Optical and structural investigation on sodium

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 534–538

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Optical and structural investigation on sodium borosilicate glasses doped with Cr2O3 E. Ebrahimi, M. Rezvani ⁎ Department of Materials Science and Engineering, University of Tabriz, Tabriz, Iran

a r t i c l e

i n f o

Article history: Received 27 May 2017 Received in revised form 23 August 2017 Accepted 12 September 2017 Available online 14 September 2017 Keywords: Borosilicate glasses Optical properties Optical filter

a b s t r a c t In this work, Sodium borosilicate glasses with chemical composition of 60% SiO2–20% B2O3–20%Na2O doped with different contents of Cr2O3 were prepared by melting-quenching method. Physical, structural and optical properties of glasses were investigated by studying density and molar volume, Fourier Transform Infrared (FT-IR) Spectra and UV–visible absorption spectroscopy. The results showed an increase in density of glasses with the increase of Cr2O3 that can be due to addition of oxide with high molar mass. The optical absorption spectra of un-doped glass reveals UV absorption due to trace iron impurities with no visible band however Cr2O3 doped glasses shows absorption in visible range that are characteristic. Increasing of Cr3+ ions in the glassy microstructure of samples provides a semiconducting character to Sodium borosilicate glass by reducing the direct and indirect optical band gaps of glass samples from 3.79 to 2.59 (ev) and 3.36 to 2.09 (ev), respectively. These changes could be attributed to the role of Cr3+ ions as the network former which asserts improvement of semiconducting behavior in presence of Cr2O3. © 2017 Published by Elsevier B.V.

1. Introduction Over the last few decades, 3d-Transition metal (TM) ion-doped glasses point out interesting spectroscopic and electrical properties because of the potency of the ions to exist in more than one valence states enabling electrical conduction to occur by the movement of carriers from lower to higher valence state [1–4]. Among oxide glasses, Borosilicate glasses have the advantages of the stability of silicate glass and the higher TM ion solubility of borate glass without producing heavy concentration quenching together that make them good candidates for TM ion hosts and thus they can find wide technological applications in solid state lasers, phosphors, solar energy converters, plasma display panels and especially optical filters which are known for their selective absorption in certain wavelength ranges [5–8]. The optical filter glasses appear to be colored if their filter effect lies within the visible light spectrum. The glasses are usually colored by transition metals (TM). The electronic absorption spectra of TM due to transitions between sublevels of the d-shells (d – d transitions) lie in the visible, near-UV, and IR frequencies of electromagnetic radiation [9]. Among various transition metal Ions, Chromium which has a (3d)9 electronic configuration in the ground state, as a paramagnetic ion when dissolved in glass matrices in very small amount changes the optical transmission and can participate in the glass network forming two valence states, Cr3+ and Cr6+, with [CrO6] and [CrO4] structural units, respectively, which have ⁎ Corresponding author. E-mail address: [email protected] (M. Rezvani).

https://doi.org/10.1016/j.saa.2017.09.031 1386-1425/© 2017 Published by Elsevier B.V.

strong influence over the insulating character and optical transmission of these glasses. In the light of these observations the aim of the present study is to investigate the influence of chromium ion dopant on the optical properties and the network structure changes of Na2O–B2O3–SiO2 glasses by using Fourier transform infrared (FTIR) spectroscopy. 2. Material and Methods 2.1. Glass Preparation Sodium borosilicate glasses with the composition of SiO2 60%, B2O3 20% and Na2O 20% (mol%) were used as a glass matrix doped with different amount of Cr2O3. The glass samples are labeled as follows. Leached SiO2 with high purity, B2O3 (Merck 1303622), Na2Co3 (Merck 10639) and Cr2O3 (Merck 5314617) were used as raw materials and weighed accurately in an electronic balance mixed thoroughly and ground to fine powder. The batches were then placed in aluminum crucibles and melted in an electrical furnace at 1450 °C for 1 h to allow the melts to become visibly homogeneous and bubble free. The homogenized melts were cast in preheated stainless steel molds to be annealed at 450 °C for 1 hour and slowly cooled to room temperature. 2.2. Glass Characterization The amorphous nature of the glass samples was confirmed by using X-ray diffraction experiments recorded with a Siemens X-ray

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diffractometer using copper Kα radiation at room temperature. The density of the samples was calculated by standard Archimedes principle using Eq. (1).
 D ¼ w1 =w1 −w2 g=cm3

ð1Þ

where w1and w2 is the weight of the glass sample in air and water. The molar volume (Vm) of each sample was calculated using Eq. (2). Vm ¼

X


 Mi =D cm3 =mol

ð2Þ Fig. 2. Variation of density and molar volume with Cr2O3 content.

where Mi is the total molecular weight of the multicomponent system given by Mi ¼ Ci Ai

ð3Þ

Hence Ci is the mole fractions of the constituent oxides, and Ai is the molecular weights of the different oxides. 2.3. Fourier Transform Infrared Spectroscopy (FT-IR) FT-IR absorption spectra of sodium borosilicate glass were recorded at room temperature using KBr disc technique. A Bruker Tensor 27 Fourier Transform Infrared Spectrometer was used to measure the spectra of all samples in a range (400–4000 cm−1) with a resolution of 2 cm−1.

