Sm3+:Ag NPs assisted modification in absorption features of magnesium tellurite glass

Journal of Molecular Structure 1079 (2015) 167–172

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Sm3+:Ag NPs assisted modification in absorption features of magnesium tellurite glass N.M. Yusoff, M.R. Sahar ⇑, S.K. Ghoshal Advanced Optical Materials Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

h i g h l i g h t s  Fully amorphous tellurite glass system by controlling the ratio of Sm2O3 and AgCl contents is obtained.  The formation of Ag NPs is stimulated via controlled melting and annealing process, thus confirmed by TEM image. 3+

 Physical parameters of Sm :Ag NPs tellurite glasses are examined. 3+

 Optical absorption characteristic of Sm

in prepared glasses is investigated.

 Structural property of investigated glasses has been studied.

a r t i c l e

i n f o

Article history: Received 18 June 2014 Received in revised form 9 September 2014 Accepted 11 September 2014 Available online 19 September 2014 Keywords: Tellurite glasses Nanoparticles Optical absorption Vibrational modes

a b s t r a c t Metallic nanoparticles (NPs) assisted enhancements in absorption and emission cross-section of tellurite glass is the present challenge. The influences of samarium (Sm3+) ions and silver (Ag) NPs ratio on physical and optical absorption properties of melt quench synthesized magnesium tellurite glasses are reported. XRD patterns verify the amorphous nature of glasses. Glass density, molar volume and ionic packing fraction are discerned to be in the range of 4.92–5.0 g cm3, 29.82–30.26 cm3 mol1 and 0.452–0.446, respectively. Moderate reduction potential of tellurite glass converted Ag1+ to Ag0 via single step process and NPs are formed. TEM image manifest the existence of NPs of average diameter 16.94 nm having Gaussian size distribution. The significant changes in structural properties in the presence of Ag NPs are discussed in terms of TeO4 tetrahedra distortion and network depolymerization process. The Sm3+:Ag NPs dependent variation in physical properties are ascribed to the alteration in the number of bridging oxygen to non bridging (NB) one. Enhancement in absorption intensity due to the local field effects of Ag NPs is attributed to the changes in Sm–O bond strength. Optical energy band gap (2.81–3.18 eV) and Urbach energy (0.18–0.24 eV) are found increase and decrease, respectively with the increase of Sm3+:Ag NPs up to 1.33 then quenches and enhances, respectively thereafter which are related to the changes in cross-link and NBO numbers. The FTIR spectra reveal modification in network structures evidenced from vibrational wave-number shifts of TeO4 and TeO3 structural units. The observed notable increase in HOH vibration mode suggests its helpfulness in promoting the absorption of water and light. It is asserted that the physical, optical and structural properties of magnesium tellurite glass can be tuned by controlling Sm3+:Ag NPs. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Tellurite based-glass is the most promising host due to several special features such as wide transmission range from UV to infrared region (340 nm–15 lm), low dispersion [1–5], high transparency with refractive index from 2 to 2.5 [6–9] and high rare earth ion solubility. Consequently, they are attractive for lasers ⇑ Corresponding author. Tel.: +60 0127381709; fax: +60 75566162. E-mail address: [email protected] (M.R. Sahar). http://dx.doi.org/10.1016/j.molstruc.2014.09.039 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

and optical amplifiers application [10–13]. These desirable features allowed intensive studies on their optical properties [14–16]. However, the modification in overall properties of noble metallic (Au and Ag) nanoparticles embedded tellurite glass containing Sm3+ ion is not much reported. Recently, Sm3+ ion is verified as a dopant while Au or Ag NPs are demonstrated as stimulating agents for the enhancements of absorption and emission properties [17]. Nelson et al. [18] have proposed two important effects of dopant ions in terms of their local environment. Firstly, each of the dopant ions can occupy an

