Synthesis, structural, optical and solid state NMR study of lead bismuth titanate borosilicate glasses

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

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Synthesis, structural, optical and solid state NMR study of lead bismuth titanate borosilicate glasses Abhishek Madheshiyaa, Krishna Kishor Deyb, Manasi Ghoshb, Jai Singhb, Chandkiram Gautama, a b

⁎

Advanced Glass and Glass Ceramics Research Laboratory, Department of Physics, University of Lucknow, Lucknow, Uttar Pradesh 226007, India Department of Physics, Dr. Harisingh Gour Central University, Sagar, Madhya Pradesh 470003, India

A R T I C LE I N FO

A B S T R A C T

Keywords: XRD Infrared spectroscopy Raman spectroscopy UV–vis spectroscopy Nuclear magnetic resonance

Various compositions (0.0 ≤ x ≤ 1.0) in the glassy system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3] doped with one mol percent of graphene nanoplatlets (GNPs) have been synthesized using the conventional melt-quenching technique. X-ray diffraction (XRD) study revealed the formation of bulk and transparent lead bismuth titanate borosilicate glasses. Density and molar volume were determined using the liquid displacement method. Structural analyses were carried out in detail using different characterization techniques such as infrared spectroscopy, Raman spectroscopy, UV–vis spectroscopy and solid state nuclear magnetic resonance (SSNMR) spectroscopy. 29Si and 11B-MAS-NMR-spectral analysis for five glass samples with different fraction of PbO reveal that as increases the content of PbO, the silicate and borate network become more polymerized.

1. Introduction Glasses are the utmost significant and high-performance amorphous materials used for different scientific applications and commercially available for several useful applications due to its incomparable optical transparency and excellent electrical insulating properties [1]. Especially, lead based bismuth substituted glasses have their manifold applications in the field of optical and optoelectronic devices such as ultrafast switches, low-loss fiber optics, infrared windows, infrared transmitting materials, optical isolators, fiber optical amplifiers and oscillators [2–7]. The substitution of heavy metal oxides like lead and bismuth increases the refractive index of the glasses and make them denser for non-linear optical properties. However, the lead oxides (PbO) not only act as glass modifiers but also reduce the viscosity of the glasses during melting. Although, PbO substituted with bismuth borate glasses; the glasses are predicted to become well stable against devitrification and showed chemically inert behavior [8]. Moreover, bismuth exhibit Bi3+ ionic state and ions can exist in Bi+, Bi4+ and Bi5+ states respectively in various glassy systems according to their fabrication methods, structural and atmospheric temperatures etc. [9]. Rao et al. synthesized a ternary glassy system, PbO-Bi2O3-B2O3 and found the presence of Bi-O-Pb linkages having a decrease in intensity of the absorption bands with increasing the magnetic susceptibility and density [10]. Chen et al. have reported the IR study of PbO-Bi2O3-B2O3 glasses and found the various vibrational modes of the borate network

⁎

that exists with different absorption peaks towards the lower wavenumber side due to the vibration of different metal cations such as Pb2+ and Bi3+ [11]. Moreover, UV–visible spectroscopy revealed the cutoff moves to longer wavelength side with increasing the mixed concentrations of PbO and Bi2O3. The reported values of density lie in the range of 6.57–8.33 g/cm3 [11]. Further, Babu et al. reported that the density and optical band gap of the various synthesized glass samples derived from the glassy system Li2O-PbO-B2O3-SiO2-Bi2O3-Al2O3 was found to be 4.12–4.45 g/cm3 and 3.39–3.41 eV respectively [12]. The IR and Raman spectroscopy of these glasses were carried out and found a decrease in the degree of disorders due to the presence of Al3+ ions that occupied tetrahedral as well as octahedral positions. Recently, Suresh et al. have synthesized a ternary glassy system, PbO-Bi2O3-SiO2 and reported the structural units for the fabrication of glassy network of [BiO3] pyramidal, [BiO6] octahedral and [SiO4] tetrahedral units [13]. The Raman spectra of these glasses explore the bands due to Bi-O-Bi and BieO stretching vibrations of BiO6 octahedral units. They also reported slightly less values of the optical band gap in comparison to the previously reported glassy system Li2O-PbO-B2O3-SiO2-Bi2O3-Al2O3 that were lying in the range of 2.76 to 3.10 eV while the density of these glass samples was found to be large i.e. 5.04 to 5.11 g/cm3 [13]. Therefore, in view of these results, it is noticeable that the values of densities are found to be large and optical band gap values are lesser. Thus, an idea has come to our knowledge to incorporate the GNPs as a dopant in lead bismuth titanate (PBT) borosilicate glasses to overcome

