Investigation of structural, physical and optical properties of CeO2–Bi2O3–B2O3 glasses

Investigation of structural, physical and optical properties of CeO2–Bi2O3–B2O3 glasses

Physica B 407 (2012) 4168–4172 Contents lists available at SciVerse ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Investi...

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Physica B 407 (2012) 4168–4172

Contents lists available at SciVerse ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Investigation of structural, physical and optical properties of CeO2–Bi2O3–B2O3 glasses Gurinder Pal Singh, Parvinder Kaur, Simranpreet Kaur, D.P. Singh n Department of Physics, Guru Nanak Dev University, Amritsar 143005, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 April 2012 Received in revised form 27 June 2012 Accepted 29 June 2012 Available online 10 July 2012

xCeO2–30Bi2O3–(70  x) B2O3 glasses are synthesized by using the melt quench technique. A number of studies such as XRD, density, molar volume, optical band gap, refractive index and FTIR spectroscopy are employed to characterize the glasses. The band gap decreases from 2.15 to 1.61 eV, refractive index increases from 2.67 to 2.93 and density increases from 4.151 to 4.633 g/cm3. The decrease in band gap with CeO2 doping approaches the semiconductor behavior. FTIR spectroscopy reveals that incorporation of CeO2 into glass network helps to convert the structural units of [BO3] into [BO4] and results in Bi–O bond vibration of [BiO6]. & 2012 Elsevier B.V. All rights reserved.

Keywords: Optical properties Bismuth glasses Borate glasses

1. Introduction Borate glasses are very important optical materials because of their low melting point, high transparency, and high thermal stability [1]. It is generally used to make insulating and dielectric materials. The addition of bismuth oxide to the borate glasses improves the chemical durability and thermal stability of the samples [2]. As the bismuth ions have small field strength and high polarizibility, they cannot work as network former. But in the presence of conventional glass former like B2O3 the glass forming is possible. It may built a glass network of BiOn (n¼ 3, 6) pyramids [3]. Bismuth borate glasses are very important in present day due to their many potential and photonic applications as ultrafast optical switches, infrared transmission components, mechanical sensors, reflecting window [4–11]. These glasses are also used in nonlinear optical devices, gamma ray absorber, scintillators detectors, tunable waveguide and tunable fiber gratings devices [12–16]. Also these glasses have high refractive index due to which they are very useful in processing devices and optical telecommunication [17]. In heavy metal glasses PbO and Bi2O3 based glasses have so much importance. But the wide use of lead in glasses is not environment friendly and it has harmful effect on health also. As it has similar properties to bismuth, so bismuth is considered as a suitable material and can replace lead in heavy metal glasses. In past, effects of rare earth metals on structural, magnetic and optical parameters have been studied [18,19]. Rare-earth

n

Corresponding author. Tel.: þ91 9914010451. E-mail address: [email protected] (D.P. Singh).

0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.06.043

containing glasses have been intensively studied due their technological applications like luminescent devices and laser technology [20–23]. Cerium doped glasses have many applications in blue luminescent optical system [24], biosensors, solid oxide fuel cells, scintillators and dielectric materials. CeO2 shows the hopping mechanism and ionic polaronic behavior in borate and lead borate glass system [25,26]. The cerium containing glasses also show conductivity which has been explained on the basis of existence of Ce3 þ and Ce4 þ [27]. In the light of above, the aim of the present work is to investigate the dominant role of CeO2 on physical, structural and optical properties in Bi2O3–B2O3 glass system. The structural and physical properties are studied by using XRD (x-ray diffraction), Fourier transform infrared spectroscopy (FTIR), density and molar volume techniques. The optical properties of glasses are determined by using UV–visible spectroscopy and refractive index measurements.

2. Experimental details 2.1. Preparation of glasses A series of glass samples of formula xCeO2–30Bi2O3–(70 x) B2O3 with 0rx r8 mol% were prepared by using the melt quenching technique. The required amount of chemicals, bismuth oxide (Bi2O3), cerium oxide (CeO2) and boric oxide (B2O3), were mixed together by grinding to obtain a fine powder. The obtained mixture was melted in a silica crucible at a temperature of 1000– 1100 1C for 60 minute until a homogenous bubble free liquid was formed. The melt was then poured into preheated steel mould

