Comprehensive study on compositional dependence

Journal of Non-Crystalline Solids 449 (2016) 107–112

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Comprehensive study on compositional dependence of optical band gap in zinc soda lime silica glass system for optoelectronic applications Mohd Hafiz Mohd Zaid a, Khamirul Amin Matori a,b,⁎, Sidek Hj. Ab Aziz a, Halimah Mohamed Kamari a, Zaidan Abdul Wahab a, Nuraidayani Effendy a, Ibrahim Mustapha Alibe b a b

Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 6 April 2016 Received in revised form 12 July 2016 Accepted 16 July 2016 Available online xxxx Keywords: Glasses Zinc oxide Structural properties Optical absorption Optical band gap

a b s t r a c t Zinc soda lime silica (ZnO-SLS) glass system with composition x(ZnO)100-x(SLS) (x = 0, 10, 20, 30, 40 and 50 wt.%) were synthesized by the conventional melt-quenching technique. The structural and optical properties of the glasses are measured using X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM) and UV–Visible (UV–Vis) absorption spectroscopy. The optical band gaps were determined by analyzing the optical absorption edge using the Mott-Davis model. A differential method based on Mott-Davis model are used to obtain the type of transition and optical band gap (Eopt) which in turn was compared with the value of Eopt obtained using the extinction coefficient. The analysis shows that in ZnO-SLS glasses, the optical band gap arises due to direct forbidden transition. Progression of ZnO content cause the absorption edge shifts toward longer wavelengths and decreases the optical band gap. This behavior can be explained in terms of changes to the Zn\\O chemical bonds with glass composition. Furthermore, in the case of glasses containing increasing amounts of ZnO, a change of the role of zinc ions in the glass matrix was confirmed from the modifier to a structureforming component. © 2016 Published by Elsevier B.V.

1. Introduction Soda lime silica (SLS) glasses are an interesting material especially in connection with their various applications and the most prevalent type of glasses and commonly used for window glass panes and glass containers but also containment of radioactive waste, as degradable tissue and bone scaffolds within the human body and also development in vitreous glass-ceramic applications [1–5]. Such a wide-ranging of SLS glass applications results from the innovation and possibility of wide range modifications of their chemical composition. Recently, glass containing zinc oxide (ZnO) is one of the most ideal glass constituents due to their decent physical and mechanical properties with low softening point compared to others glass system [6–8]. This type of glass when doped with transition metal or rare earth ions

⁎ Corresponding author at: Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. E-mail addresses: [email protected] (M.H.M. Zaid), [email protected] (K.A. Matori), [email protected] (S.H.A. Aziz), [email protected] (H.M. Kamari), [email protected] (Z.A. Wahab), [email protected] (N. Effendy), [email protected] (I.M. Alibe).

http://dx.doi.org/10.1016/j.jnoncrysol.2016.07.020 0022-3093/© 2016 Published by Elsevier B.V.

have a properties such as high transparency from UV to IR region, low refractive index and thermal expansion coefficient with high thermal stability and possibility of incorporating large number of dopant ions are suitable for many potential applications such as favorable material for optical host in solid-state laser [9]. The influence of ZnO on the structure of glasses is unique since they can act as network formers as well as network modifiers [10–13]. The different of ZnO concentrations can strongly influence the structural and optical properties of the glasses. Hence, studying the optical properties, in particular, the optical absorption and energy band gap of ZnO-SLS glasses and how these properties vary with glass composition bring great interest for practical applications. A spirited discussion occurred in the literature and works according to the influence of ZnO content in the borate, tellurite and phosphate glass system [14–16]; however the effect of increasing ZnO concentration into the SLS glass system was inadequate with no briefly explanations to the fundamental topics. Although many properties of SLS glass such as high insulating properties, good and acceptable mechanical have attracted a number of researchers because of their wide-ranging industrial and technical applications, less systematic study on structural and optical properties of SLS glass with addition of ZnO has been reported. In the present work, the investigation on the effect of compositional dependence on structural and optical properties of ZnO-SLS glass for a