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individual site which is determined by the configuration of the structural unit in the melt. Lastly, they can modify the spatial geometry of the nearby glass network to outfit their own bonding requirement. Moreover, dopants can act as network modifiers and thus promote the formation of high number of non-bridging oxygen (NBO) [19]. Conversely, the metallic NPs assisted strong modifications in the rare earth transition probabilities caused by local field effect and energy transfer are easily detected from emission measurements [20]. The composition, shape and size of NPs play significant roles towards their interaction with external radiation [20]. TeO2 glassy and crystalline states are built by co-ordination of Te4+ ions in TeO4 groups as trigonal bipyramidal (tbp) form with bridging oxygen [21]. In TeO4 tbp linkage, two oxygen atoms are located in the axial site, while the other two and the lone electron pair of tellurium are located in the three equatorial sites. Kim and Yoko [22] acknowledged that the equatorial Te–O bonds are slightly shorter than the axial bonds. Trigonal bipyramids are linked to each other by sharing their vertices which form a continuous threedimensional structure [23]. The basic structure of TeO2 glass network often changes from TeO4 to TeO3+1 and/or TeO3 in the presence of network modifier. The combined effects of Sm2O3 and AgCl on magnesium tellurite glass are far from being understood. A series of Sm3+ ion doped magnesium tellurite glasses containing Ag NPs are synthesized using melt-quenching method by controlling the ratio of Sm2O3 and AgCl contents. Glasses are characterized using XRD, TEM, FTIR and UV–Vis spectroscopy. A correlation between optical and structural properties a function of Sm3+ ion to Ag NPs ratio is established. These glasses may be nominated as potential candidates for nanophotonic applications.

Vm ¼

M

ð1Þ

q

where M is the glass molecular weight. The ionic packing density (Vt) is calculated using Makishima and Mackenzie approach [24–26],

Vt ¼



1 Vm

 X  ðV i  xi Þ

ð2Þ

where xi is the mole fraction (mol%) and Vi is packing density parameter (m3/mol). For an oxide glass of the form MxOY, the value of Vi yields [26],

Vi ¼

  4pNA ½Xr3M þ Yr 3o  3

ð3Þ

where NA is Avogadro’s number (mol1), rM and ro are the Shannon’s ionic radius of metal and oxygen, respectively. The amorphous nature of glass is examined by Siemens X-ray Diffractometer D5000 using Cu Ka radiations (k = 1.54 Å) at 40 kV and 100 mA, with scanning angle 2h ranges between 10° and 80°. The room temperature absorption spectra in the range of 200– 2000 nm are recorded using Shimadzu UV-3101PC scanning spectrophotometer (Kyoto, Japan). Transmission electron microscopic (TEM) imaging is carried out using a Philips CM12 operating at 200 kV with Docu version 3.2 image analyses. Fourier transform infrared (FTIR) transmission measurements over the range of 400–4000 cm1 are performed using Perkin Elmer FTIR 1660 spectrometer followed by standard KBr pellet disc technique. Results and discussion

Experimental Glasses with chemical composition 88.6TeO2–10MgO–xSm2O3(1.4-x)AgCl (x ranges between 0.2 and 1.0 mol%) are synthesized using melt quenching technique. Starting powdered materials of TeO2, MgO, Sm2O3 and AgCl from Sigma Aldrich (analytical grade purity 99.9%) are mixed thoroughly. An aluminium crucible containing the glass constituents is placed in a furnace at 900 °C for 25 min and the melt is poured in a brass mould after the desired viscosity is attained. Subsequently, the sample is transferred to an annealing furnace and kept for 3 h at 295 °C to remove the thermal and mechanical strains completely. The samples are then cooled down to room temperature before polishing. Finally, they are cut and polished (thickness 0.25 mm) for the structural and absorption measurements. Nominal compositions of prepared glasses (transparent and yellowish colour) with respective codes are listed in Table 1. The formation of Ag NPs is stimulated via controlled melting and annealing process in which Ag+ cations are formulated and reduced to Ag neutral NPs (Ag+ + 1e ? Ag0) during the melting process. Density (q) of each sample is measured using standard Archimedes principle (Analytical balance of specific density-PrecisaXT220A) with distilled water as an immersion liquid. The molar volume (Vm) is calculated following,