Corresponding author. E-mail address: [email protected] (C. Gautam).

https://doi.org/10.1016/j.jnoncrysol.2018.10.009 Received 2 July 2018; Received in revised form 28 September 2018; Accepted 7 October 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Madheshiya, A., Journal of Non-Crystalline Solids, https://doi.org/10.1016/j.jnoncrysol.2018.10.009

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water, W4 = weight of specific gravity bottle with distilled water. The molar volumes were calculated by using the value of density in the Eq. (1). Therefore, the molar volume equation has been given below [21]:

the high density and low band gaps, because the graphene may decrease the value of density and increase the optical band gap of synthesized glasses. Here in there are two main reasons to incorporate GNPs as a dopant in PBT borosilicate glasses: the first is large surface area, 2630 m2/g and second is high electron mobility at room temperature, 15,000 cm2 V−1 s−1 [14]. It is well reported that the lead containing bismuth titanium borosilicate glasses are technologically play very significant role in manufacturing ultra low loss waveguide devices, glass to metal seals, infrared transmitting devices and optical gratings [15,16]. More recently, Gautam et al. investigated several new perovskites of PBT borosilicate glass and glass ceramics and studied well their optical and electrical properties [17–20]. No one to the best of our knowledge has been attempted to synthesize the PBT borosilicate glasses doped with 1 mol percent of GNPs to investigate their optical and electrical properties. However, in the present investigation, attempts have been made to synthesis the various optimized compositions (0.0 ≤ x ≤ 1.0) in the glass system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs with structural, optical and solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study using various characterization techniques such as XRD, density, molar volume, IR spectroscopy, Raman spectroscopy, UV–vis spectroscopy and solid state MAS-NMR spectroscopy.

VM =

(2) th

where, Mi be the molecular weight of the i components and Xi be the molar fraction of the ith components. IR spectroscopy was used to study the structural change, bonding nature and also the source of formation of various absorption peaks by using JASCO FT/IR-5300 in transmittance mode in the wave number range from 500 to 2000 cm−1. The samples for recording the IR spectra have been prepared by mixing the powdered glass samples with KBr powder in the ratio of 1:99 into an agate mortar and then pellets were formed of the diameter of ~12 mm by pressing in a hydraulic press machine. The pellet samples were then dried in an oven at 100 °C with heating @ 2 °C/min for 30 min to remove the moisture from the pellet samples. The Raman spectroscopy was carried out to study the vibrational modes of microstructural units and to explore the structure of the glassy materials. The Raman spectra were recorded on powdered glass samples obtained by Micro Raman setup, Renishaw, UK, equipped with 1800 lines/mm diffraction grating. An Olympus (model MX-50) A/T spectrometer was employed and focused an argon ion laser beam as an excitation source of 10 mW power onto the samples having a wavelength of the order 15.4 nm. The scattered light was collected at 180o by scattering geometry and then the software GRAM-32 was used for the data collection. Raman spectra of powdered glass samples were recorded in the wave number range from 200 to 2000 cm−1. UV–vis spectroscopy of the glass samples were also performed using UV–visible spectrophotometer (Varian, Carry-50Bio) in the wavelength range from 200 to 1200 nm. The optical band gap energy, Eg, were determined by using Davis and Mott relation which concomitant with the absorption coefficient and the incident photon energy, hυ [22]:

2. Experimental procedures 2.1. Synthesis of glass samples High purity analytical reagent grade chemicals PbO, Bi2O3, TiO2, H3BO3, SiO2 and GNPs having purity > 99.8% were used for the preparation of various glass samples in the glass system 55[(PbxBi1x)TiO3]-44[2SiO2.B2O3] doped with 1 mol percent of GNPs with compositions (0.0 ≤ x ≤ 1.0). On the basis of the glass compositions, different amount of the raw materials were accurately weighed and mixed in an agate mortar using acetone as a mixing medium and then dried the powders in an oven at 100 °C for 30 min. The glass batches of 20 g were melted in high grade alumina crucible using Metrex made programmable electric furnace having silicon carbide heating elements. The melt was maintained at the casting temperature 1300 °C in the furnace for half an hour for refining and homogenization. The melt was poured into a steel mould and pressed by thick an aluminum plate and then immediately placed into a programmable preheated muffle furnace for annealing at 400 °C with heating @ 5 °C/min for 3 h to remove the residual stresses from the glasses. After annealing processes, the glasses were then cooled to room temperature within the furnace and glass samples were used for further characterizations.