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broad range of glass compositions. The influence of ZnO content on the density, structural, absorbance and optical band gaps are analyzed and discussed. 2. Experimental A series of zinc soda lime silica glass with the composition x(ZnO)100-x(SLS) with x = 0, 10, 20, 30, 40 and 50 wt.% have been prepared using conventional melt quenching technique. The x values was varied in steps of 10 wt.% producing six glass samples for ZnO-SLS glass system, respectively. Usually, glass may have different value and wide range of melting points depending on their oxide composition. In this study, the mixture is melted at selected temperature that is 1400 °C for 2 h. As all the substances melted, the substances were poured immediately into a container that contained water to obtain a transparency glass frits and then the glass frits were ground in a vibratory mill jar to obtain fine glass powder (b45 μm). The density of the glass sample was measured by the Archimedes technique using acetone as an immersion liquid. The amorphous nature (glass phase) of the glass samples was confirmed by XRD characterization using Philips X-ray diffractometer (Philips, PW3040/60) with CuKα radiation in the 2θ range from 10° to 90° using 0.02° steps. In this work, a high-resolution FESEM (FEI NOVA NanoSEM 230) was used to observe the microstructure of freshly fractured surfaces of the glass samples. The optical absorption spectra of the glass samples were recorded at room temperature using UV–Vis spectrophotometer (Lambda 35, Perkin Elmer) in the wavelength region from 300 to 800 and these measurements are made on glass and glass ceramic powder with size ≤63 μ that have been compressed in a specific holder. However, there was a limitation when using this technique which is the particle size need to be small and fine about b20 μm so that the precise data can be achieved. The application of the Tauc plot method has been used widely to determine band gap energy by using the diffuse reflectance spectrum as shown; ðαhvÞ

1=n

¼ B hv−Eopt

ð1Þ

where α is absorption coefficient, h is Planck's constant, v is frequency of vibration, B is proportional constant and Eopt is the optical band gap. From the specific software inside the UV–Vis computer, the obtained diffuse reflectance spectrum was converted to Kubelka-Munk function. Thus, the vertical axis was converted to quantity F(R∞), which is proportional to the absorption coefficient. The α in the Tauc equation was substituted with F(R∞) hence the relational expression becomes: 1=n

