Effect of ZnO incorporation on the structural, thermal and optical properties of phosphate based silicate glasses

Materials Chemistry and Physics 242 (2020) 122461

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Effect of ZnO incorporation on the structural, thermal and optical properties of phosphate based silicate glasses Refka Oueslati-Omrani *, Ahmed Hichem Hamzaoui Useful Materials Valorization Laboratory, National Centre of Research in Materials Science, Technologic Park of Borj Cedria, B.P. 73, 8027, Soliman, Tunisia

H I G H L I G H T S

� (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol) glasses was prepared by melt quenching technique. � Physico-chemical properties were carried out from density, refractive indices, ICP-AES and DSC. � Results obtained from FTIR and Raman spectroscopy revealed the cleavage of P–O–P bonds. � Band gap energy variations were correlated to the increase in the number of non-bridging oxygen. A R T I C L E I N F O

A B S T R A C T

Keywords: Zinc phosphate-silicate glasses Depolymerization Spectroscopic analysis Optical band gap energy Polarizability

Zinc based phosphate-silicate glasses with a molar composition of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol %) were successfully synthesized by melt quenching technique. The amorphous state of the glass series was checked using X ray diffraction (XRD). The variation of density, molar volume and refractive index were correlated to increase in the compactness of the vitreous network when ZnO oxide is progressively introduced. The Fourier transformed infrared and Raman spectroscopy investigations revealed that the glass network depolymerized with the increase of ZnO content. This result induces the disruption of metaphosphate chains (Q2) suggesting the formation of phosphate dimers (Q1). The DSC analysis shows the increase of Tg and Tc with the incorporation of ZnO oxide is due to the devel­ opment of a more compact network structure. Optical properties were investigated by means of UV–visible spectroscopy. The variation of the band gap energy (Eopt) of the glass series were attributed to the increase of the non bridging oxygen (NBO) resulting from the disruption of the metaphosphate chains.

1. Introduction Great interests for phosphate glasses have been considered over the past several decades because of their particular properties which make them suitable for a large variety of applications. Due to their lower melting and softening temperature, high electric conductivity and optical characteristic, phosphate glasses have been the subject of a large number of applications particularly in medicine, biology, optic, electronic [1–46]. Furthermore, alkali phosphate glasses have attracted more attention because of their high ionic conductivity, low melting point and strong glass-forming character [4]. Thus, phosphate and silicate glasses are an important materials

which can extensively used for laser sources and fiber amplifiers [3]. The phosphate network is based on corner-sharing PO4 units which form chains and rings or isolated PO4 groups [6]. Furthermore, the phosphate network is commonly described by the Qn tetrahedral sites (n ¼ 0 … 3), when n is the number of bridging oxygens (BO) per Q unit to neighbor phosphate tetrahedral [2,11–13,16,20,23,29,32,33]. It is well known that phosphate glasses have poor chemical dura­ bility, volatile and hygroscopic. These characteristics decrease the sta­ bility of glasses and limit their use in several technological applications [5]. On the other hand, the addition of transition metal oxides (TM) such as: CuO, MgO, ZnO, MnO, CaO, SrO … into the vitreous network induces the cleavage of P–O–P bridges and the formation of non-bridging oxygen

* Corresponding author. E-mail address: [email protected] (R. Oueslati-Omrani). https://doi.org/10.1016/j.matchemphys.2019.122461 Received 29 August 2019; Received in revised form 14 November 2019; Accepted 16 November 2019 Available online 18 November 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