α=

(hν − Eg )n (3)

hν

where, constant n decides the transition nature (n = 2 allowed for indirect transition and n = 1/2 allowed for direct transition). The direct optical band gap was calculated by using extrapolating the linear parts of the Davis-Mott curves, (αhυ)1/2 versus hυ. In most of the amorphous materials, the absorption coefficient, α(ν) of the optical absorption near the band edge depends on the photon energy, hν and exhibits an exponential behavior on the photon energy and it obeys the empirical relation given by Urbach [23]. This exponential equation as follows:

hν α (ν ) = αo exp ⎛ ⎞ ⎝ EU ⎠

2.2. Sample characterizations

⎜

δ=

1 α 29

(W2 − W1 ) .ρ (W4 − W1) − (W3 − W2 ) W

⎟

(4)

where αo is a constant and EU is the width of the band tails of localized states at the band edges into the energy gap, also known as Urbach energy which is associated with the amorphous nature of the materials. The skin depth or penetration depth, δ, represents the electromagnetic wave can reduce the amplitude after traversing a thickness of the sample. The skin depth related to the absorption coefficient, α by the following relation [24]:

XRD patterns of the glass samples were recorded in the 2θ range of 20o to 80o with the scanning rate of 3o per min using a Rigaku UltimaIV X-ray diffractometer with monochromatic Cu-Kα radiation (λ = 1.54 Å) and operated at 40 kV and 40 mA for confirming the amorphous nature of the glass samples. The densities of the irregularly shaped glass samples were measured by liquid displacement method (Archimedes principle) using double distilled water as the immersion liquid. Density measurements were repeated three times per glass sample. The average value of repeated density measurements of each glass samples are shown in Table 1. The density (ρ) of glass samples were calculated by using the formula [21]:

ρ=

∑ Xi . Mi ρ

(5) 11

Si and B magic angle spinning (MAS) solid state NMR experiments were also performed on JEOL ECX 500 NMR spectrometer. The spectrometer was well-equipped with a 3.2 mm JEOL double resonance MAS probe and all the experiments were performed with sample spinning speed of 10 kHz. The reference for 29Si chemical shifts measurement was done by using 2-dimethul-2-silapentane-5-sulfonate sodium

(1)

where, ρW = density of water (1 g/cm ), W1 = weight of empty specific gravity bottle, W2 = weight of specific gravity bottle with sample, W3 = weight of specific gravity bottle with sample and distilled 3

2

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Table 1 Glass samples code, compositions, density, molar volume, band gap and Urbach energy of the various glass samples in the system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]1GNPs. Glass samples code

BTG0.0 PBTG0.3 PBTG0.5 PBTG0.7 PTG1.0

Glass compositions (mol %) PbO

Bi2O3

TiO2

SiO2

B2O3

GNPs

0.0 7.85 14.57 22.99 40.61

46.95 37.87 30.11 20.37 0.0

8.05 9.28 10.32 11.64 14.39

21.72 21.72 21.72 21.72 21.72

22.28 22.28 22.28 22.28 22.28

1 1 1 1 1

Density ρ (g/cm3)

Molar volume VM (cm3/mol)

Band gap Eg (eV)

Urbach Energy EU (eV)

1.39 1.56 1.82 1.97 2.23

189.95 ± 0.9498 153.99 ± 0.7700 120.82 ± 0.6041 98.66 ± 0.4933 63.21 ± 0.3161

3.89 4.00 4.07 4.11 4.19

0.26 0.25 0.24 0.23 0.22

± ± ± ± ±

0.0070 0.0078 0.0091 0.0099 0.0112

salt (DSS) and that for 11B by using 1 M H3BO3 solution. For getting 29Si MAS NMR spectra a 90o pulse width of 3 μs was used with a relaxation delay of 30 s between two pulses. 11B MAS NMR spectra were obtained by the Half-echo method by using 1st and 2nd interval 4 ms and relaxation delay 10 s.

± ± ± ± ±

0.0778 0.0800 0.0814 0.0822 0.0838

4.2. Density and molar volume analysis

In the five letters of the glass sample code, starting the letters PBT denoted the content of lead bismuth titanate. The fourth letter G revealed the 1 mol percent of GNPs was used as an additive, while the fifth letter, i.e., 0.0, 0.3, 0.5, 0.7 or 1.0, representing the fraction of composition ‘x’ in the glass system. For example, glass sample PBTG0.3, the first three letters represents the content of the lead bismuth titanate, fourth letter G represents the 1 mol percent of GNPs was used as dopants, while 0.3 represents the value of composition, x = 0.3. The glass samples code along with the sample compositions are listed in Table 1.