G. Pal Singh et al. / Physica B 407 (2012) 4168–4172

Table 1 Nominal composition (mole%), density, molar volume, average boron–boron separation of glasses. The errors in the measurement of density is estimated to be 0.008 g cm  3. CeO2 (%)

Bi2O3 (%)

B2O3 (%)

Density (D) (g/cm3) 7 0.008 g cm  3

Molar volume (cm3/mol)

/dB–BS (nm)

Bi-1 Bi-2 Bi-3 Bi-4 Bi-5

0 2 4 6 8

30 30 30 30 30

70 68 66 64 62

4.151 4.332 4.442 4.524 4.633

45.416 43.991 43.364 43.031 42.461

0.501 0.485 0.473 0.463 0.453

Intensity [a.u]

Glass code

Bi-1

Bi-3 Bi-5

0

10

20

30

40

50

60

70

80

90

2θ[degree]

4169

Besides this, the band from 2400 to 4000 cm  1 is due to O–H vibration of water group [31–36]. The infrared spectra of CeO2–Bi2O3–B2O3 glasses are displayed in Fig. 2. The spectra consist of large, medium, weak and broad bands. In general the band at 806 cm  1 is assigned to the boroxol ring in borate glass network [37]. In the present study, this band is found to be missing. This indicates the absence of boroxol rings in borate network. Hence the glass system contains [BO3] and [BO4] groups. A weak band observed in all glass samples around 2932 cm  1 is assigned to vibration of O–H bond of water group. In sample Bi-1, a broad band centered at 1312 cm  1 is due to symmetric stretching vibration of B–O bond in [BO3] units [38,39]. Intensity of this band decreases with increase in the CeO2 concentration. This happens only when the number of trigonal [BO3] unit decreases and tetrahedral [BO4] units increases in the glass system. Band at 958 cm  1 has been assigned to stretching vibration of B–O bond in [BO4] tetrahedral units [40]. It has been observed that, the intensity of this band increases from sample Bi1 to Bi-5, with increase of cerium oxide content as shown in Fig. 2. This is due to presence of [BO4] groups. A band at 696 cm  1 stands for B–O–B bond bending vibration of bridging oxygen atoms [41,42]. In sample Bi-1, a band centered at 485 cm  1 may correspond to Bi–O bending vibration in [BiO3] or [BiO6] units [43]. As the content of cerium is increased this band shifts to lower wave number (485–472 cm  1).Thus the presence of cerium ions affects the surroundings of Bi3 þ cation favouring the formation of [BiO6] units. A band due to [BiO3] polyhedral at 840 cm  1 is absent. This also confirms the presence of only [BiO6] units in glass network [44]. Hence the continuous increase of cerium concentration helps to increase the concentration of tetrahedral [BO4] units along with the formation of [BiO6] octahedral units. This result reveals the modifier behavior of cerium in present glasses.

Fig. 1. X-ray diffraction pattern of glasses.

3.3. Density and molar volume and annealed at a temperature of 380 1C for 1 hour to avoid breaking of the sample by residual internal strains. The obtained samples were polished with cerium oxide in order to obtain maximum flatness. The nominal composition of the prepared glasses is given in the Table 1. The other experimental techniques applied to study the structural, physical and optical studies were the same as given in our previous study [28].

This measurement is simple but a powerful tool to examine the changes occurring in the structure of glasses. It is affected by the structural softening or compactness. The relation between density and molar volume for glass sample as function of cerium concentration is shown in Fig. 3. It has been indicated in Table 1 that density of CeO2–Bi2O3–B2O3 glasses increases while the molar volume decreases with increasing CeO2 concentration at expense of B2O3. It has been observed that as CeO2 is added to the

3. Results and discussion

XRD pattern of CeO2–Bi2O3–B2O3 glass samples (shown in Fig. 1) prepared with concentrations of CeO2 from 2% to 8% shows no continuous or discrete sharp peaks which reflect the characteristic of amorphous glass structure. The absence of long range atomic arrangement is a clear indication of the glassy nature of the samples [29,30].

3.2. FTIR

Transmittance [a.u]

3.1. X ray diffraction

Bi-5

696 472 952 696 472

Bi-4 1312 952

696 472 696 952 472 952

The infrared spectra of glasses are used to get more information about the presence of different structural groups. It is generally characterized by three distinguished regions. The first region extends from 1200 to 1600 cm  1 due to B–O stretching of [BO3] units, second region from 800 to 1200 cm  1 is due to B–O stretching of [BO4] units and the third region lying around 700 cm  1 is due to B–O–B bending vibration in borate network.