R. Oueslati-Omrani and A.H. Hamzaoui

Materials Chemistry and Physics 242 (2020) 122461

atoms (NBO) [2,5,11–13,16,17,19–21,25,29,32,33]. Among several oxides mentioned above, zinc oxide addition has considerable attention because these zinc oxide containing glasses have numerous applications especially in optic field as LED light sources and substrates for optical waveguides. Zinc doped glasses can also play an important role in bone formation and mineralization [21]. Furthermore, the addition of ZnO oxide to the glass network is ex­ pected to improve the chemical stability of the vitreous structure due to the formation of P–O–Zn ionic bond. This suggests there is an increase in the compactness and the rigidity of the glass network [5,21]. Depending on the content, the structural role of ZnO oxide can change from network former to network modifier. ZnO oxide acts as a network former by creating ZnO4 structural units. When it is octahe­ drally coordinated, zinc oxide acts as a network modifier [21,33,42]. Compared to phosphate glasses, silicate glasses exhibit particular physico-chemical properties that make them suitable for a large number of applications. In fact, silicate glasses are chemically durable, thermally stable and optically transparent at excitation and lasing wavelength. Furthermore, the higher viscosity of glasses allowed them to be formed, cooled and annealed without crystallization. These amorphous materials are useful especially in optics as lenses or beam splitters in optical telecommunications, micro-and optoelectronics and in near IR windows [3,7]. In addition, silicate glasses are an attractive host matrix for transition metal ions (TMI) due to their excellent optical and mechanical proper­ ties, good chemical stability, and high UV transparency [3,7]. However, referring to literature, few publications have also recently appeared that studied the influence of the addition of ZnO oxide on the structure of glasses with a mixed phosphate-silicate glasses. For this purpose, our investigation is an extension to our previous work based on the structural, thermal and optical properties of phosphate glasses doped with SiO2 [11], this work aims to investigate the effect of the addition of ZnO on the structural and spectroscopic properties of phosphate based silicate glasses having a general formula: (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol). It is anticipated that there would be significant correlation between the changes in physical properties of the studied glasses and their structures investigations that can be carried out and discussed with the variation of ZnO content.

Table 1 Analyzed and nominal glass compositions for (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol) glass series: (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (e) 25 mol% ZnO, (f) 30 mol% ZnO. Na2O (mol%) Nominal/ analyzed

P2O5 (mol%) Nominal/ analyzed

SiO2 (mol%) Nominal/ analyzed

ZnO (mol%) Nominal/ analyzed

45/45.2 � 2.3 40/44.2 � 2.2 37.5/39.2 � 2.0 35/34.5 � 1.73 32.5/33.45 � 1.70 30/28.3 � 1.41

45/46 � 2.3 40/39.5 � 2.0 37.5/38 � 1.90 35/37.54 � 1.90 32.5/33.5 � 1.70 30/33.6 � 1.70

10/8.8 � 0.44 10/9.2 � 0.50 10/9.1 � 0.50 10/9.31 � 0.50 10/9.65 � 0.50 10/9.35 � 0.50

0/0 10/8.1 � 0.41 15/14.7 � 0.74 20/18.65 � 0.93 25/23.4 � 1.20 30/28.75 � 1.44

2.3. Measurements of density Glass densities (ρ) were measured utilizing the notable Archimedes technique with relative error �0.03 gcm 3 using water as the liquid [4, 11]. The molar volume of glasses was computed from the relation Vm ¼ M/d, where M is the molecular weight and d is the density. 2.4. Variations of refractive index The samples were studied by spectroscopic ellipsometry (SE) tech­ nique in order to determine the optical properties such as the refractive index (n). Spectroscopic ellipsometry (SE) experiment was performed at room temperature using an automatic ellipsometer SOPRA GES 5. Data were collected in the 0.6–5eV region, at the incidence angle of 75� . The ellipsometer measures the ratio between the complex Fresnel reflection coefficients rp, rs, for the p- and s-polarization components as defined by the following equation [11,47,48]:

ρ¼

rP ¼ tan ψ expðiΔÞ rs

where ψ represent the amplitude ratio of the perpendicularly polarized waves after their reflection from the surface of the studied samples and Δ denotes their phase shift. The optical functions are usually in the form of a complex refractive index: N ¼ n þ ik, where N is determined from the measured reflectance ratio ρ using the following equation [11,47,48]: sffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffiffiffi ! 1 ρ N ¼ sinϕ0 1 þ tan2 ϕ0 1þρ

2. Structures 2.1. Glass preparation Glasses with a composition of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.1 mol) were prepared using analytic grade reagent of NaH2PO4.H2O (Sigma Aldrich), ZnO (Sigma Aldrich) and silica SiO2 (Sigma Aldrich) in the suitable proportions. The mixture having the desired compositions was heated in platinum crucible at 200 � C for 1 h. The temperature was progressively increased to 1200 � C and held constant for 30 min to from bubble free melt. The melt was quenched at room temperature in air to form a clear glass. In order to accomplish homogeneity, samples were annealed below their glass transition temperature for 2 h too much repetition. The amorphous state of these glasses was confirmed by an X-ray diffraction. The nominal and analyzed glass compositions are shown in Table 1. Samples were stored in a desiccator and taken out only at the time when experiments are carried out.

