Physical, structural and optical properties of erbium doped rice husk silicate borotellurite (Er-doped RHSBT) glasses

Physical, structural and optical properties of erbium doped rice husk silicate borotellurite (Er-doped RHSBT) glasses

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

Contents lists available at ScienceDirect

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Physical, structural and optical properties of erbium doped rice husk silicate borotellurite (Er-doped RHSBT) glasses S.A. Umara,b, M.K. Halimaha,⁎, K.T. Chana, A.A. Latifa a b

Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Department of Physics, Faculty of Science, Bauchi State University Gadau, Nigeria



Keywords: Rice husk silicate (RHS) Erbium Refractive index Reflectivity Polaron radius

A series of erbium doped rice husk silicate borotellurite glasses with chemical composition {[(TeO2)0.7 (B2O3)0.3]0.8 (SiO2)0.2}1 − x (Er2O3)x with x = 0.01, 0.02, 0.03, 0.04 and 0.05 mol was prepared using the meltquenching technique. The density and the molar volume were determined and found to be increasing with Er3 + concentration. The glasses were subjected to FTIR and XRD to study the structural changes in the glass. UV–Vis spectroscopy was carried out to obtain the absorption spectrum that is used in the calculation of the optical energy band gap (Direct and Indirect), the Urbach energy and the refractive index. Using the refractive index, density and molar volume, the molar polarizability, metallization criterion, polaron radius, average boron‑boron separation, inter-nuclear distance of Er3 +, surface reflection loss, transmission coefficient and oxygen packing density were determined. The density, molar volume, optical band gaps, molar refraction, transmission coefficient and metallization criterion were found to have increased with increasing concentration of Er3 + ions. While the values of the refractive index, Urbach energy and inter-nuclear distance of Er3 + ions decreased. As more Er3 + ions were introduced, the reflectivity of the glasses decreased. The polaron radius also decreased, with the values suggesting that the glass has small polaron.

1. Introduction Tellurite glasses are increasingly studied in the recent years due to their promise in various applications. The glasses are promising in the non-linear optical applications, lasers, sensors, optical fibers and solar cells, with appropriate rare earth (RE) ions doping [1,2]. Tellurium oxide (TeO2) as a conditional glass former with high refractive index, low melting point and low phonon maxima, needs modifying ions to easily form glass [1]. Boron oxide (B2O3) is excellent material for combination with TeO2 as it improves the glass quality in terms of transparency, RE ions solubility and hardness [2]. Silica (SiO2) has been used as substrates for electronic displays, optical fibers, optical disc, medical and dental implants and radiation shielding. SiO2 is mostly used in glasses to give them mechanical quality [3]. Extraction of silicate (silica) from the rice milling waste (rice husk or rice paddy) for commercial and scientific use is one of the alternative solutions to its disposal problems [4]. Different researchers have used different techniques to extract silica from the rice husk with different degrees of purity from 80 to 99% [6–9]. Erbium as an excellent glass doping element is famously used in

communication technology for optical signal amplification in the Erbium doped fiber amplifier (EDFA). Many glass scientists have carried out several works on the influence of Er3 + ions on the physical, structural, optical, electrical and mechanical properties of different glass compositions [1,10–12]. This work sorts to study the erbium doping effects on some physical, structural and optical properties of rice husk silicate borotellurite glasses. This includes the density, molar volume, optical energy band gap, refractive index, molar refractive index, reflectivity (reflection loss at the glass surface), transmission coefficient, molar polarizability, polaron radius and so on. The inclusion of the rice husk silicate is important as it provides another waste utilization alternative as well as help in providing a much stronger glass. This will also provide an alternative for a more profitable utilization of the rice waste. The rice husk silicate composition was selected and fixed as 20% molar ratio as it appeared to be the best in terms of transparency and high refractive index. 2. Experimental section A series of erbium doped silicate borotellurite glasses where

Corresponding author. E-mail address: [email protected] (M.K. Halimah). Received 14 March 2017; Received in revised form 16 June 2017; Accepted 12 July 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Umar, S.A., Journal of Non-Crystalline Solids (2017),

