Sharmila N. Shirodkar, V. Jayaram, K.P.J. Reddy, Umesh V. Waghmare, C.N.R. Rao, Chem. Phys. Lett. 582 (2013) 105–109. [4] V. Jayaram, Preetam Singh, K.P.J. Reddy, J Adv. Ceram. 3 (2014) 297–305. [5] J. Staun Olsen, Gerward, J.Z. Jiang, High Pressure Res. 22 (2002) 385–389. [6] Q.J. Li, B.Y. Cheng, X. Yang, R. Liu, B. Liu, J. Liu, Z.Q. Chen, B. Zou, T. Cui, B.B. Liu, J. Phys. Chem. C. 117 (2013) 8516–8521. [7] Quanjun Li, Ran Liu, Tianyi Wang, Xu. Ke, Qing Dong, Bo Liu, Jing Liu, Bingbing Liu, AIP Adv. 5 (2015) 097128–097136. [8] S. Kalaiarasi, M. Jose, Appl. Phys. A. 123 (2017) 512. [9] Otakar Frank, Marketa Zukalova, Barbora Laskova, Jenö Kürti, János Koltaib, Ladislav Kavan, Phys. Chem. Chem. Phys. 14 (2012) 14567–14572. [10] T. Mazza, E. Barborini, P. Piseri, P. Milani, Phy. Rev. B. 75 (2007) 0454161– 0454166.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
Shock wave induced anatase to rutile TiO2 phase transition using pressure driven shock tube S. Kalaiarasi, A. Sivakumar, S.A. Martin Britto Dhas, M. Jose ⇑ Department of Physics, Sacred Heart College (Autonomous), Tirupattur-635601, India
a r t i c l e
i n f o
Article history: Received 11 November 2017 Received in revised form 24 January 2018 Accepted 14 February 2018 Available online 14 February 2018 Keywords: Nanoparticles Phase-transformation FT-Raman X-ray techniques Crystal structure
a b s t r a c t Pressure driven shock tube was employed to switch TiO2 nanoparticles from anatase to rutile phase at an applied pressure of about 2.683 MPa. The crystal structure and surface morphology of the anatase-rutile phase transformation are investigated for pre and post loaded shock using X-ray diffraction, FT – Raman spectroscopy and transmission electron microscopy respectively. The results from the experimental investigations elucidated that the TiO2 nanoparticles under post shock facilitated the formation of new crystallographic phase (rutile) from anatase phase. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Interest in titanium dioxide has received tremendous interest among research community and industrialists in energy and environmental catalysis owing to its fascinating properties at nano dimensions [1]. Most importantly, TiO2 has found its utility in photocatalyst, chemical sensor, dielectric materials for capacitors, filters, ceramics, cosmetics and fiber manufacturing, etc [2]. Reports available in the literatures indicate that, interaction of materials with shock heated gas leads to formation of a new solid or stabilization of a material in new crystallographic phase. Phase transformation can be induced in materials under extreme pressure and temperature due to application of shock waves for a very short duration of time [3]. Phase transformation of Titanium dioxide nanoparticles (TiO2 NPs) using shock waves with different shock wave loading techniques were employed to modify its structure. Interesting crystallographic phase transformation from anatase TiO2 to N doped rutile TiO2 was demonstrated using shock compressed nitrogen gas at high temperature for short duration of 3.5 ms [4]. TiO2 NPs is found to transform from tetragonal rutile to the monoclinic baddeleyite structure between 20 and 30 GPa [5]. The morphology-tuned structural phase transition from anatase to baddeleyite phase under high pressure was found in the anatase TiO2 structure [6–7]. In this paper, we have demonstrated the impact of shock waves on the morphology and structural changes from anatase to rutile phase transition of TiO2 NPs. The pressure induced phase transi⇑ Corresponding author. E-mail address: [email protected] (M. Jose). https://doi.org/10.1016/j.matlet.2018.02.064 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
tions in TiO2 NPs is studied using powder XRD analysis and FT – Raman Spectroscopy and the corresponding morphological evolution is studied using transmission electron microscopy (TEM) analysis. The findings suggest that the anatase phase of TiO2 was stable upto 60 shocks of Mach number 2.7. Further, the stability of anatase phase below 90 shocks at applied pressure (2.367 Mpa) and temperature (987 K) was demonstrated. However, when the number of shocks reaches 90, the anatase TiO2 undergoes shock induced phase transitions to rutile which could be due to the lattice dynamical instabilities caused by the applied shock. 2. Synthetic procedure for material preparation Typical procedure was adopted to prepare the TiO2 NPs [8]. Aqueous titanium citratocomplex solution was prepared by dissolving titanium isopropoxide in anhydrous citric acid solution at 1 mol concentration and the mixture was stirred for 30 min. Then 25 wt% aqueous Hexamethylenetetramine and ammonium was added drop wise to adjust the pH at 6.0 and the above solution was transferred into teflon lined autoclave maintained at 200 °C for 12 h in an electric hot air oven. The product was decanted with ethanol and thereafter the resultant product was calcinated at 600 °C for further analyses. 3. Experimental description Indigenously designed semi automated table top pressure driven shock tube (PDST) capable of producing shock waves of about
73
S. Kalaiarasi et al. / Materials Letters 219 (2018) 72–75
Fig. 1. Schematic diagram of pressure driven shock tube.