2.5. DSC study

2.2. XRPD and ICP-AES analysis

2.7. Raman spectroscopy

The XRPD patterns of the glass series are plotted in Fig. 1. The results show the amorphous states for this glass series. The glassy compounds are labeled as Z0, Z10, Z15, Z20 and Z30.

The Raman spectra were recorded on powder of glasses using a Labram HR800 micro Raman operating in the 50-4000 cm 1 range. An internal He–Ne laser source (λ ¼ 488 nm) was used in this study.

The glass transition and crystallization temperature were determined with a Metler Toldo differential scanning calorimetry (DSC) using a heating rate of 10� Cmin-1 in air. Platinum crucible was used as sample holder. 2.6. FTIR spectroscopy Infrared spectra of the glass series were recorded at room tempera­ ture with a PerkinElmer (FTIR 2000) spectrometer using KBr pellets in the frequency range of 400-4000 cm 1. The samples were prepared by grinding about 9 mg of glass powder with 300 mg of spectroscopic grade dried KBr.

2

R. Oueslati-Omrani and A.H. Hamzaoui

Materials Chemistry and Physics 242 (2020) 122461

Intensity (a.u)

(f) (d) (c) (b) (a) 10

20

30

40

50

Fig. 1. X Ray diffraction of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol%)glass series: (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (f) 30 mol% ZnO.

2.8. UV-VIS spectrsocsopy

As shown Fig. 2, the density increases with the incorporation of ZnO

UV-VIS-NIR absorption spectra of prepared glasses were recorded with a PerkinElmer Lambda 950 spectrometer in the range of 200 and 1800 nm at room temperature using air as reference. All the optical measurements were carried out at room temperature.

.

3. Results and discussion 3.1. Density and molar volume The estimation of both density (d) and molar volume (Vm) of the studied glasses were given in Table 2. The density is an adequate parameter that depends on the compactness, the geometrical configu­ rations including the coordination number, the cross-link densities and the dimensions of the interstitial spaces of the glass [4,11].

1

Glass composition

Density (gcm 3)

Vm(cm mol )

Tg (� C)

Tc (� C)

ΔT

OPD (gatml 1)

0.9NaPO30.1SiO2 0.8NaPO30.1SiO20.1ZnO 0.75NaPO30.1SiO20.15ZnO 0.7NaPO30.1SiO20.2ZnO 0.65NaPO30.1SiO20.25ZnO 0.6NaPO30.1SiO20.3ZnO

2.45 � 0.1 2.59 � 0.1

39.90 � 1.2

291 �5 295 �5

395 �5 412 �5

104

72.67 � 2.2 73.00 � 2.2

297 �5

415 �5

118

37.00 � 1.1

117

2.66 � 0.1

35.55 � 1.1

2.70 � 0.1

34.65 � 1

300 �5

445 �5

145

72.15 � 2.2

2.79 � 0.1

33.23 � 1

–

–

–

72.2 � 2.1

2.83 � 0.1

32.30 � 1

305 �5

450 �5

144

71.23 � 2.1

3

44

2.9

42

2.8 Density (g/cm3)

3

73.12 � 2.2

2.7

40

2.6

38

2.5

36

2.4 2.3

34

2.2

32

2.1 2

0

5

10

15

20

25

30

molar volume (Cm3/mol)

Table 2 Glass composition, density, molar volume (Vm), Tg, Tc, ΔT, oxygen packing density (OPD) of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol) glass series: (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (e) 25 mol% ZnO, (f) 30 mol% ZnO.

