- Email: [email protected]
Physical and absorption properties of titanium nanoparticles incorporated into zinc magnesium phosphate glass
Materials Characterization 111 (2016) 177–182
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Physical and absorption properties of titanium nanoparticles incorporated into zinc magnesium phosphate glass S.F. Ismail, M.R. Sahar ⁎, S.K. Ghoshal Physics Department, Advanced Optical Materials Research Group, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
a r t i c l e
i n f o
Article history: Received 7 July 2015 Received in revised form 27 November 2015 Accepted 30 November 2015 Available online 02 December 2015 Keywords: Amorphous state Optical band gap Physical properties Titanium nanoparticles
a b s t r a c t We report the influences of Titania (TiO2) nanoparticles (NPs) on the physical and optical properties of melt quench synthesized zinc magnesium phosphate glasses. Five glass samples with composition (42 − x)P2O5– 50ZnO–8MgO–xTiO2, where x = 0, 1, 2, 3, 4 mol% are prepared and characterized. XRD pattern verified the amorphous nature of all samples. TEM images manifested the growth of Ti NPs of average size ≈ 5.78 nm. TiO2 NP concentration dependent variation in the physical properties including glass density, molar volume, molar refractivity, electronic polarizability and ionic packing density are determined. The values of glass refractive indices, density and ionic packing density are increased with the increase of TiO2 NP contents. Conversely, the Urbach energy, direct and indirect optical band gap are found to decrease with the increase of TiO2 NP concentration. These glass compositions may be potential for various solid state devices including laser. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Certainly, phosphate glasses are prospective over conventional silicate and borate systems due to their unique physical properties such as high temperatures expansion coefficient, low melting and transition temperatures, lower indices of refraction and prominent ultra-violet transmission. Recently, intensive research on ZnO–P2O5, MgO–P2O5 and ZnO–MgO–P2O5 glasses displaying their superior structural, physical, thermal and optical properties made them potential for diverse device applications [1–3]. Khor and co-workers [1] examined the physical and optical properties of simultaneous admixture of MgO and ZnO into the phosphate compositions and demonstrated their strong influenced on overall properties. Lately, TiO2 NPs received special attention due to its several advantages properties and potential benefits including low cost, chemical stability, non-toxicity and, high reactivity under UV light irradiation [4]. Moreover, TiO2 is a wide band gap (≈3.2 eV) semiconductor [5,6]. Generally, TiO2 NPs are widely used for photovoltaic cells, electro-chromic materials, photo-catalysis, self-cleaning glass and waste water treatment [5,7]. The reconstruction of the TiO2 occurs upon prolonged heating, where the titanium ions usually exist in Ti4 + state. These Ti4+ ions form TiO4, TiO6 and rarely TiO5 (comprising of trigonal bipyramids) structural units in the glass network [8,9]. However, some reports acknowledged the existence of Ti3+ valence state in the glass matrices [10,11]. According to Diebold [12], slightly defective TiO2 surface reveals weak feature due to the presence of small amounts of Ti3+ defects when heated to 1000 K. ⁎ Corresponding author. E-mail address: [email protected] (M.R. Sahar).
http://dx.doi.org/10.1016/j.matchar.2015.11.030 1044-5803/© 2015 Elsevier Inc. All rights reserved.
The goal of this study is to examine the effects of varying concentrations of TiO2 NPs on the physical and optical properties of zinc magnesium phosphate amorphous matrix. Ti NP mediated emergent physical and optical properties of ZnO–MgO–P2O5 glass system are inspected. Results are analyzed, compared and discussed.
2. Experimental The nominal glass compositions with their codes are listed in Table 1. About 20 g batch with analytical grade glass constituents of P2O5 (purity 98.94%), ZnO (purity 99%), MgO (99%) and TiO2 (purity 99.7%) in powder form are well-mixed and melted in alumina crucibles at 1100 °C for 2 h in an electrical furnace so that a homogeneously mixed melt is obtained. The melt is then transferred to an annealing furnace and poured on the brass mold before being annealed at 400 °C for 3 h to reduce the mechanical stress that causes embrittlement [13]. The vitreous/amorphous state of the prepared samples is examined by X-ray diffraction (XRD) measurement on a Siemen Diffractometer that used Cu-Kα (λ = 1.54 Å) as radiation source. Relatively fine glass powder is placed in a sample holder and scanned from 2θ = 10–80° with a step of 0.50° at running voltage of 30 kV and current 20 mA. The presence of NPs in the glass matrix is detected using a transmission electron microscope (TEM, Philips CM12), which operated at 200 kV with Dock version 3.2 image analyzer. Specimens for TEM are prepared by dispersing the powder in acetone via ultrasonic bath. The solution is then placed onto copper grid and allowed to dry before got ready for characterization. TEM images are analyzed to determine the NP sizes, shapes, and distribution in the glass matrix. In addition, the Energy Dispersive of X-Ray (EDX) analysis is used to analyze the sample composition.