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5. XRF analysis

prepared using the chemical formula {[(TeO2)0.7 (B2O3)0.3]0.8 (SiO2)0.2}1 − x (Er2O3)x with x = 0.01, 0.02, 0.03, 0.04 and 0.05 mol by the use of conventional melt-quenching technique. The chemical reagents used are TeO2 (Alfar Aeser, 99.9%), B2O3 (Alfar Aeser, 99.9%), Er2O3 (Alfar Aeser, 99.9%) and SiO2 (Rice Husk extracted, 98.548%). The silicate used was extracted from the rice husk (waste from rice milling). The husk was first washed three times using normal water and with deionized water the fourth time, to rid it from dust like impurities after the water is drained using a plastic bowl [5]. The drained sample was then leached in hydrochloric acid (HCl, 2 M) for about two hours [6]. The husk was then drained off the acid and washed with deionized water to get the acid off the fabrics of the husk. The husk was then drained of the deionized water for 24 h and dried in an oven at 120 °C for about 4 h. Finally, the dried rice husk was then incinerated at 700 °C for about 6 h to obtain the rice husk ash (silicate) [7]. To prepare the glasses, 12 g of the powdered chemicals were weighed in the molar proportion presented by the above chemical formula using a high precision digital weighing machine ( ± 0.0001). The weighed chemicals were mixed and stirred for 30 min to ensure homogeneity in the mixture. The homogeneous mixture was then preheated for an hour in a furnace at 400 °C and later transferred to another furnace at temperatures 900 °C for another one to two hours for melting of the sample. For glass casting, the glass was allowed to anneal at 400 °C for 1 h. The prepared glass was cut and then polished using silicon carbide [8]. The densities of the glass samples were measured based on the famous Archimedes principle using an electronic densimeter MD-300S (Alfa Mirage). The equation is given as;

ρsample =

wair ρ wwater water

As shown in Table 1, the silicate obtained has about 98.548% purity. The processes of leaching and washing were carried out at room temperature, unlike the method adapted by Mustafa and co-workers which require the use of heat [5]. The purity of the extracted silicate depends on the method and the conditions of extraction, which includes the leaching temperature, acid concentration and the burning temperature [7,13,15]. 6. Density and molar volume Structural changes in glass network due mostly to changes in crosslink density, coordination number, structural compactness, geometrical configuration, and dimension of interstitial spaces of the glass are the factors that affect the glass density value [8]. Obtained density values can be used in analyzing the physical, structural, optical, thermal and the elastic properties of glass material [11, 16 17]. From Fig. 2, the density of erbium doped rice husk silicate borotellurite (Er doped RHBTS) glass increased with addition of more Er ions in the glass network. The increase in the density may be connected to the substitution of lighter atoms of Te, B and Si with heavier atoms of Er in the network which resulted to increase in the mean molecular weight of the glass [18,19]. Density increase in the glass may also result to a change in the cross-link density in the glass [10]. The molar volume was obtained from the density and molar mass values of the glass. The molar volume as can be seen in Fig. 3 increased when more Er ions were introduced in the glass network. As the FTIR result presented in Fig. 13, the addition of more Er ions in the network led to production of more TeO3 and TeO4 units in the network and eventually increasing the number of non-bridging oxygen (NBO) in the network [10]. The increase may also be attributed to the substitution of atoms of smaller atomic/ionic radii (Te, B and Si) with Er atoms with larger atomic/ionic radius [11].


The molar volume was obtained for each sample using the equation.

Vm =

mw ρsample

(2) 7. Absorption spectrum

where ρsample, ρwater, wAir, wwater and mw are the sample density, water density, weight of sample in air, weight of sample in water and molar weight of the glass sample respectively [9]. The optical absorption spectrum for the glass samples were determined using UV–Visible spectrometer Shidamatsu Model UV-1650PC with wavelength range from 200 nm to 800 nm. FTIR spectroscopy was carried out on the powdered portion of the glasses to obtain the structural nature of the glass based on the behavior of its functional groups when the material interacts with infrared waves. This was carried out at the wave number range of 280–4000 cm− 1. The XRD spectroscopy was carried out on the powdered portion of the glass samples between 20 < 2θ < 80 to determine the crystalline or amorphous nature of the glass.