The typical X-ray diffraction pattern for sample S4 as shown in Fig. 1 in the presence of a broad hump confirms its amorphous nature. Furthermore, the absence of any sharp crystalline peak verifies the short ranged order. TEM image for sample S4 as displayed in Fig. 2(a) reveals the growth of homogeneously dispersed Ag NPs in the glass matrix having varying shape and sizes. The average diameter Ag NPs discerned to be 16.94 nm which is shown in Fig. 2(b). The calculated physical parameters of all glass samples are listed in Table 1. Figs. 3–5 shows the Sm2O3 to Ag NPs ratio (here after

Fig. 1. XRD pattern of sample S4.

Table 1 The nominal composition of glasses with codes, density (q), molar volume (Vm) and ionic packing density (Vt). Sample code

S1 S2 S3 S4

Nominal composition (mol%) TeO2

MgO

Sm2O3

AgCl

88.6 88.6 88.6 88.6

10 10 10 10

0.2 0.4 0.8 1.0

1.2 1.0 0.6 0.4

Sm3+:Ag NPs

q (g cm3)

Vm (cm3 mol1)

Vt

0.17 0.40 1.33 2.5

4.92 4.96 5.00 4.94

30.05 29.89 29.82 30.26

0.447 0.449 0.452 0.446

N.M. Yusoff et al. / Journal of Molecular Structure 1079 (2015) 167–172

Fig. 2. (a) TEM image for sample S4, and (b) Ag NPs size distribution.

Fig. 3. Sm3+:Ag NPs dependent variation in density.

Fig. 4. Sm3+:Ag NPs dependent variation in molar volume.

denoted as Sm2O3:Ag NPs or Sm3+:Ag NPs) dependent variation of glass density, molar volume and ionic packing density. Glass density (Fig. 3) is increased with the increase of Sm3+:Ag NPs up to 1.33 and decrease at 2.5. This increase in density is attributed to the substitution of AgCl with lower molecular weight (143.32 g mol1) by higher molecular weight Sm2O3 (348.80 g mol1). The net increase in molecular weight causing a strong connectivity in the glass network is responsible for such densification. However, the decrease in density is ascribed to the formation of NBO that loosen the network. Conversely, the decrease in molar volume with the increase in Sm3+:Ag NPs as displayed in Fig. 4 is due to the lack of free space in the glass network where the interstitial sites are fully occupied by samarium atoms. Moreover, the increase in molar volume (free excess space) beyond 1.33 of Sm3+:Ag NPs is due to the partial participation of samarium atoms in the glass network which leave some voids. The Sm3+:Ag NPs dependent ionic packing density as depicted in Fig. 5 follows the same trend as that of density. The increase in Vt or glass compactness is found to be directly proportional to the addition of higher density (Sm2O3 of 8.35 g mol1) substance that replaces the lower density AgCl (5.56 g cm3) in the glass [26]. This clearly demonstrates that glass samples with more rigid and highly cross linked network resulting closely packed structures are achieved. However, the decrement of both the glass density and the ionic packing density for Sm3+ to Ag NPs ratio 2.5 reflect the loosening of glass structure [27], where Sm2O3 start to play the role as dopant as well as network modifier [18,19]. Sm3+ ion acting as network modifier can alter the glass structure considerably by breaking Te–O bond in TeO4 tbp and creating a large number of NBO [19]. More the number of NBO is less the glass density and ionic packing. The room temperature UV–Vis spectra for all samples as shown in Fig. 6 exhibit ten absorption bands without any sharp absorption edge [28]. The shift of absorption edge towards higher or lower wavelength is intimately related to the structural rearrangements [28]. This change in absorption characteristics is related to the alteration in oxygen bonding in glass network especially the formation of NBO [29]. Shobe [30] demonstrated that the light absorption in the UV range is determined by the interaction of photon with the oxygen ions in the glass. The observed shift of the absorption edge or cut-off wavelength towards lower value with increasing Sm3+ to Ag NPs ratio up to 1.33 is ascribed to the stronger binding of O2 ions. These tightly bound oxygen ions require higher photon energy for interaction and thereby shifts the absorption edge towards lower wavelength. Conversely, the observed red shift in the absorption edge for Sm3+ to Ag NPs ratio greater than 1.33 is attributed to the formation of NBO. The cut-off wavelength red-shift as summarized in Table 2 is mainly caused by the weakly bound O2 ions [19]. The optical absorption coefficient, a(m) can be determined using,