178
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
Table 1 Chemical compositions of the synthesized glasses with codes. Chemical composition (mol.%) Sample code S1 S2 S3 S4 S5
P2O5
MgO
ZnO
TiO2
42 41 40 39 38
8 8 8 8 8
50 50 50 50 50
0 1 2 3 4
Room temperature UV–Vis–NIR measurements in the wavelength range of 200–700 nm (Shimadzu 3101PC UV–Vis–NIR scanning spectrophotometer) are carried out on well-polished (up to 1200 μm grid size using diamond paste), highly transparent and shiny glass samples of thickness 2 mm. The absorption spectra are further exploited to evaluate optical band gaps, Urbach energies, and refractive indices of all prepared samples. The photon energy (hv) dependent optical absorption coefficient, α(ν) near the absorption band edge yields, αðvÞ ¼
2:303A d
ð1Þ
where A is the absorbance and d is the thickness of the sample. Following Davis and Mott [14], for amorphous and semiconducting materials the values of α(v) as a function of photon energy can be calculated from, αðvÞ ¼
n C hv−Eopt hv
ð2Þ
where C is a constant, Eopt is the optical band gap energy with n = 2 for indirect transition and n = 1/2 for direct transition. The Urbach energy (ΔE) that characterizes the extent of the exponential tail of the absorption edge can be written as hv αðvÞ ¼ B exp ΔE
ð3Þ
where B is a constant. The Urbach energy measures the width of the band tails of the localized states. The refractive index (n) is related to the indirect optical band gap energy EIopt via sffiffiffiffiffiffiffiffi EIopt n2 −1 : ¼ 1− 2 20 n þ2
ð4Þ
Fig. 2. (a) TEM image of S5; (b) size distribution of TiO2 NPs in S5.
The density (ρ in g/cm3) of glass samples is determined via Archimedes method with toluene (insoluble in water and does not react with glass samples) as immersion liquid. The estimated error in density measurement is ≈ ± 0.001 g/cm3. The sample weight in the air and within the immersion liquid is measured using a digital balance. Glass density is calculated using, ρ ¼ ρT
WA WA −WL
ð5Þ
where ρT is the density of toluene (0.8669 g/cm3), WA and WL are the weight of the sample in the air and liquid respectively. The molar volume (Vm in cm3/mol) is calculated from Vm ¼
X ni mi ρ
ð6Þ
where ni and mi denote the molar fraction and molecular weight of ith component, respectively. The molar refractivity (Rm), the electronic polarizability (αe) and the ionic packing density (Vt) are determined from the value of molar volume using the following relations: Rm ¼
n2 −1 ðV m Þ n2 þ 2
ð7Þ
3 Rm 4πNa
ð8Þ
X V t ¼ ð4=3πÞ ðr 3i ni Þ=V m
ð9Þ
αm ¼
i
Fig. 1. Typical XRD pattern for S1, S3 and S5 glass samples.
N ¼ np Na ρ =M
ð10Þ
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
179
Fig. 3. EDX spectrum of S5 glass sample.