Fig. 4 shows the UV–Vis absorption spectra of Er3 + doped RHSBT glass samples in the wavelength interval of 400 to 800 nm. The characteristic absorption bands corresponding to the transitions from the ground states to excited states of Er3 + are 651, 522,488, 448.5 and 403 nm are attributed to transition from the ground state of 4I15/2 to the excited states of 4F9/2, 2H1/2, 4F7/2, 4F3/2 and 2G11/2 respectively (Fig. 4). The absorption intensity is highest around 521 nm and decreases in the order of 651, 488, 403 and 448.5 nm [2,10,24]. The weak absorption recorded around 534 nm may be due to the overlap of 4S3/2 with the upper 2H11/2 excited states [12]. The characteristic absorption peaks position may be slightly affected by the medium or the hosting material and the nature of the bonding around the transition element in the study [13].

3. Result and discussion 8. Other optical and structural and physical features The structure of the silicate obtained was found to be amorphous from the XRD analysis. As with the hypothesis that the crystalline or amorphous nature of the silica depends on the incineration temperature at which it was extracted on; that at burning temperatures below 700 °C, the rice husk silica obtained was amorphous [7].

The optical band gap (Direct and Indirect) was determined using the Davis and Mott expression for the absorption coefficient α (υ) as;

α (υ) = B

(hυ − Eopt )n hυ


where Eopt is the optical band gap, n is a number, with n = 2 for indirect and n = 1/2 for direct allowed transitions and B is a constant [10,25]. The value of α (υ) is the absorption coefficient obtained using the expression;

4. XRD analysis The amorphous or crystalline nature of the glass determined from the XRD pattern as shown in Fig. 1 for the Er doped RHBTS glass samples revealed a broad diffused scattering around 2θ = 20°–30°. This exhibits the amorphous nature of the glass and shows absence of long range atomic arrangements [16,22].

α (υ) = 2.303

A t


where t is the sample thickness, A is the value of the corresponding 2

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Fig. 1. The XRD pattern of Er doped RHSBT glasses.

increase in the concentration of Er3 + ions in the network. The Urbach energy decrease shows an increase in the glass fragility due to increase in the TeO3 concentration which changes the structural networking in the glass. This indicates defects formation and hence causes the observed increase in the optical band gap energy value [17,18,19]. The refractive index (n) of the Er3 + doped RHSBT glasses was calculated from the indirect optical energy band gap values (Eindir) using the equation in [16].

Table 1 The XRF analyzed result showing the various elements/oxides and their concentrations in the rice husk ash (RHA). Element (oxide)

Amount/percentage (%)

SiO2 SO3 CaO Fe2O3 K2O MnO ZnO

98.548 0.793 0.407 0.129 0.079 0.035 0.009

n2 − 1 =1− n2 + 2

Eopt (6)

20 3+

ion molar The refractive index as presented in Fig. 8 against Er concentration shows a decreasing pattern with increase in the amount of Er3 + ions in the glass network. This may be due to substitution of a TeO4 + with Er3 + ions with higher polarizability [20]. The molar refractive index (Rm) and Oxygen Parking Density (OPD) of the glass samples were calculated using the equations in [1,23].

optical absorbance. Tauc's plots were drawn between (αћω)1/n and the photon energy (ћω) [14]. The optical band gap values where obtained by extrapolating the linear part of the curves at (αћω)2 = 0 and (αћω)1/2 = 0 for direct and indirect transitions respectively [12]. Figs. 5 and 6 represent the direct and indirect optical band gap plots against the Er3 + molar concentration. In both cases, the values increased with the increase in the Er3 + ions concentration which may be attributed to bridging of the free space in the glass. This results to an increase in the structural compactness in the glass network [27,28]. The Urbach energy (Δ E) is the width of the localized states that corresponds to optical transition between the localized tail states in the adjacent valence band and the extended state in the conduction band lying above the mobility edge [15]. The absorption coefficient, α (υ) of the glasses can be expressed as the exponential function of photon energy, hυ with B as a constant value as in [16];

α (υ) = B exp

hυ ΔE

n2 − 1 ⎞ Vm Rm = ⎛ 2 ⎝n + 2 ⎠


ρxO mw



where ρ = density, mw = molar weight of the glass sample and O = number of oxygen atoms in a unit formula. The variations of Rm and OPD with Er3 + concentrations are presented in Figs. 9 and 10 respectively. As can be observed from both the figures, the values of Rm and OPD increased and decreased respectively with increase in the Er3 + concentration. This is due to the structural changes in the glass network with the formation of TeO3 and TeO4 suppressed and more non-bridging oxygen formed [21]. Figs. 11 and 12 show the optical transmission coefficient (T) and the reflectivity/reflection loss (RL) variations respectively, due to addition of more Er3 + ions in glass series. The reflection loss from the glass


The values of the Urbach energy as observed in Fig. 7 decreases with

Fig. 2. Density against Er3 + molar concentration.