aðmÞ ¼

2:303A d

ð4Þ

where A is absorbance and d is the thickness of the sample.

Fig. 5. Ionic packing density versus Sm3+:Ag NPs.

169

Fig. 6. Absorption spectra of all glass samples.

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Table 2 The cut-off wavelength ðkcutoff Þ, optical energy band gap (Eopt), Urbach energy (DE), refractive index (n) and electronic polarizability (am) for all glass samples. Sample No.

kcutoff (nm)

Eopt (eV)

DE (eV)

n

am ðÅ Þ

S1 S2 S3 S4

454 440 437 444

3.01 3.13 3.18 2.81

0.22 0.19 0.18 0.24

2.39 2.36 2.35 2.45

7.30 7.17 7.11 7.51

3

The photon energy (hm) dependent absorption coefficient, a (m) for direct and indirect transition is calculated following Davis and Mott relation [31],

Fig. 8. Variation of optical energy band gap as a function of Sm3+:Ag NPs.

n

aðmÞ ¼

Aðhm  Eopt Þ hm

ð5Þ

where n = ½ for allowed direct transition and n = 2 for allowed indirect transition, Eopt is optical band gap energy and A is a constant called band tailing parameter. The typical variation of (ahm)1/2 as a function of hm (Tauc’s plot) for sample S1 is shown in Fig. 7. The value of Eopt for indirect transition is obtained by extrapolating (linear part) (ahm)1/2 = 0 as indicated. The values of Sm3+ to Ag NPs ratio dependent band gaps and Urbach energies are furnished in Table 2. The noticeable increase in the optical band gap energies with the increase of Sm3+:Ag NPs up to 1.33 as depicted in Fig. 8 is attributed to the enhancement of Sm–O bond strengths that require higher excitation energy for electron. In contrast, the decrease in the optical band gap energies for Sm3+:Ag NPs of 2.5 is related to the formation of Sm–O–Te linkages via O–Te–O bonds which results an increase of NBO. The depolymerisation of TeO4 tbp to TeO3+1 and/or TeO3 tp plays a pivotal role in the increase of bond defects thus decrease in Eopt. The Urbach energy is determined using the empirical relation [32,33],

aðmÞ ¼ B exp



hm DE



ð6Þ

where DE is the Urbach energy, B is found as the inverse slope of the ln a versus hm curve as shown in Fig. 9. Urbach energy being a measure of the width of localized states can further be used to characterize the degree of disorder in amorphous and crystalline system. The reduction in Urbach energy with increasing Sm3+:Ag NPs up to 1.33 as shown in Fig. 10 is ascribed to the escalation of long range order (crystallinity) in the glass. The glasses with the smaller Urbach energy would have lower disorder and more compactness (less fragile) [28]. Furthermore, the increment in Urbach energy at 2.5 of Sm3+ to Ag NPs ratio implies the augmentation of local short range order. Refractive index (n) being most important property in characterizing optical glasses is calculated following the relation proposed by Dimitrov and Sakka [34],

n2  1 ¼1 n2 þ 2

Fig. 9. ln a versus hm for sample S1.

rffiffiffiffiffiffiffiffi Eopt 20

ð7Þ

Fig. 10. Sm3+:Ag NPs dependent Urbach energy.