30

ZnO%mol Fig. 2. Density and molar volume variations of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol%) series glasses. Blue plot is density and red plot is the molar volume. 3

R. Oueslati-Omrani and A.H. Hamzaoui

oxide from 2.45 to 2.84gcm 3. Whereas, the molar volume, Vm, de­ creases linearly from 40 to 32.30cm3mol-1 when x rises from 0 to 30 mol %. This result can be explained by the increase in the compactness of the structure due to the formation of P–O–Zn ionic bonds.

320 315 310 Tg (°C)

305

The structure of the glass series can be studied using the concept of oxygen packing density (OPD) which defines the fractional filling of space by oxygen ions within the glass. This parameter can be esti­ mated by the formula:

285 280 275

where C is the number of oxygen atoms in the composition [11,42, 49–51]. It is a measure of tightness of packing of the oxide network with the increase of ZnO proportion. The calculated values are presented in Table 2. Inspecting these data, one can note that the OPD decreases with the introduction of ZnO content in the structure. This variation indicated that the structure became more distorted due the increase in the number of non-bridging oxygen (NBO’s) resulting from the cleavage of P–O–P bridges when ZnO oxide is progressively incorporated [49–51].

200

300

Tc

25

30

35

(b) (a)

Rm ¼

(c)

Tg

20

The refractive index is an important physical parameter which can control the changes in physical properties with the variation of glass composition [3]. For glassy compounds, refractive index is a fundamental parameter that is strongly relevant to optical device performance and reliability. This has prompted a large number of researchers to carry out in­ vestigations to ascertain the relation between refractive index and glass composition [10]. The refractive index is one of the fundamental properties of mate­ rials, because it is closely related to the electric polarizability of ions and the local field inside the material [3,10,11]. For zinc phosphate based silicate glasses with a general formula (0.9x)NaPO3-xSiO2-0.1ZnO (0 � x � 0.1 mol) glass series presented in a previous work, the refractive index value is low and does not exceed 1.49 which is characteristics of phosphate glasses [11]. For this glass series, the refractive index values show that this parameter is between 1.31 and 1.45 when x rises from 0 to 30 mol% of ZnO oxide which indicated that the refractive index values are closely related to the glass density [3,10,11]. Furthermore, there are many factors which could influence the refractive index value such as density, polarizability of the first neighbor ions coordinated with it (anion), coordination number of ion, electronic polarizability of the oxide ion and optical basicity [3,10,11]. The molar refractivity (Rm) was estimated from the refractive index and the molar volume (Vm) using the Lorenz-Lorenz equation [3,10,11].

(d)

-10

15

3.3. Refractive index measurements

(f)

0

10

matrix in composition (a) has the lowest thermal stability indicating a tendency towards crystallization. A crucial parameter for technological application is the thermal stability [42]. Glasses with ΔTi100 � C will require a large amount of heat energy to activate the crystallization processes and are considered stable [42]. A ΔTi100 � C provides broad working change, which means that the glass can be heated above Tg without causing crystallization [42]. Table 2 shows that ΔT increases from 104 � C to 145 � C when x rises. This indicates that glasses with higher ZnO content are thermally stable which induces a stronger inhibition to nucleation and crystallization process [11–13,15,29,32,33].

30

10

5

Fig. 4. Glass transition (Tg) and crystallization (Tc) temperature of (0.9-x) NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol%) series glasses.

Effect of the chemical composition on the glass transition (Tg) and crystallization temperature (Tc) is shown in Table 2. The DSC curves for the glass series are depicted in Fig. 3. With increasing ZnO content, glass transition temperature, Tg, in­ creases linearly for all glass compositions as shown in Fig. 4. This behavior is undoubtedly due to some changes in the nature of bonding in the vitreous network. Tg and Tc values were affected by the cation field strength. As a result an increase in Tg values for the glassy compounds. On the other hand, the cation bond strength can be strictly related to the bond length which defined the charge the cation field divided by the square of the cation-oxygen distance [11–13,15,29,32,33,42]. The variations in Tg and Tc values indicate the increase of the rigidity of the glass network due to formation of P–O–Zn bonds that strength­ ened the vitreous structure. Based on these parameters, the thermal stability (ΔT) of the glass series was determined according to Dietezel. This parameter (ΔT) can be expressed by the temperature difference between Tg and Tc, ΔT ¼ Tc - Tg. The increase in ΔT values delays the nucleation process which indicated a better stability of the glass [29]. Inspecting these data, one can note that the undoped ZnO oxide glass

