Effect of silver nanoparticles incorporated with samarium-doped magnesium tellurite glasses

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Effect of silver nanoparticles incorporated with samarium-doped magnesium tellurite glasses N.M. Yusoff, M.R. Sahar

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Received date: 4 June 2014 Revised date: 22 August 2014 Accepted date: 26 August 2014 Cite this article as: N.M. Yusoff, M.R. Sahar, Effect of silver nanoparticles incorporated with samarium-doped magnesium tellurite glasses, Physica B, http://dx.doi.org/10.1016/j.physb.2014.08.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of silver nanoparticles incorporated with samarium-doped magnesium tellurite glasses N. M. Yusoff and M. R. Sahar* *Advanced Optical Material Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia.

*Corresponding author E-mail: [email protected] Tel.: +60 0127381709

Fax: +60 75566162

Abstract Silver nanoparticles (Ag NPs) are incorporated in samarium doped tellurite glass of a composition (89-x)TeO2-10MgO-1Sm2O3-xAgCl where, 0.0≤x≤0.6 mol% by melt quenching technique. It is found that all the glasses are amorphous in nature, and the existence of Ag NPs with an average size of 16.94 nm is confirmed by Transmission Electron Microscopy. Meanwhile, their physical properties such as glass density, molar volume and ionic packing density are computed utilizing the normal method. The density and ionic packing density are observed to decrease with an increasing Ag NPs, but increases as the Ag NPs are beyond 0.2 mol%. On the other hand, the molar volume behaves exactly opposite to the increase in Ag NPs content. It decreases as the Ag NPs content value is more than 0.2 mol%. The optical energy band gap and Urbach energy are evaluated from the absorption spectra in the range of 200-900 nm at room temperature. It is also observed that the direct and indirect optical energy band gap reduces with Ag NPs content, but enhances as the Ag NPs are beyond 0.2 mol%. Meanwhile, the Urbach energy is found to increase as the Ag NPs content is increased, but decreases when Ag NPs is 0.2 mol%. The refractive index is deduced from indirect optical energy band gap. Meanwhile, molar refraction and electronic polarizability have been calculated from the Lorentz-Lorentz relation. Refractive index and electronic polarizability are also observed to raise with Ag NPs content, but drop off as Ag NPs content 1

is more than 0.2 mol%. In this paper, all properties are discussed with respect to the Ag NPs concentration. Keywords: Tellurite glasses; Samarium oxide; Nanoparticles; Transmission electron microscopy (TEM); Optical energy band gap; Electronic polarizability

1.

Introduction Among motivated studies of glassy materials, tellurite based glasses drew much

interest because of their unique properties such as high dielectric constant and excellent transmission in the visible as well as IR wavelength regions, good mechanical strength and chemical durability [1-4]. These glasses also possess higher refractive index, which is approximately in the range of 2.0 to 2.5 [5-8], and their low melting temperature (about 800oC) contributes to the high possibility of stable glass forming using a conventional melt quenching method [4]. Although, pure tellurium oxide cannot form glass by itself but needs another element known as glass modifier such as alkali metal, alkaline earth metal oxide and transition metal oxide (TMO) to improve the network connectivity to produce a stable tellurite glass [9-10] with increasing non-bridging oxygen [9]. In fact, it is believed that the properties of oxide glasses strongly depend on the nature and the concentration of the constituent oxides [11]. Tellurite glasses contain Te-O bonds, connecting each other forming a normal glass network. However, Te-O bonds can be easily broken and therefore, can accommodate heavy metal oxides or rare earth ions (RE) [12]. TeO2 glass is good for hosting rare earth ion since it provides low phonon energy (~750 nm), which minimizes non-radiative losses [13-14]. Samarium oxide is one of the rare earth families that are used as a dopant to create a lasing character of TeO2 glass. Ravi et al. [15] has proposed certain composition of Sm2O3 to use in laser and photonic devices. Further investigations towards glass containing RE ions 2