ð F ðR∞ÞhνÞ

¼ B hν−Eopt

ð2Þ

Table 1 Chemical composition of ZnO-SLS glass system (wt.%). Glass sample/oxide

SLS

SiO2 Na2O CaO Al2O3 MgO K2O ZnO Others Total

69.5 12.5 11.3 2.8 2.0 1.5 – 0.4 100

10ZnO 90SLS 62.6 11.3 10.2 2.4 1.9 1.3 10.0 0.3 100

20ZnO 80SLS 55.6 10.0 9.1 2.2 1.7 1.2 19.9 0.3 100

30ZnO 70SLS 48.7 8.8 7.9 1.9 1.5 1.1 29.8 0.3 100

40ZnO 60SLS 41.7 7.5 6.8 1.6 1.3 0.9 39.9 0.3 100

The chemical composition of precursor glasses were analyzed by Energy Dispersive X-ray Fluorescence (EDXRF). The chemical analysis can be seen in Table 1 and all the elements are measured in oxide form. As can be seen in Table 1, the major components of the precursor glass are ZnO, SiO2, CaO and Na2O which these oxide elements comprise around 95 wt.% from the total weight composition of the precursor glasses. Other oxide elements such as K2O and MgO are minor constituents and account for a small percentage of the bulk composition. All other elements such as BaO, Cr2O3, Fe2O3, and B2O3 collectively seldom exceed 1 wt.% of the bulk composition. The addition of ZnO content into the SLS glass matrix has affected the percentage of other elements. The increase of ZnO content in the precursor glasses has decreased the percentage of other major oxides such as SiO2, CaO and Na2O. The decrease of these oxides has been optimized principally for physically and elasticity and has a good durability provided by the low CaO content. The EDXRF analysis shows that with the progression of ZnO content into the SLS glass has decrease the percentage of other major elements in the precursor glass samples. The composition of SiO2 element is decrease from 69.5 to 34.9 wt.%, Na2O decrease from 12.5 to 6.3 wt.% and CaO decrease from 11.3 to 5.7 wt.%. Other elements in the glass samples also decrease with the addition of ZnO content. As shown in the Table 1, the percentage of Al2O3 decrease from 2.8 to 1.3 wt.%, K2O decrease from 1.5 to 0.7 wt.% and MgO decrease from 2.0 to 1.0 wt.%. Density is a tool in revealing the degree of change in the structure with the change in the glass composition. The average density was frequently measured in order to understand the molecular packing inside the material. The density of the glass samples was measured using the Archimedes technique. A plot of density of ZnO-SLS glass sample is shown in Fig. 1. As shown in Fig. 1, the density of the glasses is increases from 2.520 to 2.842 g/cm3 with the addition of ZnO content. The increasing in density of the glasses is due to the heavier Zn atomic mass compare to the other element in the glass samples [16]. The atomic mass of Zn is 65.390 amu which is heavier compare to the atomic mass of Si (28.086 amu), Ca (40.078 amu) and Na (22.989 amu). Besides, an increase of density of the glass samples also results in the changes of the crosslink density [17]. The increases in density of the

In order to find the type of transition, the Eopt values are calculated by extrapolation of the linear parts of (αhv)1/n vs. hv curves to (αhv)1/ n = 0 for different values of transition. Here, the unit of hν is eV (electron volts), and its relationship to the wavelength λ (nm) becomes hv = 1239.7/λ. Later, a line is drawn tangent to the point of inflection on the curve, and the hν value at the point of intersection of the tangent line and the horizontal axis is determined the Eopt values. The type of transition can be obtained from the value of n. 3. Results and discussion A series of ZnO-SLS glass frits were successfully melted and formed into a glass from pure ZnO powder and SLS glass waste powder via conventional melt and quenching technique. Most of the glass frits were transparent, light greenish in color, bubble-free and homogeneous. The color of the glass frits become more greenish with increasing of ZnO content.

50ZnO 50SLS 34.9 6.3 5.7 1.3 1.0 0.7 49.8 0.3 100

Fig. 1. The average density of ZnO-SLS glass system.


glass samples are attributed to the formation of new linkages in the ZnO-SLS glass structure. Generally, in the glass structure, ZnO can be act as a network former or network modifier. When acting as a network former, ZnO can enter the glass network as ZnO4 structural units, where zinc is linked to four oxygen's ions in a covalent bond configuration. However, different role occur when ZnO played a network modifier, it may breaks Si\\O\\Si bonds and leads to the formation of non-bridging oxygen's (NBO's) atoms together with the defects known as dangling bonds [18]. In recent work, the Zn2 + ion tends to occupy interstitial sites within the highly open glass network. In addition, the increasing in density of the glass sample with the addition of ZnO content effect the structure of the glass samples become loose. The addition of Zn2+ ions rearrange the bonding of the SLS glass and causing the splitting of Si-O-Si bond, hence the bridging oxygen's is converted to non-bridging oxygen's. The formation of NBO's decrease the connectivity of the glasses and the glass structures become weakened [19]. The X-ray diffraction patterns of all glasses are shown in Fig. 2. For all the glass samples, broad halos at angles ≈ 20°–40° are observed. The XRD patterns of the glass samples shows no continuous or discrete sharp peaks but presence of a broad feature (amorphous halo), with long-range structural disorder which reflected the amorphous nature of the glasses. From the Fig. 2, its can be observed that the broad peak was shifted from lower diffraction angle (27°) to higher angle around (33°) and became less broadening with the increasing of ZnO content. The less broadening trend of the amorphous halo indicated a decrease in the lattice constant as the ZnO content increased in the SLS glass matrix [20]. The observation might be attributed to the substitution of Na+ or Ca2+ ions in the SLS glasses with Zn2+ ions. Besides, the shift of the broad peaks approaching to the diffraction angles associated with ZnO content was observed and could be related to the substitution of smaller ionic radius of the Zn2+ ions (~72 pm) with more higher ionic radius of Na+ ions (~102 pm) and Ca2+ ions (~100 pm). Fig. 3 shows the structural morphology for based ZnO-SLS glass system. FESEM micrograph showed the glassy surface and no crystal growth on the surface of the precursor glass samples, which indicate an amorphous phase or glassy nature of the glass samples. Depends on the ZnO content in the structure of glasses, the structural role of ZnO in many oxide glasses is unique since ZnO can act both as a glass former and a glass modifier. As a glass former, ZnO enters the network with ZnO4 structural units. As a network modifier, zinc ion is coordinated in octahedral form and behaves like conventional alkali oxide modifiers [13]. This is a good agreement with an amorphous nature results shown by the XRD measurement. There are no significant changes on the microstructure of the glass samples can be seen as the samples have been prepared with different percentage of ZnO content. The research of optical absorption in the ultraviolet region is an advantageous technique in order to understand the electronic band structures in the materials. In the optical absorption spectrum of glasses, a rapid rise in the absorbance toward the lower wavelengths can be