20

0

ZnO%mol

3.2. DSC study

Heat flow (mw)

295 290

OPD ¼ 1000C x dGlass /MGlass

100

300

465 455 445 435 425 415 405 395 385 375

Tc (°C)

Materials Chemistry and Physics 242 (2020) 122461

ðn2 1Þ Vm ðn2 þ 2Þ

The molar electronic polarizability αm was calculated using the relation of Clasius-Mosotti as follows [3,10,11]:

400

Temperature (°C)

3 4

αm ¼ ΠNRm

Fig. 3. DSC curves of (0.9-x)NaPO3-xZnO-0.1SiO2 glass composition: (0 � x � 0.3 mol%): (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (f) 30 mol% ZnO.

where N is the Avogadro number. The value of 34ΠN is known as the 4

R. Oueslati-Omrani and A.H. Hamzaoui

Materials Chemistry and Physics 242 (2020) 122461

m

-25

/Vm (*10 )

Lorentz function. Table 3 summarizes the values of Rm and αm. This latter shows an increase of the two parameters (Rm and αm) with the addition of ZnO oxide. Inspecting this data, Rm is in the range of 10.52–11.47cm3mol-1 and αm value is between 4.17 and 4.55 Å when x rises from 0 to 30 mol% of ZnO. The presented results confirms that the refractive index depends essentially on both density and molar electric polarizability of glass [3, 10,11,42]. Since, the ion oxide polarizability (α) represents the state of the average polarizability of glass matrix that can be defined by the capacity to donate electrons to the neighbor cations [42]. In the present work, we found that the refractive index (n) depends on the ratio (Vαmm ). This parameter shows that the refractive index (n) for the glass series increases linearly versus the ratio (Vαmm ) as shown Fig. 5.

This result can be probably correlated to the electronic polarizability of oxide ions which is an important factor affecting the refractive index for each glass composition [3,10,11]. In Na2O ionic rich glasses, the polarizability of oxygen ions has the lowest value (αO2 ¼2.45 Å) compared to CuO oxide based glasses (αO2 ¼2.55 Å3) [3,10,11].

1,30

1,32

1,34

1,36

1,38

1,40

1,42

1,44

Refractive index (n)

Fig. 5. variations versus the refractive index (n) of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol%) glass series.

Duffy et al. suggested that increasing the optical basicity ( Λ ¼ � � 1:67 1 α 12 ) indicates an increase in the effective electronic O

density of the oxide ions and accordingly, increasing covalency in the oxygen-cation bonding [3,10,11].

Transmission (a.u)

760

For this glass series, the incorporation of ZnO oxide causes a decrease in the molar volume making the structure more compact. 3.4. Spectroscopic analysis 3.4.1. FTIR spectroscopy In order to investigate the structural role of ZnO oxide, FTIR spec­ troscopy was carried out and the spectra are presented in Fig. 6. The characteristics features of 0.9NaPO3-0.1SiO2 are: the PO2 asymmetric stretching vibration band (υas(PO2)) near 1280 cm 1, the PO2 symmetric stretching vibration band (υs(PO2)) at 1150 cm 1, the υas(PO3) groups (chain end groups) at 1100 cm 1, the υs of PO3 groups at 1000 cm 1, the υas of P–O–P groups at 880 cm 1, the υs of P–O–P groups at 780 and 720 cm 1 and the deformation mode of P–O-(PO34 ) groups at 535 and 480 cm 1 [11–13,15,29,32,33]. The FTIR spectra of the glass series revealed the appearance of some bands assigned to phosphate-silicate glasses in the range 1000–1300 cm 1. The band situated at 1100 cm 1 is assigned to asymmetric

n

αm

Rm(cm3mol

0.9NaPO30.1SiO2 0.8NaPO30.1SiO2-0.1ZnO 0.75NaPO30.1SiO20.15ZnO 0.7NaPO30.1SiO2-0.2ZnO 0.65NaPO30.1SiO20.25ZnO 0.6NaPO30.1SiO2-0.3ZnO