incorporating metallic nanoparticles (NPs) such as gold (Au) or silver (Ag) NPs are extensively conducted. Metallic nanoparticles show great diverse fascinating properties compare to usual glass system without NPs. The striking distinction of incorporated metallic NPs is addressed during the interaction of metallic NPs with electromagnetic light. Such interaction generates a collective oscillation of metallic NPs’s conduction electrons at interface of NPs and surrounding of glassy medium, which is known as localized surface plasmons (LSP). It produces enhanced on the local field inside and near the NPs [16], hence raises the electromagnetic field that is very important in enhancing optical process. It should be noted that, the size and shape of metallic NPs as well as surrounding medium play important roles in enhancing optical properties. Yeshchenko et al. [17] has reported that the scattering rate of the conduction electrons becomes increase with the smaller size of Ag NPs and results in a nonlinear red shifted of SPR energy. This gains a paramount importance to surface enhanced Raman spectroscopy [18]. Naranjo et al., [19] have reported the enhancement of down conversion luminescence by embedding Ag NPs in Pr3+ doped germinate glass system. Meanwhile, Kassab et. al [20] found a substantial increment of ion optical stimulated second harmonic generation in tellurite glass containing Ag NPs. A recent research by Jimenez et al., [21] described the use of Ag NPs in the optical interaction of metal and rare earth in dielectric host. Optical energy band gap as well as Urbach energy are important glass properties. The optical absorption is directly related to the size of particles in which the optical energy band gap is slightly blue and shifted toward the higher energy, presumably due to the decreasing of the particle size, until it reaches the nanosize region [22-24]. However, it is also important to note that the concentration of different nanoparticles plays a different role in tuning the optical energy band gap [25], which is due to the broadening or narrowing of the valence band or multivalence structures. It is also found that electronic polarizability is one of

3

important properties that show direct nonlinear response of materials when intense light beam is incident to the sample. Refractive index and molar refraction for isotropic materials are governed by electronic polarizability [26-28]. Refractive index reduces when ion in the system are less polarized. Commonly, the polarization is directly proportional to field strength, whereas smaller the field strength of ion, the ion will be less polarized thus decreases the refractive index. However, it is observed that refractive index depends on glass composition [29]. According to the best of authors’ knowledge based on literature survey, there is lack of reports about the Ag NPs concentration effect on this glass. Therefore, the present study investigates many desirable features such as density, molar volume, ionic packing density, optical energy band gap, Urbach energy, polarizability and refractive index. The main objective of study is to observe the interesting changes in the said properties by incorporating Ag NPs.

2.

Experimental procedure Series of glasses based on (89-x) TeO2-10MgO-1Sm2O3-(x) AgCl, where 0≤x≤0.6

(mol%) composition have successfully been prepared by melt quenching technique. A 15 gram batch with a proportional amount of TeO2 (purity 99%), MgO (purity 99.9%), Sm2O3 (purity 99.2%) and AgCl (purity 99.999%) in powder form was mixed in a platinum crucible. Then the mixture was melted at 900oC in an electrical furnace for one hour. During the melting process, chlorine was removed as gas molecules while, Ag+ cations were formulated and reduced to Ag neutral NPs (Ag+ + 1e- → Ag0), thus produced Ag NPs. The melts was quenched between two brass plates before annealing at 300oC for three hours and then allowed to cool down to room temperature. The glass was then cut, grounded, and polished to a thickness of 2.5 mm to produce a shiny and scratch free surface to use for optical

4

measurements. The amorphous nature of glass was confirmed using a Siemens X-ray Diffractometer D5000 with a scanning angle 2θ ranging between 10 and 80o. A tube voltage of 30 kV and current of 20 mA were used. Meanwhile the Energy Dispersive of X-Ray (EDX) analysis was used to analyse the actual composition of the glass. The glass density was measured by the Archimedes method using distilled water as an immersion liquid. The density, ρ (g/cm3) of each sample is determined by a relation [30],

ρ = ρL

W1 W1 − W2

(1)

where, ρ L is the density of distilled water (0.9975 g/cm3), w1 and w2 are the weight of the sample in the air and in water, respectively. Meanwhile, the molar volume, Vm is calculated using a relation [31], Vm = ∑

xi M i

i

(2)

ρ

where xi and M i denotes the molar fraction and molecular weight of the ith component respectively. According to Makishima and Mackenzie [32-34], the ionic packing density, Vt can be expressed as, ⎛ 1 Vt = ⎜ ⎝ Vm

⎞ ⎟ * ∑(Vi * xi ) ⎠

(3)

where Vm is the molar volume, xi is the molar fraction (mol%), Vi is packing density parameter (m3/mol). For an oxide glass MxOY, the Vi can be obtained from the following relation [34],

⎛ 4π N A ⎞ 3 3 Vi = ⎜ ⎟ [ XrM + Yro ] 3 ⎝ ⎠

(4)

5

where N A is Avogadro’s number (mol-1), rM and ro are the Shannon’s ionic radius of metal and oxygen, respectively. The occurrence of Ag NPs in a glass matrix can be observed under a Phillips CM12 Transmission Electron Microscope (TEM). The absorption spectra were obtained with Shimadzu 3101PC UV-VIS-NIR spectrophotometer in the range of 200-900 nm at room temperature.