observed. This rapid rise of absorption coefficient phenomenon is referred to the fundamental absorption edge known as UV “cut off” [21]. If the measurements conduct beyond this UV “cut off” value, the glasses will be opaque to the electromagnetic radiation. Normally, most of the oxide glasses are opaque at wavelengths shorter than 200 nm. This opaque situation happens when the photon energy becomes greater than the optical band gap that exists between the valence and conduction band of the materials. In glasses, measuring the energy band gap can provide information regarding the structural alterations and the nature of chemical bonds in the glass matrix. The width of the localized states in the band gap which arises due to the disorder in the matrix can also be determined from the analysis of the optical absorption spectra [22]. Rapid rises in the absorbance toward lower wavelengths are present in the optical absorption spectrum of glass material. This rapid rise of absorption coefficient can be referred as fundamental absorption edge known as UV cutoff. Fig. 4 shows the optical absorption spectra of precursor ZnO-SLS glass system, respectively. It was clear that no sharp absorption edge which corresponds to the characteristic of glassy state was observed. It is also observed that the fundamental absorption edge (cut-off wavelength (λcutoff)) shifts to higher wavelength side with an increase of ZnO content. As the concentration of ZnO increases, the position of the fundamental absorption edge shifts from 320 to 350 nm. The absorption edge of these glasses is not as sharp as in crystalline compounds due to the disorder in the matrix [23]. The shifts in the optical absorption edge to longer wavelengths indicate that the Eopt values will decrease with increase of ZnO content in the glass structure. There are some models to analyze the structure and correlate the optical band gaps of the oxide glass. The measurement of energy band gap can provide information regarding structural alterations and the nature of chemical bonds in the glass matrix [24]. The width of the localized states in the band gap which arises due to the disorder in the glass matrix can also be determined from the analysis of the optical absorption spectra. Basically, there are two types of transitions can occur at the fundamental absorption edge of both amorphous and crystalline materials; direct transitions and indirect transitions. Both types of transition involve the interaction of an electromagnetic wave with an electron in the valence band, which is raised across the fundamental gap to the conduction band [25]. For the direct optical transitions, it is essential that the wave vector for an electron remains the same as it absorbs a photon. On the other hand, the indirect transitions will also involve simultaneous interaction between the electron and lattice vibrations. In this case, the wave vector of the electron can change in an optical transition due to the absorption or emission of a vibration. In other words for the indirect transitions, the minimum of the conduction band lies in a different k-space direction compared to the maximum value of the valence band [26]. There are two approaches which have been given in the literature about the absorption edge of the material. The first one by Urbach (1953), then the other approach followed by Tauc et al. (1966) and Davis and Mott (1970) [22,27,28]. Analyzing the behavior of the optical absorption coefficient (α), near the fundamental absorption edge is a standard method for investigating optically induced electronic transitions in many materials. The experimental optical band gap can be obtained directly by using relation between absorption coefficient and extinction coefficient which is given by [29]; k¼

Fig. 2. X-ray diffraction pattern of ZnO-SLS glass system.