The average calculated values of density and molar volume of PBT borosilicate glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 are enlisted in Table 1. Supplementary Fig. S1 shows the variations of density and molar volume with different mol % of PbO of all glass samples. For a microscopic point of view, increase in density of the glass samples with different amounts of PbO might be due to an increase in the number of non-bridging oxygen (NBO) atoms which attributed to a replacement of a low-density oxide (Bi2O3, 8.90 g/cm3) by a high-density oxide (PbO, 9.53 g/cm3). Therefore, the density of glass sample PTG1.0, x = 1.0 was found to be maximum, 2.23 g/cm3 while it was found to be minimum, 1.39 g/cm3 for glass composition, x = 0.0. On the other hand, the molar volume is defined as the volume occupied by the unit mass of the glass [25]. Thus, it is also observed from Supplementary Fig. S1(b) that the molar volume decreases from 181.40 cm3/mol to 57.88 cm3/mol for glass samples BTG0.0 and PTG1.0 as PbO content increases from 0.0 to 40.61 (mol%).

4. Results and discussion

4.3. IR spectroscopy

4.1. XRD analysis

Infrared spectroscopic measurements were performed to understand the molecular structure/bond formation mechanism of the synthesized glass samples. The IR spectra of various PBT borosilicate glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 doped with 1 mol percent of GNPs are shown in Fig. 2(a-e). Five different absorption bands were observed that are lies in the wave number range from 1737 to 676 cm−1. The various peak positions in the IR spectra obtained for all the glass samples are listed in Table 2. The peak positions are represented in descending order as 1, 2, 3, 4, 5. The first absorption peak was observed in the all IR patterns within the wave number range of 1718–1737 cm−1 and occurred due to asymmetric stretching relaxation of BeO bonds of trigonal BO3 units [26]. However, the second absorption peak lies in the wave number range from 1361 to 1372 cm−1, which assigned due to the stretching vibration of BeO bond in BO3 groups that is the chains or rings of metaborate, pyroborate and orthoborate groups [27,28]. The third absorption peak was observed at 1217 cm−1 for all IR spectra of the glass samples, attributed to the BeO bond stretching vibrations and BeO bridging between B3O6 and BO3 triangles [26]. The fourth absorption peak lies in the wave number range from 987 to 1015 cm−1 corresponds to stretching vibration of groups containing the tetrahedral BO4, that is the groups di, tri-, tetra- and pentaborate groups [29,30] and also due to increase in the number of non bridging oxygen and depolarization of the glassy network [31]. Particularly, the presence of fifth absorption band at the wave number 676 cm−1 could be assigned as B-O-B bond-bending vibrations from pentaborate group or bending vibrations of BO3 triangles [32] and combined vibrations of BO4 and PbO4 groups [33,34]. It is observed that the varying content of PbO increases the broadness of the absorption peaks and act as a glass network former. However, it has been reported that Bi2O3 act as network former while PbO itself acts as network modifier as well as the network former [34,35]. The addition of GNPs is slightly increasing the

3. Nomenclatures of glass samples

XRD patterns of the specified glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 have been shown in Fig. 1(a-e) respectively. The spectra of each glass samples show a broad diffuse scattering at different angles instead of Bragg's diffraction crystalline peaks; hence, this confirms the short range structural order characteristics of a glassy amorphous nature.

Intensity (a.u.)

(a)

(b)

(c) (d) (e) 20

30

40

50

60

70

80

2θ (Degree) Fig. 1. XRD patterns of glass samples (a) BTG0.0, (b) PBTG0.3, (c) PBTG0.5, (d) PBTG0.7 and (e) PTG1.0 in the glass system 55[(PbxBi1-x)TiO3]44[2SiO2.B2O3]-1GNPs. 3

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(a)

x=0.0

561 1361

1217

(b)

1737

1366

1217

489

676

x=0.3

992

676

(c)

x=0.5

(b)

1608

1368

472

(c) 447

1363 1217

994

(e)

(d)

x=0.7

2000

1723

1367 1217

1004

676

1015

1500

1250

800

400

Fig. 3. Raman spectra of glass samples (a) BTG0.0, (b) PBTG0.3, (c) PBTG0.5, (d) PBTG0.7 and (e) PTG1.0 in the glass system 55[(PbxBi1-x)TiO3]44[2SiO2.B2O3]-1GNPs.

1372 1217

1750

1200

-1

(e)

1718

1600

Raman Shift (cm )

x=1.0

2000

460

776

1203

676

(d)

769

1216 1728

(a)

1139

1368

987

Intensity (a.u.)