2.4.1. Absorption and Extinction Coefficients Absorption coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light. The absorption coefficient α(ν) of depends on the material and also on the wavelength of light which is being absorbed can be calculated using Eq. (4). α¼

ð4Þ

where d is the thickness of the sample, Io and IT are the intensities of incident and transmitted radiations respectively. In addition, extinction coefficient (imaginary part of the complex index of refraction, which also relates to light absorption) of material can be determined by using Eq. (5).

2.4. UV–Vis Spectra and Optical Properties Determination k¼ Optical transmittance in UV–Vis spectrum range were carried out for perfectly polished glass samples using a recording double beam spectrophotometer (shimadzu UV–Vis 1700 scanning spectrophotometer) at room temperature covering the wavelength range from 190 to 1100 nm. According to the results obtained from transmittance spectra, absorption and extinction coefficient, Fermi energy level, direct and indirect optical band gaps and Urbach energy of the samples were determined.

1 I0 ln IT d

αλ 4π

ð5Þ

where α is the absorption coefficient and λ is the variable wavelengths probing the sample [10]. 2.4.2. Determination of Fermi Energy Level of Glasses According to stronger UV absorption bands than visible photons [11], extinction coefficient for different glass samples obeys from Fermi–Dirac distribution function as kðλÞ ¼

1    1 þ exp E f −E kB T

ð6Þ

where Ef is the Fermi energy, E is the variable photon energy and kB is the Boltzmann constant [12,13]. Fermi energy of samples are achievable from K vs. hν plots and least square fittings of Eq. 2.4.3. Direct and Indirect Band Determination The optical band gap in the amorphous system is closely related to the energy gap between valence and conduction bands. An expression for the absorption coefficient (α) as a function of photon energy (hν) for direct and indirect optical transitions is given by Mott and Davis

2

α¼β

 n hν−Eopt g

ð7Þ

hν

Table 1 Physical properties of Sodium borosilicate glasses with different amounts of Cr2O3 dopant. Sample

Fig. 1. X-ray diffraction pattern of SiO2–B2O3–Na2O: Cr2O3 glasses.

C0 C1 C2 C3 C4

Glass composition (mol%) SiO2

B2O3

Na2O

Cr2O3

60 60 60 60 60

20 20 20 20 20

20 20 20 20 20

0.0 0.2 0.4 0.6 0.8

D (g·cm−3)

Vm (cm3·mol−1)

2.50 2.51 2.54 2.57 2.61

24.94 24.77 24.64 24.54 24.27

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E. Ebrahimi, M. Rezvani / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 534–538 Table 2 FT-IR spectral data of Sodium borosilicate glasses doped with different amount of Cr2O3. Assignment

C0

C1

C2

C3

C4

B\ \O - bonds stretching vibrations in BO3 units B\ \O - bonds stretching vibrations in BO4 unit Bending vibrations of various borate segments Si\ \O\ \Si and O\ \Si\ \O bending modes of bridging oxygens (Q4)

1385 1032 711 469

1426 1017 718 462

1412 1022 – 463

1429 1031 – 464

– 1039 – 471

using Eq. (8) and least square fitting of Lnα against hν curves in the tailing part of localized states.   hν α ¼ β exp Eu

ð8Þ

3. Results and Discussion 3.1. Glass Characterization

Fig. 3. FTIR absorption spectral curves of Sodium borosilicate glasses doped with different amount of Cr2O3.

where β is a constant related to the extent of the band tailing, hν is the incident photon energy that could have variable amounts and n is the index which can have different values 2, 3, 1/2 and 1/3 corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions, respectively. Therefore, direct and indirect band gap can be calculated by plotting (αhν)2 and (αhν)1/2 against photon energy. The intercept of the obtained line divided by slope, is equal to energy band gap of optical transitions. 2.4.4. Urbach Energy In many amorphous materials, variation of absorption coefficient with photon energy could be discussed in three different regions. The first region is known with almost constant absorption due to exciton– phonon coupling due to transitions between tail and tail. The second part which is also known as Tauc region corresponds to high absorption from which optical band gap could be calculated. This region is associated with interband transitions. The third region is an exponential one called Urbach. Urbach energy corresponds to the width of localized states, is used to characterize the degree of disorder in amorphous and crystalline systems. Urbach energy (EU) of samples has been calculated

Fig. 4. Transmission spectra of SiO2-B2O3-Na2O containing different amounts of Cr2O3.