Fig. 2. X-Ray diffraction intensities of TiO2 NPs.
1–5 Mac numbers is shown in Fig. 1. Input pressure unit is employed to raise the pressure in driver section and at a critical pressure, the diaphragm raptures, consequently, shock wave propagates into the driven section and tends to impact the freshly prepared TiO2 NPs which is placed 1 cm apart from the open end of the driven section. For this investigation, the sample is loaded with number of shock waves viz,. 30, 60 and 90 with Mach number 2.7 at applied dynamic pressure (2.367 MPa) and temperature (987 K) and the influence of shock waves on the structural and morphology changes of the TiO2 nanoparticles was systematically analyzed. 4. Result and discussion 4.1. Powder XRD analysis and Raman spectroscopy analysis Investigations of XRD analysis (Fig. 2a) indicates that as synthesized TiO2 NPs comprises of mixed phase with a characteristic anatase phase at 2h = 25.12°(1 0 1), 37.9°(0 0 4), 47.6°(2 0 0), 53.4°(1 0 5), 54.08°(2 1 1), 62.8°(2 0 4), 68.7°(1 1 6), 70.3°(2 2 0),75.05°(2 1 5) and 82.3°(3 0 3) and a reflection of brookite peak at 2h = 43.22°(2 1 2) (JCPDS No. 86-1157, space group = I41/amd, lattice parameter, a, b = 3.7913(3) and c = 9.527(2) Å). XRD patterns obtained for the sample with shock number at 30 and 60 do not show any additional peaks, however, complete relapse of the brookite peak takes place (Fig. 2b and c). Less impact over reduction in interlayer spacing and the collapse of the atomic particles endures the formation
Fig. 3. Raman spectra of TiO2 NPs.
of phase pure anatase TiO2 NPs and anatase phase remains stable upto 60 numbers of shocks. Interestingly, when the number of shocks is increased to 90, anatase phase disappears completely and rutile phase is evolved which is evident by the appearance of diffraction peaks (Fig. 2d) corresponding to the planes (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0) and (3 0 1) (JCPDS No.87-0920, space group: P42/mnm, lattice parameter a, b = 4.591(3) Å, and c = 2. 959(2) Å). The sharp and intense peaks suggest that transformed rutile TiO2 is highly crystalline. The calculated grain size of as prepared TiO2 NPs is 21 nm and shock induced anatase phase TiO2 NPs using Scherrer equation at shock 30, 60, 90, is 18 nm, 31 nm and 44 nm respectively, which clearly elucidates the impact of shock in phase transformation. The shocks induce the grain growth of the particle, as demonstrated by TEM and crystallite sizes analyses from Scherrer equation. Thus, when it reaches the critical particle size, from the thermodynamic point of view, anatase transforms to rutile. This happens because the phase stability in nanoscale is directly related to the grain sizes of the particles. Fig. 3(a–d) shows the Raman spectra of as prepared and post shock loaded TiO2 NPs. Initially, the as synthesized TiO2 NPs contains mixed phase as evidenced from the appearance of four Raman modes at 142 cm 1, 397 cm 1, 515 cm 1, 638 cm 1 corresponding to anatase phase and one peak corresponding to brookite phase at 194 cm 1 (Fig. 3a) The high intensity peak at 142 cm 1 is in good agreement with the Raman spectrum of polycrystalline anatase TiO2 which is due to the linear combination of asymmetric bending of OATiAO bonds
74
S. Kalaiarasi et al. / Materials Letters 219 (2018) 72–75
Fig. 4. (a–l). TEM Micrographs of TiO2 NPs at pre and post shock condition. (a–c) As prepared, (d–f) – 30 shocks, (g–i) – 60 shocks and (j–l) – 90 shocks.