1.31 � 0.05 1.36 � 0.05 1.38 � 0.06

4.17 � 0.2 4.38 � 0.2 4.39 � 0.2

10.52 � 0.43

1.40 � 0.06 1.43 � 0.06

4.46 � 0.2 4.52 � 0.2

11.24 � 0.45

1.45 � 0.06

4.55 � 0.2

11.47 � 0.5

(Å3)

1

11.03 � 0.44 11.07 � 0.44

11.40 � 0.5

)

(αm/Vm) x10 25

Eopt (ev)

1.05 � 0.02 1.20 � 0.02 1.24 � 0.02

1.30 � 0.03 1.50 � 0.03 1.80 � 0.04

1.30 � 0.03 1.36 � 0.03

2.00 � 0.04 –

1.41 � 0.03

2.10 � 0.04

(e) (d) (c) (b) 1280 1400

1200

880

1150 1000

760 720 800

520 480

(a)

600

-1

Wavenumber (cm ) Fig. 6. Infrared spectra of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol%): (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (e) 25 mol % ZnO, (f) 30 mol% ZnO.

stretching vibration of SiO4 and PO4 tetrahedrons. It was observed that the weaker band at 840 cm 1 is assigned to the symmetric stretching vibration O–Si–O with two non bridging oxygen per SiO4 tetrahedron (Si–O–2NBO) called also Q2 groups. The band located at 1080 cm 1 is attributed to symmetric stretching vibration Si–O–Si. The band situated at 460 cm 1 is attributed to the bending vibration of Si–O–Si and O–Si–O bonds [11–13,15,29,32,33]. FTIR spectra of (0.9-x)NaPO3-0.1SiO2-xZnO, (0 � x � 0.3 mol) glass series are presented in Fig. 6. This later shows that when ZnO oxide is introduced, the asymmetric band of PO2 shifts from 1280 cm 1 shift to lower frequency indicating the depolymerization of phosphate chains when x increases [11–13,15,29,32,33]. The FTIR spectra revealed that the asymmetric band of P–O–P stretching mode shifted to higher frequency from 880 cm 1 for 0.9NaPO3-0.1SiO2 to 900 cm 1 in 0.6NaPO3-0.1SiO2-0.3ZnO glass composition. This result can be explained by the formation of P–O–Zn ionic bond. However, one can note that for 0.6NaPO3-0.1SiO2-0.3ZnO glass composition, FTIR spectrum revealed only a single band at 760 cm 1 assigned to P–O–P linkage in diphosphate groups (P2O47 ). These spec­ tral changes could be attributed to the reduction of infinite phosphate groups (P2O26 ) into shorter phosphate groups such as: P2O47 and PO34 [11–13,15,29,32,33].

Table 3 Refractive index, molar refractivity (Rm), molar electronic polarizability (αm) and band gap energy of (0.9-x)NaPO3-xSiO2-ZnO (0 � x � 0.3 mol) glass series: (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (e) 25 mol% ZnO, (f) 30 mol% ZnO. Glass composition

(f)