The X-ray diffraction spectra of glass samples are shown in Fig. 1. No sharp Bragg peaks are observed in the spectra which indicate the amorphous nature of the glasses. The values of density and molar volume of all the glass samples have been calculated and their values are shown in Table 1. Dependence of density and molar volume with mol% of Cr2O3 is shown in Fig. 2. Density is found to increase with increasing Cr2O3 content in all investigated range. This is most likely related to addition of oxide with high molar mass. The molar volume VM decreases by increasingtheCr2O3 content. It means that the glasses become more compact. It may conclude from such behavior that Cr3+ has a contracting effect on the structure Cr3+ ions play a role in modification of glassy structure by probably creating of Cr2SiO5 structures, leading to an increase in the glass densities. 3.2. FTIR Normalized FTIR absorption spectral curves of SiO2–Na2O–B2O3 glasses doped with Cr2O3 content from 0.0 to 0.8 mol%, are shown in Fig. 3. In the present investigations, the observed bands and their corresponding assignments are presented in Table 2. It can be observed that the base ternary Na2O–B2O3–SiO2 glass system have four main absorption bands at around 470, 711, 1032 and 1385 cm−1.The band at around 470 cm− 1 is assigned to Si\\O\\Si and O\\Si\\O bending modes of bridging oxygens (Q4) overlapped with B\\O\\B linkages [14]. The weaker band at 650–750 cm−1 is assigned to the bending vibrations of bridging oxygen (BO) between trigonal BO3 groups [15,16]. The absorption band in the range of 1020–1032 cm−1 is attributed to asymmetric stretching vibrations of NBO of SiO−4 tetrahedral (Q3) and the

Fig. 5. UV–visible optical absorption coefficient versus wavelength of samples containing different amounts of Cr2O3.

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which are achieved from least square fittings of Eq. (6). Different values of fermi energy level for all samples are listed at Table 3. Decreasing of Fermi energy level in presence of more Cr2O3 indicates more semiconducting properties.

Fig. 6. Extinction coefficient vs. energy plots of glasses containing different amounts of Cr2O3.

3.3.3. Direct and Indirect Band Determination Table 3 contains direct and indirect allowed optical band gap of glass samples with different amounts of Cr2O3 dopants. As it is shown, Eopt values systematically decrease with increasing Cr2O3 content in glass. Presence of Cr2O3 may cause the creation of energy levels in forbidden gap that could reduce band gap of glass. The decrease in Eopt to lower energies with increasing Cr2O3 content can be also related to the progressive increase in the concentration of BO4 units and creation of bridging oxygen (BO).

peak at around 1385 cm−1 is assigned to stretching vibrations of NBO of BO3 triangles [17]. It can be seen that the intensity of asymmetric stretching vibrations decreases with increase of Cr2O3 concentration. Furthermore, the position of this broad band slightly shifted towards higher wavenumbers with increasing the concentration of Cr. The decrease in intensity of this band is indicative of the fact that the BO3 borate groups and NBOs are decreasing in the glass structure.

3.3.4. Urbach Energy Urbach energy (EU) of samples, which corresponds to the width of localized states, is used to characterize the degree of disorder in amorphous and crystalline systems, has been calculated using Eq. (8) and least square fitting of Lnα against hν curves in the tailing part of localized states. Table 3 represents the Urbach energy values of Sodium borosilicate glasses doped by different amounts of Cr3+ ions. As it can be seen, the addition of chromium ions, decreases the Urbach energy values. It can be attributed to a decrease in the broadening due to static disorder-related part [18].

3.3. Evaluation of Optical Constants

4. Conclusion

3.3.1. Transmission Spectra Fig. 4 reveals the UV–visible optical transmission spectra of SiO2– B2O3–Na2O glasses containing different amounts of Cr2O3. It can be noticed that the parent borosilicate glass (C0) shows a strong UV absorption band. Introduction of low concentrations of Cr2O3 to the base glass lead to the transmittance decrease with increasing Cr2O3 content. Moreover, it is observed that the positions of the fundamental absorption edge and cut-off wavelength shifted towards higher wavelength as the content of Cr2O3 increases. The combined UV and visible absorption spectra of chromium doped glasses indicate the presence of both of the +3 and +6 oxidation states of chromium in the studied glasses. The ultraviolet absorption is attributed to hexavalent chromium and the broad visible absorption band refers to the spin-allowed transitions of octahedral Cr3+ from 4A2(F) to 4T2(F) and 4T1(F), to increase energy.

Sodium borosilicate glasses with different amounts of Cr2O3 were prepared to investigate the influence of chromium ion on physical, structural and optical properties.

3.3.2. Absorption, Extinction Coefficients and Determination of Fermi Energy Level Fig. 5 illustrates the variation of absorption coefficient in UV–Vis part of electromagnetic radiation. As it is obvious the value of absorption Coefficient for doped glasses in all UV–Vis spectrum is more than base glass. On the other hand, with increasing of Cr2O3 absorption edge becomes more vertical. Obeying the extinction coefficient function of samples from the Fermi–Dirac distribution function due to stronger absorption of UV than visible photons, the Fermi energy level can be calculated applying the Eq. (6). Fig. 6 depicts K vs. hν plots for different glass samples. The calculated values for Fermi energy of samples are presented in Table 3,

Table 3 Optical properties of Sodium borosilicate glasses with different amounts of Cr2O3 dopant. Sample

C0 C1 C2 C3 C4

Energy (ev) Ef

Eindirect

Edirect

Eu

4.04 2.97 2.88 2.87 2.83

3.36 2.42 2.36 2.33 2.09

3.79 2.76 2.70 2.61 2.59

0.33 0.28 0.26 0.25 0.22

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