109

αλ 4π

ð3Þ

By extrapolating the linear region of the extinction coefficient to zero, the experimental optical band gaps can be obtained. The extinction coefficient is plotted as a function of energy, hv. By extrapolating the linear region of the extinction coefficient to zero, the experimental optical band gaps for different glass compositions can be obtained as shown in Fig. 5.

110


Fig. 3. FESEM micrograph of precursor ZnO-SLS glass system, a) SLS, b) 10ZnO90SLS, c) 20ZnO80SLS, d) 30ZnO70SLS, e) 40ZnO60SLS and f) 50ZnO50SLS.

Fig. 4. Optical absorption spectra of precursor ZnO-SLS glass system.

Fig. 5. Plot of extinction coefficient vs. (hv) for precursor ZnO-SLS glass system.


111

Besides, Urbach reports that the absorption coefficient α(v) may depends exponentially on the photon energy (hv) of the materials. The Urbach rule is given by [27]: α ðvÞ ¼ B exp

σ ðhv−Eopt ÞÞ KT

ð4Þ

where v is the angular frequency of the radiation, B is a constant and σ/ KT gives a measure of the steepness of the absorption edge and it is sometime interpreted as the width of the tail of localized state in the band gap [30]. The origin of the exponential part in Equation (2) is not very clear and then Tauc et al. (1966) has suggested that the origin is arise from electronic transitions between the localized states where the density of the state are totally depends on the energy [28]. In 1970, Davis and Mott modified and simplify the rule and recommended the new expression for direct transition can be written as:

Fig. 6. (αhv)2/3 as a function of energy for (hv) precursor ZnO-SLS glass system.

n

ðhv−Eopt Þ α ðvÞ ¼ B hv

ð5Þ

where B is a constant, Eopt is the optical band gap, and n is the index determined by the nature of the electronic transitions during the process of absorption. The optical band gap in amorphous materials proposed by Davis and Mott can be explained as the width of the localized states near the mobility edge which is turn depends on the degree of disorder and defects present in the amorphous structure. For direct transitions, the value of n = 1/2 or 3/2 depending on whether the transition is allowed or forbidden and for indirect transitions, the value of n = 2 or 3 depending on whether the transition is allowed or forbidden, respectively. The type of transition can be obtained from the value of n. In order to determine the unique value of n, the differential method has been used as following equation: d ½ ln ðαhvÞ n ¼ d ðhvÞ hv−Eopt

ð6Þ

The differential curve has a discontinuity at a particular energy value (hv = Eopt) which gives the optical band gap. In order to find the type of transition, the Eopt values are calculated by extrapolation of the linear parts of (αhv)1/n vs. hv curves to (αhv)1/n = 0 for different values of transition as shown in Table 2. By comparing the value from the extinction coefficient and differential curve with the optical band gap obtained using different values of n, a good agreement with Eopt values for n = 3/2 were achieved. Hence, from the comparison of n values, it is can conclude that for precursor ZnO-SLS glass system, the optical band gap arises due to the direct forbidden transition. Thus, the experimental optical band gap obtained directly from the extinction coefficient matches well with the Mott and Davies relation for n = 3/2 transition. Fig. 6 illustrates the plots of (αhv)2/3 vs. hv for precursor ZnO-SLS glass samples and shows the direct forbidden optical band gaps of the precursor glass samples calculated from these plots. The obtained values of optical energy gap were decreased from 3.80 to 3.56 eV with the increasing of ZnO concentration. The addition of ZnO to SLS glass network has caused the breakdown of a continuous SiO4 network,