1312

Bi-3 1312

Bi-2 1312

Bi-1

696 485

958

500

1000

1312

1500

2000

2500

3000

Wave Number [cm1] Fig. 2. FTIR spectra of CeO2–Bi2O3–B2O3 glasses.

3500

4170

G. Pal Singh et al. / Physica B 407 (2012) 4168–4172

4.7

46 density molar

5 Bi-5

44

4.4 4.3

4

Absorption [a.u.]

4.5

Molar Volume [cm3/mol]

Density [g/cm3]

4.6

Bi-4 Bi-3

3 Bi-2 2

4.2

Bi-1 1 42

4.1 0

2

4 Mol % of CeO2

6

8

0 400

500

600 700 800 Wavelength [nm]

Fig. 3. Dependence of density and molar volume of glasses on CeO2 contents.

V Bm ¼

Vm 2ð1X B Þ

where Vm is molar volume, XB molar fraction of B2O3 !1=3 V Bm /dB2B S ¼ NA

ð1Þ

1000

Fig. 4. Optical absorption of CeO2–Bi2O3–B2O3 glasses.

4.0 3.5 Bi-5 3.0 [αhν]1/2(cm-1 eV)1/2

glasses, packing fraction is able to achieve a higher value. Due to the addition of cerium oxide contents a large number of oxygen ions are available in glass structure. These oxygen ions help to convert three coordinated boron units to four coordinated boron units. The [BO4] tetrahedral are considerably denser than the symmetric [BO3] triangle [45]. This is also due to replacement of higher molecular weight CeO2 with lower molecular weight B2O3 of glasses system. It has been observed that the molar volume of glasses decrease which is attributed to decrease of interatomic spacing of glass network and results in compaction of glass structure [46]. Decrease of interatomic spacing is also confirmed by calculating the average boron–boron separation /dB–BS [47]. The boron atoms are the central atoms BO3/2 with negatively charged tetrahedral BO4/2 units, thus the volume V Bm corresponds to the volume that contains one mole of boron within the given structure and has been found as

900

Bi-1

Bi-4

2.5

Bi-2

Bi-3

2.0 1.5 1.0 0.5 0.0 1.5

2.0 hν (eV)

2.5

3.0

Fig. 5. Optical band gap of CeO2–Bi2O3–B2O3 glasses.

ð2Þ

where NA is Avogadro number. This is observed that the value of /dB–BS decreases continuously with increase of CeO2 contents (Table 1). Hence, the presence of CeO2 leads to contraction of glass network [28]. Above mentioned factors are responsible for an increase of density followed by decrease in molar volume [17]. The graph showing the increase in density with decrease in molar volume reveals the change in the structure of glass with increasing CeO2 contents. 3.4. UV–visible spectroscopy The study of optical absorption and band edge is a helpful method for getting information about the band structure and energy gap of crystalline and amorphous materials. Presently, it has been observed that due to increase in cerium concentration, the optical absorption edge shifts towards the longer wavelength from 419 nm to 512 nm. Presence of cerium helps to shift the band edge of prepared glasses as shown in Fig. 4. Shifting of band edge is due to conversion of the [BO3] groups into [BO4] groups [48,49].

The plot between (ahn)½ and energy (hn) has been used to calculate the band gap energy as shown in Fig. 5 where a, h and n denote absorption coefficient, Planck constant and frequency respectively. The absorption coefficient (a) has been determined from the relation, a ¼ 1/t log (I0/I), where t¼thickness of sample, log (I0/I)¼absorbance. The decreasing values of optical band gap energy upon increasing the CeO2 content is understood in terms of structural changes which are taking place in studied glasses. In an initial stage from sample Bi-1–Bi-3 decrease of band gap is due to conversion of large number of trigonal [BO3] units to tetrahedral [BO4] units of borate. Due to addition of CeO2 excess oxygen ions are available in glass system which are utilized for the conversion of [BO3] to [BO4] groups. The tetrahedral [BO4] groups are strongly bonded in glasses network because the bond strength of B–O (808.7 kcal/mol) is greater than Ce–O (795 kcal/mol) which results in a decrease in optical band gap [50]. Band gap is showing a continuously decreasing trend in samples Bi-4 and Bi-5 with an increase in cerium concentration. This is due to the presence of Ce4 þ or Ce3 þ groups of cerium. These parameters (BO4 groups, Ce4 þ or Ce3 þ ions) have shifted the absorption edge to the lower energy that leads to a significant compaction in the band gap.