900

1220

5

R. Oueslati-Omrani and A.H. Hamzaoui

Materials Chemistry and Physics 242 (2020) 122461

3.4.2. Raman spectroscopy Raman spectroscopy is an adequate technique for structural analysis of glass matrix. Raman bands are generally characteristics of structures involving chains of linked tetrahedral that may be found in crystalline, glassy phosphates and silicate because it can detect the local changes in the environment of Si–O–Si and P–O–P bonds [1,2]. Raman spectro­ scopic analysis have been recorded for (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol) glass compositions as shown Fig. 7a. For non doped phosphate-silicate glass composition (0.9NaPO30.1SiO2), Raman spectrum revealed a sharp peak at 1164 cm 1 assigned to the PO2 symmetric vibrations (νs(PO2)) of the non-bridging oxygen atoms (NBO) bonded to phosphorus atoms (O–P–O) in metaphosphate chains(Q2) as shown Fig. 7b. The large band situated at 685 cm 1 is attributed to the stretching vibration band of the bridging oxygen atoms connecting two PO4 tetrahedrons (P–O–P) in metaphosphate groups. The wide band at 1020 cm 1 is attributed to the symmetric stretching vibration of PO3 groups in diphosphate groups (Q1) [11–13,15,29,32, 33]. With increasing ZnO content, the bands situated at 1274, 1164 and 685 cm 1 decrease simultaneously as was observed with magnesium, manganese and zinc phosphate glasses. Addition of ZnO causes progressive spectral changes. In particular, the band situated at 1274 cm 1 disappear for x ¼ 30%mol. These changes can be attributed to the disruption of P–O–P bridges and the depolymerization of phosphate chains leading to the formation of phosphate dimers (Q1) [11–13,15,29,32,33]. At higher ZnO proportion, a new bond appeared for x ¼ 10%mol at about 1020 cm 1 attributed to the symmetric stretching vibration νs(PO3) of the NBO in PO4 tetrahedron in pyrophosphate groups (Q1) [12,13,15,29,32,33]. These spectral modifications could be correlated to the high P-NBO π character due to the low condensation of phosphate chains when ZnO oxide is incorporated.

Intensity (a.u)

0.9NaPO3-0.1SiO2 glass composition

400

Intensity (a.u)

600

800

1000

1200

1600

1800

� � I I0

corresponds to

the absorbance. It was observed that there is no absorption sharp edge, which indi­ cated the vitreous nature of samples. This observation has been detected in phosphate glasses [4,10,11,14,16,37,41–43]. According to Davis and Mott, the expression of the absorption co­ efficient α (ν) as a function of photon energy (hν) for direct and indirect optical absorption, was given by the relation as follows: �n A hν Eopt αðνÞ ¼ hν where A is an energy-independent constant, Eopt is the optical band gap energy and n is a constant which determines the type of the optical transition. For direct allowed transition n ¼ 2 and in the case of indirect allowed transition n ¼ 12. For glassy materials, the indirect transitions are valid according to Tauc relations [4,10,11,14,16,37,41–43]. Fig. 8 represents the variation (αhν)2 versus photon energy (hν) for all glass composition. Plotting (αhν)2 versus photon energy (hν), we obtain the Eopt values by extrapolation the linear region of (αhν)2 against photon energy (hν) plots at (αhν)2 ¼ 0. From these results, the best are obtained for n ¼ 1 2which indicates that the indirect allowed transition is responsible for the band gap energy for this glass series. The band gap value, obtained by extrapolation are reported on Table 3. Inspecting these data, one can notice that Eopt increases with the incorporation ZnO from 1.3 to 2.1ev. This quantity is not only influenced by the chemical composition also by the structural rearrangement in the glass matrix [4,10,11,14,16,22,37, 41–43]. Fig. 8 shows clearly that the Eopt values are not only dependent on the composition of the glass but also on the oxygen bonding in the vit­ reous network [4,10,11,14,16,22,37,41–43]. Any changes of oxygen bonding leading to the formation of non bridging oxygens (NBOs) causes a change of the absorption character­ istics of the glass [4,10,11,14,16,22,37,41–43]. Generally, there are two contributions to the conductivity: the

(a) 1600

1400

where d is the thickness of the glass sample and ln

(b)

1400

1200

materials, the transition is almost described by indirect transition [4,10, 11,14,16,37,41–43].The optical absorption coefficient α(hν) of the prepared glasses was calculated at different wavelengths by using the relation [4,10,11,14,16,37,41–43]. � � 1 I α ¼ ln d I0

(c)

400

1000

Fig. 7b. Raman spectrum of 0.9NaPO3-0.1SiO2 (a) glass composition.