3.

Results and Discussion

3.1

X-Ray Diffraction Table 1 shows a nominal chemical composition of the (89-x) TeO2-10MgO-1Sm2O3-

xAgCl prepared glass. Meanwhile, Figure 1 shows a typical X-ray diffraction pattern for S1 and S4 glass sample. The glasses exhibit a wide halo, which shows the characteristic of an amorphous nature of the glass. Notably, the (111) largest diffraction peak of crystalline Ag expected at 2θ = 38.784o [35-37] is missing from the two spectra. This might be due to the small amount of Ag NPs embedded in the glass system.

3.2

Glass morphology Figure 2 (a) shows a TEM image for the glass containing Ag NPs. It can be seen that

the spherical and non-spherical particles are dispersed homogeneously in the glass matrix. Figure 2 (b) shows the distribution of particle size using Gaussian plot. The calculated average diameter of Ag NPs is around 16.94 nm. Meanwhile, Figure 2 (c) shows the EDX spectrum for S3. The analyses for the other compositions have also been done and the results for the actual chemical composition are inserted in Table 2. From Table 2, it can be seen that S3 possesses an actual amount of 0.2 mol% of AgCl out of possible 0.4 mol%. From this analysis, the most possible reduction process can be deduced as follows,

0.4 AgCl → 0.2Ag 0 ( NPs ) + 0.2AgCl ( glass ) + 0.1Cl 2 ↑ (removed) 6

(5)

This process is confirmed as 0.2 mol% of AgCl is detected in S3 suggests the formation of 0.2 mol% Ag NPs.

3.3

Density, Molar Volume and Ionic Packing Density By using the results presented in Table 3, a plot of density and molar volume against

Ag NPs concentration is drawn and shown in Figure 3. From Figure 3, it can be seen that the density decreases with the increasing concentration of Ag NPs till 0.2 mol% Ag NPs. After that, the density starts to raise. The decrease in density is due to the substitution of higher molecular weight TeO2 (atomic mass = 159.60 g/mol) by a lower molecular weight AgCl (143.32 g/mol). The weak connectivity in the glass structure causes a drop off in density as reported by Pavani et. al [38] and Choudari et. al [39]. The increase in density is due to the increasing of the glass network rigidity. This would reflect the increase in the numbers of bridging oxygen in the glass network. The molar volume is inversely proportional to density, but directly proportional to the molecular weight. Therefore, an increase in molar volume reflects the increase of free volume. This is because of the substitution of higher radii and bond length substances (Ag) into the system with the expense of TeO2 (lower radii and lower bond length). A similar kind of results is reported elsewhere [7]. The glass compactness can also be explained by ionic packing density [40] that can be calculated using Eqn (3). The variation of ionic packing density with Ag NPs concentration is shown in Figure 4. A reduction in the ionic packing density is clearly evident with the increasing concentration of Ag NPs. The dependence of density on the ionic radii of atom is well known. In this case, the ionic radius of Ag (1.09Å) is larger than the other components (Te4+=0.66 Å, Mg2+=0.72 Å and Sm3+=1.079 Å) [41]. Larger the ionic radius, higher will be possibility that ion fill the space of excess volume. As a result, the compactness of the glass is reduced. However, as the Ag NPs concentration is further increased, the ionic packing

7

density is also increased. This reflects that the Ag ions have fully filled most of the space of excess volume. As a result, the compactness of the glass increases.

3.4

Optical properties Mott and Davis [42] relate the optical energy band gap with the absorption coefficient

α as,

α (ν ) =

A(hν − Eopt ) n

(6)

hν

where, A is constant, hν is photon energy, Eopt is optical energy band gap, n =1/2 for direct transition and n =2 for indirect transition [43-44]. The direct and indirect optical energy band gaps are estimated by the intercepts on the abscissa of fits to linear regions in two curves. Most of tellurite glass, the electronic transition is controlled by the direct and indirect transition [30, 45-47]. Figure 5 shows a typical Tauc plot for direct and indirect optical energy band gap for S1. From Figure 5, the optical energy band gap for both direct and indirect transitions may be obtained by extrapolating the linear part of the curve to the x-axis. The values for Eopt for direct and indirect have been achieved and listed in Table 4. Using data of Table 4, a plot of Eopt against Ag NPs concentration is drawn, and the result is shown in Figure 6. From Figure 6, it can be seen that the Eopt for both direct and indirect transitions are reduced with the increasing of Ag NPs up to 0.2 mol%. This is perhaps due to the increasing number of non-bridging oxygen (NBO). It is reported that NBO is more negatively charged than those of bridging oxygen (BO) [48-49]. Therefore, more electrons can easily be transferred from the valence band to the conduction band. This reflects a reduction of Eopt in the glass. In case of Er-Cl, as reported by Qi et.al [50], it is expected that the Sm-Cl will be formed as a result of adding AgCl to the glass. Since the Sm-Cl is ionic, the release of an electron is easier than that of Sm-O [51]. As a result, the value of Eopt is decreased. However, 8