1734

% of Transmittance

1368

1368 cm−1, that is similar in frequency to the stretching vibrations of BeOe bond in BO4 units from different borate groups [41–43]. The band appeared in the wave number range of 1139–1216 cm−1 occurs due to asymmetric vibration of SiO4 tetrahedra [44] and also due to pyro-borate groups [45,46]. Apart from these few wide band peaks around 769 and 776 cm−1 for the glass samples PBTG0.7 and PTG1.0 which corresponds to the symmetric breathing vibration of six member rings with one or two BO3 triangles replaced by BO4 tetrahedra [47–51]. Among low wave number regions 447–561 cm−1, the bands are usually assigned to bending vibrations of the bridging oxygen (BO) bonds of Si-O-Si [52], stretching vibrations of BieOe and also due to BO33− vibrational mode [53,54]. The assignments of infrared and Raman bands in the spectra of various glass samples in the glass system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3] -1GNPs are listed in Table 3.

676

1000

750

500

-1

Wave Number (cm ) Fig. 2. IR spectra of glass samples (a) BTG0.0, (b) PBTG0.3, (c) PBTG0.5, (d) PBTG0.7 and (e) PTG1.0 in the glass system 55[(PbxBi1-x)TiO3]44[2SiO2.B2O3]-1GNPs.

broadness of the absorption peaks and also modifying the glassy network structures.

4.4. Raman spectroscopy Raman spectrum of PBT borosilicate glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 are shown in Fig. 3(a-e) and their peak positions have been listed in Table 2. Fig. 3(a) shows the Raman spectra of lead free glass sample BTG0.0, which exhibit a narrow peak at 1368 cm−1. When lead is substituted for the composition, x = 0.3 (Fig. 3b) the position of the above mentioned peak remains unchanged while another extra peak arises at 1139 cm−1. The similar changes were observed in the Raman spectra of the glass sample PBTG0.5, but for this composition, x = 0.5 the low-frequency component of the fundamental band shifts to a high wave number at 1608 cm−1 and at the same moment the intensity of the high-frequency component increases. The band at 1608 cm−1 is assigned due to BeOe stretching in metaborate rings and chains [36–40]. Raman spectra of glass samples BTG0.0, PBTG0.3 and PBTG0.5 have a narrow peak at

4.5. UV–Vis spectroscopy analysis The optical absorption spectra of various synthesized glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 were recorded at room temperature in the wave length range from 200 to 1200 nm. The UV–Vis absorption spectrums of various PBT borosilicate glass samples are shown in Fig. 4. As decreasing the concentration of Bi2O3, the absorption edges were slightly shifted towards the lower wavelengths side. Further, the amorphous nature of the glass samples was confirmed and well consistent with the results of XRD. Fig. 5 shows the Davis and Mott plots of all PBT borosilicate glass samples from which the optical band gap values were calculated. The calculated band gap values are clearly shown in the inset of Fig. 5. However, the optical band gap

Table 2 Peak positions in infrared and Raman spectra of various glass samples in the system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs. Glass samples code

BTG0.0 PBTG0.3 PBTG0.5 PBTG0.7 PTG1.0

IR peaks position (cm−1)

Raman peaks position (cm−1)

1

2

3

4

5

1

2

3

4

5

1734 1737 1728 1723 1718

1361 1366 1363 1367 1372

1217 1217 1217 1217 1217

987 992 994 1004 1015

676 676 676 676 676

– – 1608 – –

1368 1368 1368 – –

– 1139 – 1216 1203

– – – 769 776

561 489 472 447 460

4

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Table 3 Assignments of infrared and Raman bands in the spectra of various glass samples in the glass system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs. Wave number (cm−1) IR

Raman

1718–1737 1361–1372

1608 1368

1217

1139–1216

987–1015

769–776

676

447–561

IR assignments

Raman assignments

Asymmetric stretching relaxation of BeO bonds of trigonal BO3 units. Stretching vibration of BeO bond in BO3 groups that is chains or rings of metaborate, piroborate and ortoborate groups. B-O bond stretching vibrations and BeO bridging between B3O6 and BO3 triangles.

B-O– stretching in metaborate rings and chains. Stretching vibrations of BeOe bond in BO4 units from different borate groups. Asymmetric vibration of SiO4 tetrahedra and also due to pyroborate groups. Symmetric breathing vibration of six member rings with one or two BO3 triangle replaced by BO4 tetrahedra.