(d)

1274

800

Wavenumber (cm )

(f)

1164

600

-1

The study of optical absorption, particularly the absorption edge, is useful for the investigation of optically-induced transitions and for getting information about the band gap energy [4,10,11,14,16,37, 41–43]. This is an interesting parameter for the applications of the studied material. It is known that optical transition occurs through the region between conduction and valence bands (optical band gap) directly or indirectly. The absorption in the UV and VIS spectral range are caused by electron transitions from unexcited to excited states. For vitreous

1020

1164

685

3.5. Optical absorption

685

1274

1020

1800

-1

Wavenumber (cm ) Fig. 7a. Raman spectra of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol%): (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (f) 30 mol % ZnO. 6

R. Oueslati-Omrani and A.H. Hamzaoui

Materials Chemistry and Physics 242 (2020) 122461

2

2

( h ) cm ev

2

(f)

(f) (d) 0

1

2

Eopt

3

h (ev)

4

5

6

2

cm ev

2

(c)

2 as a function of photon energy of (f) The of 0.6NaPO3-0.1SiO2-0.3ZnO glass composition *The Eopt values were determined by the extrapolation 2 against photon energy of the linear region of 2=0. plots at

h

(b)

(a)

1

h

ev

2

3

Fig. 8. The (αhν)2 as a function of photon energy of hν of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol) glasses: (a) 0 mol% ZnO, (b) 10 mol% ZnO, (c) 15 mol% ZnO, (d) 20 mol% ZnO, (f) 30 mol% ZnO. *Obtaining the lines corresponding to the curves of (αhν)2 against photon energy (hν) is probably due to the superposition effect for all the glass compositions.

electronic conductivity and ionic conductivity. For glasses containing transition metal oxide (TMO), the electronic conduction is dominant. This property was explained by the ability of ions to exist in one than more valence state. It occurs by the electron transfer from ions in a lower valence state to those in a higher valence state. But, in our case, the zinc ion exists in one valence state as Zn2þ. So, the conductivity is governed by the ionic conduction. The last one is affected by the number of mobile ions and their mobilities. For this glass series, the contribution of electronic conductivity is negligible. This result can be explained by the formation of ionic bond P–O–Zn when ZnO is progressively incorporated inducing the increase in the compactness and the rigidity of the vitreous structure [4,44]. On the other hand, the increase in the band gap energy (Eopt) value from 1.3ev to 2.1ev when x rises. This variation could be attributed to

the structural disorder inducing the increase of the number of NBOs resulting from the disruption of P–O–P bridges when ZnO oxide is pro­ gressively introduced [4,15,22]. Furthermore, the shift to higher energy could be correlated to the increase of the cross linking density in the glassy matrix [4,10,11,14,16, 22,37,41–43,46]. Because the NBOs bonds are predominantly ionic character and consequently have a lower bond energy [4,10,11,14,16, 22,37,41–43,46]. The higher value of the band gap energy revealed the increase of the cross-linking network due the introduction of ZnO oxide [4,10,11,14,16,22,37,41–43,46]. From Fig. 8, one can note that with the addition of ZnO content, Eopt increases linearly from 1.3 to 2.1 ev. This result can be explained by the structural rearrangement in the vitreous network. Furthermore, the addition of ZnO may enhance the degree of polymerization of the

2,4 2,2

Eopt ( eV)

2,0 1,8 1,6 1,4 1,2

0

5

10

15

20

25

30

%x ZnO Fig. 9. Variation of optical band gap energy of (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol) glass series. 7

R. Oueslati-Omrani and A.H. Hamzaoui

Materials Chemistry and Physics 242 (2020) 122461

structure of zinc phosphate-silicate glasses by the formation of P–O–Zn ionic bond.Fig. 9.

[21]

4. Conclusion

[22]

The influence of ZnO addition on the structure, physical and optical properties has been studied for phosphate based silicate glasses having a general formula: (0.9-x)NaPO3-0.1SiO2-xZnO (0 � x � 0.3 mol). Amorphous state was investigated by means of FTIR, Raman and UV–visible spectroscopy in order to study the structural role of ZnO oxide. The decrease in the molar volume indicates a structural modification of the structure with increasing ZnO content. Spectroscopic analysis revealed the formation of pyrophosphate groups (Q1) resulting from the depolymerization of infinite meta­ phosphate groups (Q2) when the ZnO oxide is gradually incorporated. Furthermore, the indirect optical band gap energy of this glass series increases with the addition of ZnO oxide. This suggests the increase in the NBOs resulting from the cleavage of P–O–P bridges which induces the shortening of the metaphosphate chains.

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