Following Lorentz–Lorenz relation [35,36] the measured refractive index is used to determine the electronic polarizability (am) of the glass [37],

Rm ¼

n2  1 am 4pNA Vm ¼ n2 þ 2 3

Figs. 11 and 12 displays the Sm3+ to Ag NPs ratio dependent variation of refractive index and electronic polarizability, respectively. The decrease in refractive index with increase of Sm3+:Ag NPs up to 1.33 is due to the lower ionic radii of Sm3+ (1.079 Å) in comparison to Ag+ (1.09 Å) possessing higher field strength. Consequently, the addition of Sm3+ ions leading to lower polarization decreases

where Eopt is obtained from the absorption data.

Fig. 7. (ahm)1/2 versus hm for sample S1.

ð8Þ

Fig. 11. Sm3+:Ag NPs dependent refractive index.

N.M. Yusoff et al. / Journal of Molecular Structure 1079 (2015) 167–172

171

ratio. This enhanced water content is responsible for higher light absorption in the glass as noticeable in Fig. 6. Conclusion

Fig. 12. Sm3+:Ag NPs dependent electronic polarizability.

the refractive index. The decrease in electronic polarizability is clearly evident from Fig. 12. However, the increase in refractive index at 2.5 of Sm3+:Ag NPs manifests the glass structural modifications. The FTIR spectra of all glasses as shown in Fig. 13 exhibit several absorption bands accompanied by shift towards lower wave-number. Different structural groups that build the TeO2 network are clearly evidenced. The shift in absorption frequency with the increase of Sm3+:Ag NPs indicates the alteration of TeO2 structural unit. The broad and intense absorption peak located around 600–690 cm1 is assigned to TeO4 tbp unit with bridging oxygen [38–40]. Meanwhile, the peak around 700–780 cm1 is allocated to TeO3 tp unit with NBO [38–40]. A broad absorption peak appeared around 723–734 cm1 is assigned to TeO3 tp group which is accompanied with overlapping absorption peak of TeO4 tbp and TeO3 tp group. This suggests the formation of large number of TeO3+1 polyhedra. Furthermore, the occurrence of the peaks at 663 cm1 and 772 cm1 are related to the structural transformation of TeO3+1 to TeO4 tbp and TeO3 tp units, respectively which is in well agreement with the earlier findings [41–45]. The stretching and bending vibrational modes of H–O–H in water are evidenced around 3355 and 1633 cm1, respectively. Fig. 14 illustrates the Sm3+:Ag NPs dependence of relative area under H–O–H stretching vibrational mode. The water content in the glass is found to increase with the increase of Sm3+ to Ag NPs

Fig. 13. FTIR spectra for all samples in the range of 450–4000 cm1.

Fig. 14. Relative area of H–O–H stretching vibration as a function of Sm3+:Ag NPs.