whereby a significant shift of the absorption edge to longer wavelengths was observed [31]. The shifting of optical band gap were more likely related to structural rearrangements of the glass and relative concentrations of various fundamental units. The movement of absorption band to a lower energy was due NBO's, whereby the electrons were loosely bonded to NBO's than BO's. The results obtained in the Fig. 7 have indicated a decreased in optical energy gap as the ZnO content was increased, whereby breaking of SLS glass structure happened due to the increment of NBO's ions content [32]. Optical band gap of ZnO-SLS glasses represents the excitation energy of electrons from the hybridized orbitals of 4 s zinc and 2p non-bridging oxygens states to the conduction band (4p states of zinc forms the conduction band). The Eopt depends on the energy level of the upper valence band edge which is determined by the separation of 4 s zinc and 2p oxygen states. A theoretical study of the energy bands based on ionic model, indicates that the separation between 4 s zinc and 2p oxygen states is very sensitive to the charge on the oxygen ion [33]. As the ionicity of the oxygen decreases, the energy of the 4 s zinc state decreases and the 2p oxygen state increases, so the hybridization of these two states increases. Besides, it is found that the chemical shift of Si units changes with ZnO concentration. This change shows that the ZnO units which are formed due to the addition of ZnO to SiO2, bridge preferentially to Si units. This results in the formation of Zn-OSi linkages, which arise as a result of the increasing covalency of the Zn\\O bond. The increase in covalency of the ZnO bond is a consequence of the high polarizability of Zn2+ ions and the directional nature of the ZnO bond which effectively reduces the optical band gap [34]. 4. Conclusion A series of x(ZnO)100-x(SLS) glasses have been successfully prepared and synthesized. The effect of compositional dependence on structural and optical properties of each glass sample was carried out. The density of glasses increased with the addition of ZnO as heavier zinc atomic mass and due to the increase of the atomic distance of the composition. The XRD pattern of the glass samples shows no continuous or discrete sharp peaks which reflected the amorphous nature of the

Table 2 Variation of Eopt for precursor ZnO-SLS glass system. Optical band gap Eopt (eV)

SLS

10ZnO 90SLS

20ZnO 80SLS

30ZnO 70SLS

40ZnO 60SLS

50ZnO 50SLS

Eopt (experimental) Eopt from differential curve Direct allowed transition n = 1/2 Direct forbidden transition n = 3/2 Indirect allowed transition n = 2 Indirect forbidden transition n = 3

3.90 ± 0.04 3.78 ± 0.04 4.15 ± 0.04 3.80 ± 0.04 3.65 ± 0.04 3.40 ± 0.04

3.84 ± 0.04 3.70 ± 0.04 4.10 ± 0.04 3.74 ± 0.04 3.56 ± 0.04 3.32 ± 0.04

3.79 ± 0.04 3.68 ± 0.04 4.05 ± 0.04 3.71 ± 0.04 3.52 ± 0.04 3.26 ± 0.04

3.73 ± 0.04 3.60 ± 0.04 4.00 ± 0.04 3.62 ± 0.04 3.45 ± 0.04 3.20 ± 0.04

3.67 ± 0.04 3.56 ± 0.04 3.96 ± 0.04 3.60 ± 0.04 3.38 ± 0.04 3.14 ± 0.04

3.65 ± 0.04 3.54 ± 0.04 3.95 ± 0.04 3.56 ± 0.04 3.36 ± 0.04 3.12 ± 0.04

112


References

Fig. 7. Compositional dependence of experimental optical band gap and optical band gap of ZnO-SLS glass system from Mott and Davis Model for n = 3/2 transition.

glasses and supported by phase morphology from FESEM micrograph. The optical band gap for ZnO-SLS glass system using Mott-Davis model from the optical absorption edge measurements has been obtained. In order to find the type of transition, optical band gap is obtained by the differential method using Mott-Davis model and compared with the experimentally obtained value from the extinction coefficient. From the analysis, the absorption edge of the glasses arises due to direct forbidden transitions (n = 3/2). The Eopt is found to decrease from 3.80 to 3.56 eV in glasses with increasing concentration of the ZnO. The reduction of Eopt with the increasing of ZnO content is due to the increase in the covalent nature of Zn\\O bond.

Acknowledgments The researchers gratefully acknowledge the financial support for this study from the Malaysian Ministry of Higher Education (MOHE = 5524817) and Universiti Putra Malaysia through the Fundamental Research Grant Scheme (FRGS) and Inisiatif Putra Berkumpulan (IPB = 9412601) research grant.

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