beyond 0.2 mol% of Ag NPs, optical energy band gap for both direct and indirect transition increases, which reflects the formation of bridging oxygen in the glass matrix. The width of band tails at the edge of conduction bands described by Urbach rule is given as [51], α (ν ) = B exp (

hν ) ΔE

(7)

where B is a constant, ΔE is the Urbach energy, which indicates the width of band tails of localized states and ν is the frequency of radiation [52]. Figure 7 shows a plot of ln (α) versus photon energy. From the slope of linear region of curve, the Urbach energy may be estimated. The variation of Urbach energy against Ag NPs concentration is shown in Figure 8. It is evident that that the Urbach energy increases as the concentration of Ag NPs increases up to 0.2 mol% (Fig.8), which reflects the rising of local short range order caused by the presence of Ag NPs. However, as the concentration of Ag NPs is kept increasing, the Urbach energy slightly reduces, which suggests the formation of nano-crystalline particle in the glass matrix.

3.5

Refractive index and electronic polarizability Refractive index and electronic polarizability entities are very important in non-linear

optical properties. Refractive index, n can be calculated from the indirect optical energy band gap by using the following equation, I Eopt n2 −1 = 1− n2 + 2 20

(8)

I is the calculated optical energy band gap. Lorentz-Lorentz [53-55] has derived where, Eopt

the relation of molar fraction by using the value of the refractive index as, Rm =

n2 −1 Vm n2 + 2

(9)

9

where Rm (cm3 mol-1) is molar refraction, Vm is the molar volume. This equation gives an average molar refraction for isotropic substance. Molar refraction is related to the structure of glass and, proportional to the electronic polarizability of material. The electronic polarizability, α m can be determined through the following equation [56] ⎛ 3 ⎞ αm = ⎜ ⎟ Rm ⎝ 4π N A ⎠

(10)

where N A is Avogadro’s number. The calculated refractive index, molar refraction and electronic polarizability are listed in Table 4. Figure 9 reveals refractive index and electronic polarizability against Ag NPs concentration and exhibits that the insertion of Ag+ into the glass structure results in an increasing refractive index but up to 0.2 mol% only. The increase in refractive index is probably due to larger ionic radii (1.09Å) of Ag+ than that of Te4+ (0.66Å). When the field strength (field strength = Z/r2, Z = oxidation number, r is the ionic radius) is calculated for both Te4+ and Ag+, it is found that the field strength for Te4+ (9.18Å2

) is greater than that of Ag+ (0.84 Å-2). This indicates that Ag+ can be easily polarised

compared to Te4+ and results in increasing the refractive index. This is in a good agreement with the results of Abdel Baki et. al [57]. A decrease in refractive index as the Ag NPs content is more than 0.2 mol% is expected because at this composition, the Ag atom starts to take place in formation of glass structural networks.

4.

Conclusions From the above discussions, following conclusions may be drawn. The glass of

composition (89-x) TeO2-10MgO-1Sm2O3-xAgCl, where 0≤x≤0.6 in mol% has successfully been prepared. All glasses are amorphous in nature, and nanoparticles of size around 16.94 nm are confirmed. The glass density, molar volume and ionic packing density are found to be in the range of (4.94-5.51) g cm-3, (30.26-27.13) m3 mol-1 and (0.45-0.50) respectively. 10

Meanwhile, the direct and indirect optical energy band gap is in the range of (3.16-3.29) eV and (2.81-2.97) eV respectively, it decreases as the amount of Ag NPs increases. It is also observed that the Urbach energy is in the range of (0.18-0.24) eV, increases with the increasing amount of Ag NPs. Refractive index and electronic polarizability are found in the range of (2.41-2.45) and (6.68-7.51) Å 3 , respectively. All properties show reverse behavior as the amount of Ag NPs is more than 0.2 mol %.

Acknowledgement The authors gratefully acknowledge to the financial support of Research, University Grant (06J39), ERGS (4L032), FRGS (4F083) and the Ministry of Higher Education.