We report the effects of Sm3+ ions to Ag NPs concentration ratio on physical and optical absorption features of magnesium tellurite glasses. The occurrence of both spherical and non-spherical Ag NPs with homogeneous distribution is evidenced from the TEM micrographs. The alteration in structural properties is majorly attributed to the distortion of TeO4 tetrahedra and network depolymerization process. A correlation between Sm3+:Ag NPs variation and physical properties is established. Enhancement in absorption intensity is attributed to the changes in Sm–O bond strength and increase in relative area under H–O–H stretching vibrational mode. The increase in optical energy band gap, Urbach energy and the shift in the absorption peaks with the increase of Sm3+:Ag NPs are attributed to the alteration in cross-link and generation of NBO. The decrease in Urbach energy with increasing Sm3+:Ag NPs implies a decrease in amorphous nature due to lack of NBO. FTIR spectra displaying a shift in frequency support this observation. Structural change of TeO2 is revealed with the creation of bridging oxygen through the transformation of TeO4 ? TeO3+1 ? TeO4. The observed decrease in refractive index and electronic polarizability with the increase of Sm3+:Ag NPs is interpreted via the presence of lower number of NBO and higher field strength. Our optimized method for controlling the ratio of Sm3+ to Ag NPs in improving the structural and optical characteristics of magnesium tellurite glasses may constitute a basis for their large scale synthesis useful for sundry of applications. Acknowledgement The authors are grateful to Ministry of Education, Malaysia for the financial support via Research University Grant (06J39 and 05H36), ERGS (4L032) and FRGS (4F083). References [1] E.J. Stanworth, J. Soc. Glass Technol. 38 (1954) 425. [2] H. Nasu, O. Matsushita, K. Kamiya, H. Kobayashi, K. Kubodera, J. Non-Cryst. Solids 124 (1990) 275. [3] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [4] M. Tatsumisago, T. Minami, Y. Kowada, H. Adachi, Phys. Chem. Glasses 35 (1994) 89. [5] Y. Himei, Y. Miura, T. Nanba, A. Osaka, J. Non-Cryst. Solids 211 (1997) 64. [6] P.G. Pavani, S. Suresh, V.C. Mouli, Opt. Mater. 34 (2011) 215–220. [7] S. Sakida, T. Nanba, Y. Miura, Mater. Lett. 60 (2006) 3413–3415. [8] P.G. Pavani, K. Sadhana, V.C. Mouli, Phys. B 406 (2011) 1242–1247. [9] E. Yousef, M. Hotzel, C. Russel, J. Non-Cryst. Solids 342 (2004) 82–88. [10] G.C. Righini, S. Pelli, SPIE Proc. 4453 (2001) 93. [11] P. Madasamy, G. Nunzi Conti, P. Poyhonen, Y. Hu, M.M. Morrel, D.F. Geraghty, S. Honkanen, N. Peyghambarian, Opt. Eng. 41 (5) (2002) 1084. [12] V.A.G. Rivera, E.F. Chillce, E.G. Rodrigues, C.L. Cesar, L.C. Barbosa, SPIE Proc. 6116 (2006) 190. [13] V.A.G. Rivera, E.F. Chillce, E.G. Rodrigues, C.L. Cesar, L.C. Barbosa, SPIE Proc. 6116 (2006) 184. [14] K. Maheshvaran, K. Linganna, K. Marimuthu, J. Lumin. 131 (2011) 2746–2753. [15] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta Part A 59 (2003) 917–925. [16] T. Sasikala, L. Rama Moorthy, A. Mohan Babu, Spectrochim. Acta Part A 104 (2013) 445–450. [17] T. Som, B. Karmakar, Plasmonics 5 (2010) 149. [18] C. Nelson, I. Furukawa, W.B. Nelson, Mater. Res. Bull. 18 (1983) 959. [19] G. Turky, M. Dawy, Mater. Chem. Phys. 77 (2002) 48–59. [20] V.A.G. Rivera, Y. Ledemi, S.P.A. Osorio, D. Manzani, Y. Messaddeq, L.A.O. Nunes, E. Marega Jr., J. Non-Cryst. Solids 358 (2012) 399–405. [21] V. Dimitrov, J. Solid State Chem. 66 (1987) 256–262. [22] S.H. Kim, T. Yoko, J. Am. Ceram. Soc. 78 (1995) 1061–1065. [23] Y. Zhou, Y. Yang, F. Huang, J. Ren, S. Yuan, G. Chen, J. Non-Cryst. Solids 386 (2014) 90–94. [24] A. Makishima, J.D. Mackenzie, J. Non-Cryst. Solids 12 (1973) 35–45. [25] Y.B. Saddeek, Physica B 344 (2004) 163–175. [26] S.K. Ahmmad, M.A. Samee, A. Edukondalu, S. Rahman, Results Phys. 2 (2012) 175–181.

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