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Fig. 3. Molar volume against Er3 + molar concentration.

Fig. 4. Absorption spectrum of Er3 + doped RHSBT glasses.

Fig. 5. Direct optical band gap against Er3 + molar concentration.

Fig. 6. Indirect optical band gap against Er3 + molar concentration.


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Fig. 7. Urbach energy against Er3 + molar concentration.

Fig. 8. Refractive index against Er3 + molar concentration.

Fig. 9. Molar refractive index, Rm against Er3 + molar concentration.

glass network [21]. Polarizability in general terms is the degree of how electrons in a material respond to an electromagnetic field [24]. The molar polarizability is given as;

surface was obtained using the Fresnel's formula from the refractive index (n) as reported in [14,31]. The relationship between T and R shows that at a constant absorption, the value of T increases when R decreases and vice versa. Hence the increase in the T value with increase in the Er3 + ions led to the observed decrease in the R value, making the glasses more transparent in the UV–Vis region [22,23].

n − 1 ⎤2 RL = ⎡ ⎣n + 1⎦


2n n2 + 1



3 ⎞ Rm αm = ⎛ ⎝ 4πN ⎠


Metallization criterion of any material tells about the non-metallic nature of the material on the basis of its band gap energy and is obtained as reported by [19] as;


Rm Vm


As shown in Table 2 above, the metallization criterion (M) for the Er doped RHSBT glasses are in the range of 0.359–0.3774. The value for metallization criterion of oxide glasses with good optical non-linearity

The molar polarizability (αm) expressed in Table 2, shows an increasing pattern with increase in Er3 + concentration. This can be due to higher polarizability of Er3 + ions against TeO4 + ions substituted in the 5

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Fig. 10. Optical parking density against Er3 + molar concentration.

Fig. 11. Optical transmission coefficient against Er3 + molar concentration.

Fig. 12. Optical reflection loss against Er3 + molar concentration.

Table 2 Molar polarizability, αm ( ± 0.0001), metallization criterion, M ( ± 0.0001), molar volume of boron atoms, VB ( ± 0.001), average boron-boron separation, < dB-B > ( ± 0.001) and ion concentration of erbium, N ( ± 0.001). M. fraction

αm (Å3)


VB (cm3)

< dB-B > (m)

N (ions cm− 3)

0.01 0.02 0.03 0.04 0.05

8.8142 8.8241 8.8498 8.8945 8.8145

0.3590 0.3603 0.3645 0.3678 0.3744

22.750 22.751 22.896 23.062 23.024

3.35596E − 08 3.35601E − 08 3.36315E − 08 3.37123E − 08 3.36937E − 08

1.73604E + 20 3.46105E + 20 5.14244E + 20 6.78617E + 20 8.4703E + 20

Table 3 Polaron radius (rp), Inter-nuclear distance of erbium (ri), field strength of Er3 + yields (F), average coordination number (m) and number of bond per unit volume (nb). M. fraction

rp (Å)

ri (Å)

F (× 1016 cm− 2)

m (av C N)

nb (× 1022)

0.01 0.02 0.03 0.04 0.05

7.226 5.742 5.032 4.587 4.261

17.93 14.25 12.48 11.38 10.57

1.302 2.063 2.686 3.231 3.746

4.02 4.04 4.06 4.08 4.10

6.979 6.991 6.959 6.922 6.946

B 1 3

V < dB − B > = ⎡ m ⎤ ⎢ NA ⎥ ⎣ ⎦

falls in the range of 0.30–0.45 [10,16]. Thus the glasses may be good for non-linear optical applications. Other parameters in Table 2 were obtained as reported by [25] as follows; Average Boron-Boron separation, < dB-B > ;



VmB =


Vm 2(1 − XB )


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Fig. 13. The FTIR spectra of the Er doped RHSBT glass samples.