Stretching vibration of groups containing the tetrahedral BO4, that is, the groups di, tri-, tetra- and pentaborate groups and also due to increase of the number of non bridging oxygen and depolarimezation of the glassy network. B-O-B bond-bending vibrations from pentaborate group or bending vibrations of BO3 triangles and combined vibrations of BO4 and PbO4 groups.

the formation of non bridging oxygen. These results revealed that the optical band gap values changed with the variation of compositions [55,56]. The value of optical band gap was found to be lies in the range of 3.89–4.19 eV.

x=0.0 x=0.3 x=0.5 x=0.7 x=1.0

Absorbance (a.u.) 200

Bending vibrations of the bridging oxygen (BO) bonds of Si-OSi, stretching vibrations of BieOe and also due to BO33− vibrational mode.

4.6. Analysis of Urbach energy and skin depth

400

600

800

1000

Urbach energy is a scale of disorder in the system and materials with larger Urbach energy have a greater tendency to convert weak bonds into defects [57]. The plots of the log of absorption coefficient Vs photon energy for varying amounts of PbO are shown in supplementary Fig. S2. The values of Urbach energy, EU of the synthesized GNPs doped borosilicate glasses were obtained from the slope of the straight line of plotting ln (α) against the incident photons energy [58]. The reciprocal of the slope of the obtained straight lines leads to determine the Urbach energy EU. The calculated values of Urbach energy are listed in Table 1. It is found that the value of the band tail decreases from 0.26 to 0.22 eV with increasing the content of PbO which is due to the formation of bonding defects and non-bridging oxygen. The small range of Urbach energies suggests the possible existence of minimum defects in borosilicate glass system. It is evident that by increasing the concentration of PbO in the present glass system, Urbach energy values decreased due to the further disorder in the glass [59]. Glassy matrix attributed the behavior of Urbach energy with increasing concentration of PbO and leads to the disorderness of the atoms along with defects in the structural bonding. Fig. S3 (supplementary) shows the dependence of skin depth upon the incident photon energy for lead substituted bismuth titanate borosilicate glasses doped with 1 mol percent of GNPs. From Fig. S3 it is noticed that skin depth, δ decreases with increasing the photon energy upto a certain value and subsequently it moves towards zero which is known as cut-off energy, Ecut-off and the equivalent wavelength is called cut-off wavelength, λcut-off. The value of Ecut-off of all the glass samples was found to be ~4.19 eV at a cut off wavelength 294 nm. In addition, a comparison of the calculated values of density, molar volume, band gap and Urbach energy of the GNPs doped glass samples with the other glass systems as reported in the present literature is shown in Table 4. It is observed from the table that GNPs doped glass samples show the less value of density and better optical properties in comparison to other reported glassy systems [2,5,7,20,22,28].

1200

Wavelength (nm) Fig. 4. UV–vis spectra of glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 in the glass system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs.

4.7. Nuclear magnetic resonance study Fig. 5. Davis and Mott plots of glass samples BTG0.0, PBTG0.3, PBTG0.5, PBTG0.7 and PTG1.0 in the glass system 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]1GNPs.

Magic angle spinning NMR is the most powerful microscopic tool to grasp the structure of silicate network in the glass. Different Q(n) species ranging from n = 0 to n = 4 in silicate network can be separated by measuring their isotropic chemical shifts [60–64]. Fig. 6 shows 29Si MAS NMR spectra of 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs with different fraction of composition ′x′ as 0.0 (referred as BTG0.0), 0.3

values, Eg for all glass samples have been listed in Table 1. It is observed that the values of optical band gap increase with increasing the concentration of PbO. The difference in the band gap values occurs due to 5

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Table 4 Comparison of obtained various properties in the present glass system with other existing glass systems. Glass systems

Density ρ (g/cm3)

Molar volume VM (cm3/mol)

Band gap Eg (eV)

Urbach Energy EU (eV)

References

55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs PbO-Bi2O3-P2O5 PbO-B2O3 Bi2O3-B2O3 PbO-Bi2O3-B2O3 Na2O-B2O3-PbO Na2O-B2O3- Bi2O3 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1La2O3 59[(PbxCa1-x)O.TiO2]-40[2SiO2.B2O3]-1Fe2O3 (60-x)[SrTiO3]-(40–1)[2SiO2.B2O3]

1.39–2.23 3.95–5.70 3.77–7.21 5.11–7.63 4.83–8.23 3.87–5.35 3.50–6.72 2.76–4.30 1.18–3.86 1.45–2.80

63.21–189.95 – 25.60–28.70 33.00–42.90 28.30–41.10 40.86–53.42 61.11–76.40 33.65–92.41 23.10–54.47 40.76–69.70

3.89–4.19 2.96–3.71 3.16–4.25 3.04–3.50 2.69–3.57 – – 3.45–3.57 1.25–2.37 3.46–3.53