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Table 1. A nominal chemical composition of (89-x)TeO2-10MgO-1Sm2O3-xAgCl glass system.

Table 2.

An actual chemical composition of (89-x)TeO2-10MgO-1Sm2O3-xAgCl glass

system as analysed by EDX.

Table 3. The density, ρ, molar volume, Vm and ionic packing density, Vt. I D Table 4. Indirect optical energy band gap, Eopt , direct optical energy band gap, Eopt and

Urbach energy, ∆E, indirect refractive index, n, molar refraction, Rm and electronic polarizability, αm of the prepared glass.

Fig.1.

The X-Ray diffraction pattern for S1 and S4 glass sample

Fig.2.

(a) TEM image of S3 (b) Distribution size of Ag NPs in S3 (c) EDX spectrum for S3

Fig.3.

Density and molar volume against Ag NPs concentration (mol%)

Fig.4.

Ionic packing density against Ag NPs concentration (mol%)

Fig.5.

A typical Tauc plot for direct and indirect optical energy band gap for S1

Fig.6.

Indirect and direct optical energy band gap against Ag NPs concentration (mol%)

Fig.7.

ln (α) vs hν . The Urbach energy can be estimated by the slope of the linear part of the curve.

Fig.8.

Urbach energy against Ag NPs concentration (mol%)

Fig.9.

The refractive index and electronic polarizability against Ag NPs concentration (mol%)

15

Table 1 Sample No

Nominal chemical composition (mol%) TeO2

MgO

Sm2O3

AgCl

S1

89

10

1

0

S2

88.8

10

1

0.2

S3

88.6

10

1

0.4

S4

88.4

10

1

0.6

Table 2

Sample No

Actual chemical composition (mol%) TeO2

MgO

Sm2O3

AgCl

S1

89

10

1

0

S2

88.8

10

1.1

0.1

S3

88.6

10

1.2

0.2

S4

88.4

10

1.3

0.3

16

Table 3

Sample No

ρ

Vm

(g cm-3)

(m3 mol-1)

S1

5.38

27.80

0.49

S2

5.05

29.61

0.46

S3

4.94

30.26

0.45

S4

5.51

27.13

0.50

17

Vt

Table 4

Sample No

I Eopt

(eV)

D Eopt

(eV)

∆E

n

(eV)

Rm

αm

(cm3 mol-1)

( Å3)

S1

2.97

3.29

0.18

2.41

17.09

6.78

S2

2.96

3.27

0.19

2.41

18.22

7.23

S3

2.81

3.16

0.24

2.45

18.92

7.51

S4

2.88

3.22

0.23

2.43

16.83

6.68

18

Figure 1

19

Figure 2 (a)

20

Figure 2 (b)

Frequency (%)

40

Gaussian fit Average diameter ~16.94 nm

30 20 10 0

8

12

16 20 Particles size (nm)

21

24

28

F Figure 2 (c)

22

Figure 3

31 30

5.4

29

→ 28

5.2 5.0 4.8 0.0

←

27 0.1 0.2 Ag NPs concentration (mol%)

23

26 0.3

Molar volume (m3/mol)

3

Density (g/cm )

5.6

Ionic packing density, Vt

Figure 4

0.50

0.48

0.46

0.44 0.0

0.1 0.2 Ag NPs concentration (mol%)

24

0.3

Figure 5

Indirect transition Direct transition

2 -1

←

8

4

4.0x10

2

1/2

(αhν)

4

6.0x10

(αhν) (cm eV)

12

-1

(cm eV)

1/2

16

4

2.0x10

4 0 2.0

→ 2.4

2.8 hν (eV)

25

3.2

0.0 3.6

Figure 6

3.30

2.95 Direct transition

→

2.90

ED opt (eV)

I E (eV) opt

3.00

3.25

3.20 2.85

←

Indirect transition 2.80 0.0 0.1 0.2 Ag NPs concentration (mol%)

26

3.15 0.3

Figure 7

-1

ln (α) (cm )

4

3

2 3.0 3.0

3.2 hν (eV)

27

3.4

Figure 8

0.24

ΔE (eV)

0.22

0.20

0.18 0.0

0.1 0.2 Ag NPs concentration (mol%)

28

0.3

Figure 9

Refractive index

2.45 7.4 2.44

7.2

→

2.43 2.42

←

6.8

2.41 0.0

7.0

6.6 0.1 0.2 Ag NPs concentration (mol%)

29

0.3

Electronic polarizability

7.6