Er3 + ion concentration, N is expressed as;

X x ρxNA N = Er mw

materials mostly from functional groups in crystalline and amorphous arrangements [2]. Fig. 13 presents the FTIR plots for the erbium doped RHSBT glass samples. The absorption bands shown are 1385–1585 cm− 1, 1150–1350 cm− 1 and 600–700 cm− 1. The absorptions peaks in the range of 1350–1378 cm− 1 and 1220–1240 cm− 1 are assigned to stretching vibrations of BeO in an isolated trigonal BO3 units [8] and BO3 units in boroxol rings [30] respectively. While the characteristic band 600–700 cm− 1 may be attributed to both stretching vibrations of TeeO bonds in TeO3 and TeO4, with 600–650 cm− 1 absorption representing TeeO bond vibration in TeO4 units. The IR absorption in the range of 650–700 cm− 1 may be due to the TeeO bending vibrations in the TeO3 and TeO6 units [10,23]. The frequency of vibrations in TeO3 bonds is higher than those in TeO4. The TeO3 absorption intensity is observed to have increased with the in Er3 + ions in the glass network. The increasing intensity and the shift in the absorption peaks toward 700 cm− 1 indicates increase in the concentration of TeO3 units in the glass network and thereby increasing the number non-bridging oxygen in the network [31,21].



where NA, Vm , XB and XEr are the Avogadro's number, molar volume of boron atoms, molar fraction of boron and molar fraction or erbium respectively. 9. Polaron radius Polaron is a quasi-particle formed by a conduction electron or hole in a polar semiconductors, ionic crystals or alkali oxides/halides with its self-induced polarization. Polarons are studied to describe the polar interaction between an electron and the longitudinal optical phonons [26,29]. The polaron radius is the linear atomic/ionic displacement field of a polaron. When the radius is of the order of the lattice constant, the polaron is called small polaron. When the radius is much larger than the lattice constant of the material, it is termed as large polaron [26]. The polaron radius (rp) for the Er doped RHSBT glasses was obtained using the equation as reported by Deepa et al. and Mahraz et al. [2,14] as;

rp =

1 π 1 ⎛ ⎞ 2 ⎝ 6N ⎠

11. Conclusion



Erbium doped rice husk silicate borotellurite glasses were fabricated using the chemical composition {[(TeO2)0.7 (B2O3)0.3]0.8 (SiO2)0.2}1 − x (Er2O3)x with x = 0.01, 0.02, 0.03, 0.04 and 0.05. The glass samples were subjected to FTIR, XRD characterizations and UV–Vis spectroscopy for structural and optical analysis. Density was obtained from the famous Archimedes principle and the values were used to get the molar volume. The direct and indirect optical band gaps were determined and refractive index obtained from the values of the indirect optical band gap. The optical band gaps, density and molar volume, transmission coefficient increased with increase in Er3 + concentration while the refractive index decreased. The increase in the density may be due to the substitution of lighter ions with the Er3 + ions which are heavier. Increase in the optical band gap may be associated with the formation of more TeO3 units which suppresses the formation of TeO4 in the glass network. Reflectivity of the glass and the value of the polaron radius were found to decrease with increase in the molar concentration of erbium, suggesting a decrease in the optical energy lost due to reflection with increasing Er3 + ions in the glass network.

In Table 3 below, the value of the polaron radius decreased with increasing concentration of Er3 + ions in the glass from 7.226–4.261 Å. The values suggest that the polaron is a small polaron as the values does not exceed the lattice constants of the individual oxides in the glass [26,29]. The decrease in the polaron radius may be associated with the increase in the glass polarizability and compactness. This may eventually increase the glass electrical conductivity [27,28,29]. The Inter-nuclear distance of the Er3 + (ri), Field Strength of Er3 + yields (F), Average coordination number (m) and Number of bond per unit volume where calculated using Eqs. (17), (18), (19) and (20) respectively as reported in [1,14].

1 1 ri = ⎛ ⎞ ⎝N ⎠



F = Z (rp2)


m = Σi nci x i


nb =

NA m Vm



where Z is the atomic number of erbium, nci is the coordination number of the cations and xi is molar concentration of the cations.

The authors appreciate the contributions of the reviewers in improving the quality of the work and the paper. The authors appreciate the financial support for the work from the Ministry of Higher Education of Malaysia through FRGS 5524817.

10. FTIR analysis The FTIR spectroscopy is used to study the structural features of 7

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