0.22–0.26 – – – – – – – – 0.30–0.33

Present work [2] [5] [5] [5] [7] [7] [20] [22] [28]

that the degree of polymerization within silicate network is increased due to the increase of bridging oxygen in silicate network. This result is consistent with the results obtained from UV–Vis spectroscopic analysis. Silicate melts were formed by a silicate network and associated with silicates tetrahedral which are interconnected to each other via oxygen bonds (Si-O-Si). When cations are inserted into the silicate melts, then it breaks Si-O-Si bonds and NBOs are generated. Anionic equilibrium is

(referred as PBTG0.3), 0.5 (referred as PBTG0.5), 0.7 (referred as PBTG0.7) and 1.0 (referred as PTG1.0). The 29Si MAS NMR spectra of five glass samples were fitted by Gaussian distribution using nonlinear least-square spectral fitting software dm fit [65], where the mean position of the Gaussian distribution for Q(2), Q(3) and Q(4) species were taken as −82.00 ppm, −89.20 ppm and − 96.50 ppm respectively. 29Si MAS NMR spectra in Fig. 6 (a-e) exhibit that the chemical shifts become more negative due to the higher concentration of PbO, which suggests

Fig. 6. 29Si MAS NMR spectra of glass samples (a) BTG0.0, (b) PBTG0.3, (c) PBTG0.5 (d) PBTG0.7 and (e) PTG 1.0 represent the compounds with PbO content x = 0.0, 0.3, 0.5, 0.7 and 1.0 respectively. 6

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is.

Table 5 Relative concentration (in %) of the Qn species in 55[(PbxBi1-x)TiO3]44[2SiO2.B2O3]-1GNPs glasses with compositions, x = 0.0, 0.3, 0.5, 0.7 and 1.0. (2)

(3)

Samples code

Q

Q

BTG0.0 PBTG0.3 PBTG0.5 PBTG0.7 PTG1.0

32.28 28.25 0.45 1.92 3.55

60.97 71.29 65.98 42.23 38.30

Q

(4)

6.76 0.45 33.57 55.86 58.15

Kn =

K3 0.0587 0.0025 0.00347 0.06014 0.14073

K3 =

[Q (4) ][Q (2) ] [Q (3) ]2

(9)

The value of Kn is nearly equal to zero for 55[(PbxBi1-x)TiO3]44[2SiO2.B2O3]-1GNPs glasses (0.0 ≤ x ≤ 1.0). This suggests that the dominant role in silicate network modification is played by only two Q(n) at a time and it is close to the binary system. Table 5 shows for x = 0.0 and 0.3 Q(n) species distribution is mostly dominated by Q(2) and Q(3), whereas x = 0.5, 0.7 and 1.0 for the phenomenon are completely altered as it is dominated by Q(3) and Q(4) species. The steep increment of Q(4) species with the sudden fall of Q(2) species suggests that the silica-enriched phase is developed within the glassy material with the increase of PbO concentration. Thus, the equilibrium constant basically measures the configurational entropy of the glassy system [68]. 11 B MAS NMR studies was done on 55[(PbxBi1-x)TiO3]-

(6)

and

2Q (0) = 2Q (1) + (O)2

(8)

where, n ranging from 0 to 4. For n = 3, the equilibrium constant is.

maintained within silicate melts by following the equations among different Q(n) species [66–68].

2Q (n) → Q (n − 1) + Q (n + 1)

[Q (n + 1) ][Q (n − 1) ] [Q (n) ]2

(7)

where, (O)2 represents the oxygen which is not bound with the silicon tetrahedra and called as a free-oxygen. The equilibrium constant

Fig. 7. 11B MAS NMR spectra of glass samples (a) BTG0.0, (b) PBTG0.3, (c) PBTG0.5, (d) PBTG0.7 and (e) PTG 1.0 represent the compounds with PbO content x = 0.0, 0.3, 0.5, 0.7 and 1.0 respectively and (f) represent the fraction of boron in four coordination with respect to x = 0.0, 0.3, 0.5, 0.7 and 1.0 fractional doping of PbO. 7

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Table 6 11 B MAS NMR parameters obtained from Cz Simple model of dmfit software for in 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs glasses with x = 0.0, 0.3, 0.5, 0.7 and 1.0. Samples code

Relative concentration (in %) BO3 species

Relative concentration (in %) BO4 species

Fraction of Boron in four coordination

Quadrupolar coupling constant Cq (in Hz)