respectively [9]. Eventhough the spectra recorded for the TiO2 NPs with shock numbers at 30 and 60 (Fig. 3b and c) do not reflect any additional peaks, the brookite peak indexed for the as prepared
TiO2 NPs with vibration band at 194 cm 1 seem to disappear eventually. As the number of shock pulse is increased to 90 (Fig. 3d), the bands at 145, 445 and 620 cm 1 increasingly sharpen substantiat-
S. Kalaiarasi et al. / Materials Letters 219 (2018) 72–75
75
ing the rutile phase transformation without forming any intermediate structures which can be ascribed to the asymmetric bending of OATiAO bonds at rutile phase (1 1 0), caused due to the reverse movement of O atoms created by the accelerated shock across the OATiAO bond [10].
impact of internal energy advancing catastrophic damage which can be evidenced by the exhibited diffraction spacing from Fig. 4 (c, f, i and l).
4.2. Morphological analysis
Shock wave influenced phase transition in TiO2 NPs was experimentally demonstrated. Structural transformation from anatase and rutile phase of TiO2 was investigated using powder XRD, FT – Raman spectra and TEM analysis. The findings suggest that the anatase phase TiO2 was stable till 60 shocks of Mach number 2.7 and becomes rutile phase completely by loading of 90 shock pulses. Below 90 shocks, the anatase phase was very stable even at applied pressure (2.367 Mpa) and temperature (987 K). When the number of shocks reaches 90, it was very clear that the anatase phase of TiO2 undergoes shock induced phase transitions which is due to the lattice dynamical instabilities caused by the applied shock.
Fig. 4(a–c) shows the TEM images and the SAED patterns of as prepared TiO2 NPs without the acceleration of shock wave exhibited tetragonal like morphology and the calculated interplanar spacing is 3.51 Å (1 0 1) which corresponds to anatase phase TiO2 NPs. Lattice fringes of shock treated TiO2 NPs at shock number 30, 60 and 90 evidenced from Fig. 4(e, h and k) show the spatial relationship between the calculated d-spacing corresponding to anatase and rutile phase respectively, which are in good agreement with JCPDS card no. 86-1157 and 87-0920. Moreover, existence of rutile phase was ascertained by SAED pattern (Fig. 4l) with interplanar spacing of 3.26 Å (1 1 0) which is evidently discerned by diffraction spots of reciprocal lattice indicating the reduction of d-spacing’s due to shock further enhancing the shrinkage of unit cells. Statistical analyses of transmission micrographs depict the crystallite size of the nanoparticles. However, the as prepared samples have tetragonal subunits, up to approximately 30 nm in size and it is very clear that particulate size decreases to 26 nm when loaded with 30 shock pulse. Obviously, the crystallite size seems to increase significantly to 44 nm with increased shock numbers to 60. Nucleation sites that are closely associated at the interfaces of anatase nanoparticles undergo phase transformation to rutile at shock 90 with size of about 57 nm (Fig. 4j and k). Further, under concurrent pressure (2.367 MPa) and temperature (987 K), unstable atomic sites undergo phase change inorder to reduce the
5. Conclusions
References [1] Yu. Bai, Ivan Mora-Sero, Filippo De Angelis, Juan Bisquert, Peng Wang, Chem. Rev. 114 (2014) 10095–10130. [2] Jing Bai, Baoxue Zhou, Chem. Rev. 114 (2014) 10131–10176. [3] K. Vasu, H.S.S.R. Matte, Sharmila N. Shirodkar, V. Jayaram, K.P.J. Reddy, Umesh V. Waghmare, C.N.R. Rao, Chem. Phys. Lett. 582 (2013) 105–109. [4] V. Jayaram, Preetam Singh, K.P.J. Reddy, J Adv. Ceram. 3 (2014) 297–305. [5] J. Staun Olsen, Gerward, J.Z. Jiang, High Pressure Res. 22 (2002) 385–389. [6] Q.J. Li, B.Y. Cheng, X. Yang, R. Liu, B. Liu, J. Liu, Z.Q. Chen, B. Zou, T. Cui, B.B. Liu, J. Phys. Chem. C. 117 (2013) 8516–8521. [7] Quanjun Li, Ran Liu, Tianyi Wang, Xu. Ke, Qing Dong, Bo Liu, Jing Liu, Bingbing Liu, AIP Adv. 5 (2015) 097128–097136. [8] S. Kalaiarasi, M. Jose, Appl. Phys. A. 123 (2017) 512. [9] Otakar Frank, Marketa Zukalova, Barbora Laskova, Jenö Kürti, János Koltaib, Ladislav Kavan, Phys. Chem. Chem. Phys. 14 (2012) 14567–14572. [10] T. Mazza, E. Barborini, P. Piseri, P. Milani, Phy. Rev. B. 75 (2007) 0454161– 0454166.