G. Pal Singh et al. / Physica B 407 (2012) 4168–4172

2.2

The refractive index, molar refraction and molar polarizibility of the glasses have been calculated by using the relation given by Dimitrova and Komatshu and Duffy [51,52].

2.1



ð3Þ

 2  n 1 Vm Rm ¼ n2 2

aM ¼



ð4Þ



3 Rm 4pNA

ð5Þ

3.6. Optical basicity The optical basicity, L, of an oxidic medium, is the average electron donor power of all the oxide atoms comprising the medium. Increasing basicity results in increasing the negative charge on the oxygen atoms and, accordingly, increasing covalency in the oxygen-cation bonding. The optical basicity can be calculated from the glass composition and from the basicity moderating parameters of the different cations present. The theoretical optical basicity, LTh, is calculated using the following expression [53]:

LTh ¼

X

2.0

2.85

1.9

2.80

1.8

2.75 Refractive Index

1.7

xi LI

ð6Þ

where x1, x2, yy xn are equivalent fractions (mole%) of different oxides, i.e., the amount of each oxide oxygen contributes to the overall glass stoichiometry and L1, L2,y.. Ln are the optical basicity values assigned to the constituent oxides. As the optical basicity of CeO2 (1.01) has greater value than B2O3. Hence the addition of CeO2 on the expense of B2O3 increases the optical basicity value. The graph between refractive index, band gap and mol% of CeO2 shown in Fig. 6 confirms the opposite behavior with an increase in cerium concentration.

2.70

1.6

2.65 0

where n, Eg, Rm, NA and am are the refractive index, optical band gap, molar refraction, Avogadro number and polarizibility respectively. The calculated values of refractive index, molar refraction and molar polarizibility of the glasses have been given in Table 2 which shows that refractive index increases progressively and molar refraction and molar polarizibility correspondingly decreases by following the band gap of the glasses. This is due to conversion of trigonal [BO3] to tetrahedral [BO4] groups. Also the higher value of polarizibility of Bi3 þ (1.508 A˚ 3) than Ce4 þ (0.738 A˚ 3) helps to obtain the above discussed results [28].

2.90

Band Gap

Band gap [eV]

rffiffiffiffiffiffi Eg n2 1 ¼ 1 20 n2 þ 2



2.95

Refractive Bandgap

Refractive Index

3.5. Refractive index, molar refraction, molar polarizibility

4171

2

4 CeO2[mol %]

6

8

Fig. 6. Dependance of refractive index and optical band gap of glasses on CeO2 contents.

4. Conclusion The optical band gap energy, density, molar volume, refractive index and FTIR spectroscopy results are in good agreement with each other and reveal that cerium acts as a structural network modifier. The conclusions drawn from these studies for CeO2–Bi2O3–B2O3 glasses are summarized as follows: (1) X ray diffraction shows the amorphous behavior of the prepared samples. (2) Decrease in average boron–boron separation with CeO2 incorporation confirms the presence of [BO4] and bismuth ions in the glass network which results in compaction of glass structure. These factors are responsible for increase in density and refractive index. Molar volume decrement also indicates the decreases in interatomic spacing among the atoms of glass network. (3) The optical band gap energy value decreases in semiconductor region and refractive index increases with incorporation of cerium oxide. This confirms the modifier role of cerium in glass system. (4) FTIR confirmed the coexistence of trigonal and tetrahedral borate groups with an establishment of Bi–O bond vibration of [BiO6] units. Acknowledgment All the authors are thankful to Instrumental centre, Department of physics, Guru Nanak Dev University, Amritsar, for providing the XRD facility.

References Table 2 Optical band gap, refractive index, molar refraction, polarizibility and optical basicity of the glasses sample. Glass code

Optical band gap Refractive (Eopt.) (eV) 7 0.01 index

Molar refraction (cm3/mol)

Polarizibility (A˚ 3)

Optical basicity

Bi-1 Bi-2 Bi-3 Bi-4 Bi-5

2.15 1.98 1.90 1.83 1.61

30.49 30.19 29.99 29.99 30.43

1.21 1.20 1.19 1.19 1.21

0.658 0.667 0.677 0.686 0.697

2.67 2.75 2.78 2.81 2.93

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