BTG0.0

87.12

12.88

0.1478

PBTG0.3

86.84

13.16

0.1515

PBTG0.5

90.29

9.71

0.1075

PBTG0.7

91.13

8.87

0.0973

PTG1.0

90.77

9.23

0.1016

1802 (BO3 species) 500 (BO4 species) 1402 (BO3 species) 500 (BO4 species) 1402 (BO3 species) 500 (BO4 species) 1402 (BO3 species) 500 (BO4 species) 1402 (BO3 species) 500 (BO4 species)

44[2SiO2.B2O3]-1GNPs glasses (0.0 ≤ x ≤ 1.0). The spectral analysis was done by using Cz Simple model of dm fit software [65,69]. It is shown in Fig. 7 that for five compounds with the different fraction of PbO, BO4 sites have very small quadrupolar broadening with coupling constant Cq, 0.5 MHz (Table 6) and isotropic chemical shifts ranging from 1.5 to 2.5 ppm. On the contrary, BO3 sites have comparatively large quadrupolar coupling constants value from 1.4 MHz to 1.8 MHz and isotropic chemical shifts ranging from 12 to 18 ppm. The fraction of four coordinated boron in five glasses are calculated from an integrated area corresponding to BO4 and BO3 sites by using Cz Simple model of dm fit software [65,69]. These parameters extracted from the fitting of MAS NMR spectra of five glasses are in close agreement with previously reported results on borosilicate glasses [70–72]. The variation of the fraction of four coordinated boron (area of BO4 line divided by the area of BO3 line) with the different fraction of PbO is shown in Fig. 7(f). For x = 0.0 and x = 0.3 the fractional content of PbO it is nearly 0.15 (Table 6) and it dropped to nearly 0.1 for other three glass samples of fractional content x = 0.5, 0.7 and 1.0. That means three coordinated boron dominate over tetrahedral boron with increasing concentration of PbO in the glass 55[(PbxBi1-x)TiO3]-44[2SiO2.B2O3]-1GNPs system, which indicates the number of non-bridging oxygen increases within the borate network. As a result, the connectivity within borate-network is increased.

analysis suggests that the number of bridging oxygen is increased within the silicate network. Acknowledgments The authors are gratefully acknowledged to the Council of Scientific and Industrial Research-Human Resource Development Group, CSIR Complex, Pusa, New Delhi (India) for financial support under the ‘Senior Research Fellowship’ vide letter no. 09/107(0380)/2016-EMR-I (Ack. No. 124250/2K15/1). The authors are also express sincere thanks to Centre of Excellence Scheme of Uttar Pradesh State Government for providing PXRD facility at the Department of Physics, University of Lucknow. Manasi Ghosh is indebted to Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India (File no. EMR/2016/000249) and UGC-BSR (File no. 30-12/2014 BSR) for financial support. We are also grateful to Sophisticated Instrumentation Centre (SIC) of Dr. Hari Singh Gour Central University for providing solid state NMR facility. This work was partly carried out with the constant support of Prof. R. K. Shukla and Prof. Poonam Tandon to extend the UV–Vis and IR spectroscopy measurement facilities at Department of Physics, University of Lucknow, Lucknow (India). Appendix A. Supplementary data

5. Conclusions Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jnoncrysol.2018.10.009.

Various bulk and transparent glasses in the system 55[(PbxBi1were successfully synthesized using a melt-quenching technique. XRD results confirmed the amorphous nature of the prepared glass samples. The density of glass samples increased from 1.39 g/cm3 to 2.23 g/cm3. The IR study confirms that the absorption peaks occurred mainly due to asymmetric stretching relaxation of BeO bonds of trigonal BO3 units, groups of chains or rings of metaborate, piroborate and ortoborate groups, stretching vibrations and BeO bridging between B3O6 and BO3 triangles respectively. It is concluded that PbO itself acts as network modifier as well as network former while Bi2O3 act as network former. The UV–Vis spectroscopic result shows that absorption edge is slightly shifted towards the lower wavelength side as decreasing the amount of Bi2O3. The band gap values were found to be in the range of 3.89 eV to 4.19 eV. It is also concluded that the dependence of the absorption coefficient α (ν) on photon energy follow the Urbach rule. The width of the tails of localized states, EU, varied from 0.22 to 0.26 eV depending on the concentration of PbO. The 11B MAS NMR results confirmed that BO4 sites have very small quadrupolar broadening with coupling constant 0.5 MHz and BO3 sites have comparatively large quadrupolar coupling constants having values from 1.4 MHz to 1.8 MHz. With increasing concentration of PbO, three coordinated boron dominate over tetrahedral boron indicates the number of non-bridging oxygen increases within the glassy borate network however 29Si MAS NMR spectral x)TiO3]-44[2SiO2.B2O3]-1GNPs

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