where ri is the ionic radii of the ith component, np is the mol% of TiO2, Na is the Avogadro's number and M is total molecular weight. Ionic radius of Shannon and Prewitt [15] is used for Vt calculation. 3. Results and discussions The XRD pattern of the prepared samples (Fig. 1) being devoid of any sharp Bragg peaks shows lack of crystalline order. Furthermore, the presence of a broad hump in the range of 20°–30° confirms their amorphous nature [16]. Conversely, the existences of NPs hardly detected by XRD due to their fairly low concentration compared to that of host and modifier contents. The typical TEM image of the sample as shown in Fig. 2(a) clearly reveals the presence of spherical and non-spherical shaped TiO2 NPs (black spots) in the glass matrix. The NP size distribution is found to be Gaussian with the average diameter ≈5.78 ± 0.1 nm (Fig. 2(b)). Fig. 3 shows the EDX spectrum for S5 and it confirmed the existence of Ti NPs as observed in the TEM image. The analysis for the composition has been done and the result summarized in Table 2. As can see that nominal and actual result for sample S5 are in good conformity. The room temperature UV–Vis–NIR spectra of the samples in the range of 200–700 nm as displayed in Fig. 4 clearly manifest the strong absorption from TiO2 NPs with characteristic UV wavelength [17]. The intensity of the surface plasmon resonance (SPR) peaks evidenced in between 208 and 324 nm are enhanced with the increase of TiO2 NP concentration accompanied by red shifts. This prominent absorption is ascribed to the transition from the valence to the conduction band of TiO2 NPs. It is worth noting that TiO2 being an ideal photo-catalyst in decomposing organic contamination are suitable for environmental applications. It is well known that TiO2 has strong oxidation power under UV illumination. Additionally, the surface of titanium nanoparticles embedded zinc magnesium phosphate glass becomes hydrophilic or hydrophobic with a water contact angle less or more than 90°, making the surface easily self-cleaned [18,19]. These two characteristics of TiO2 glass are responsible for achieving outstanding self-cleaning property when exposed outdoor. Using the absorption edges, optical band gap energy for indirect and direct transition (Table 3) are evaluated via Tauc plot [20–23]. From Fig. 5(a) and (b) the value of Eopt for indirect and direct transition are obtained by extrapolating the linear part of the curve to the x-axis = 0. Fig. 6 shows the reduction in Eopt values for indirect 3.56–3.32 eV and direct 3.71–3.55 eV with the increase of TiO2 NP contents. This reduction in Eopt is ascribed to the increasing number of
non-bridging oxygen (NBO). Consequently, the electrons localization states in the bang gap are minimized [20,24]. The increase in the degree of localization of electrons causes an enhancement of the donor centers in the glass network. The emergence of higher concentration of these donor centers reduced the optical band gap and shifts the absorption edge gradually towards higher wavelength [25,26]. Therefore, more electrons can easily be transferred from the valence band to the conduction band. In the amorphous materials, the lack of crystalline long-range order and the presence of disorder dominate the phonon assisted processes associated with the tailing of the density of states into the normally forbidden energy band gap [21,22,27]. The Urbach energy (Table 3) being a measure of the width of band tails of localized states is used to characterize the degree of disorder in amorphous and crystalline systems. The Urbach energy as depicted Fig. 7 is found to decrease with the increase of TiO2 NP contents, which is in good agreement with earlier observation [28,29]. This decrease is primarily due to the reduction of the width of the localized density of states in the forbidden gap. In fact, the density of electron with higher energy is reduced because of the enhanced electronic transition from the valence band to the conduction band. The glasses with the smaller Urbach energy would have greater tendency to minimize the static disorder in the glass structure. As illustrated in Fig. 8, the refractive index displayed an increase with increasing TiO2 NP concentration. The higher values of glass refractive index reflect smaller band gap and more compact or regular network structure. This is due to the generation of less defect and strengthening of covalency. The enhancement in the refractive index with the increase of TiO2 NP contents is related to the formation of
Table 2 Nominal/actual composition of 38P2O5–50ZnO–8MgO–4TiO2 (S5).
Nominal Actual
P2O5
ZnO
MgO
TiO2
38 50.38
50 40.68
8 6.40
4 2.58
Fig. 4. Absorbance spectra all glasses.
180
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
Table 3 Indirect and direct optical band gap energy, Urbach energy and refractive index of studies samples. Sample code
EIopt (eV)
ED opt (eV)
ΔE (eV)
n
S1 S2 S3 S4 S5
3.56 3.40 3.38 3.35 3.32
3.71 3.60 3.59 3.56 3.55
2.83 0.23 0.19 0.16 0.15
2.27 2.29 2.30 2.31 2.32
Fig. 7. Urbach energy of glasses versus TiO2 concentration.
Fig. 5. A typical Tau plot for (a) indirect and (b) direct optical band gap for S2, S3, S4 and S5.
more Ti4+ ions in the glass networks [27,30]. Consequently, the increase in the number NBO tends to make the glass more compact. Furthermore, the empty d-shell of Ti4+ metal ions contributes to large increase of the linear and non-linear indices of the glasses. Table 4 enlists the TiO2 NP concentration dependent various physical parameters. Fig. 9 depicts the variation of density and molar volume as a function of TiO2 NP concentration. The observed inverse relationship between density and molar volume is also reported by Raghavaiah et al. and Satyanarayana et al. [26,31]. The increase in glass density with the increase of TiO2 NP contents modifies the glass network rigidity. This is majorly due to the larger relative masses and ionic radius of titanium ions than the phosphate ions in the glass host [20,32]. According to Shannon, ionic radii for Ti4+ and P5+ ions are 0.605 and 0.29 Å, respectively [15]. The decrease in free volume and the inter-atomic distances also increases the rigidity of the glass. Fig. 10 shows the effects of TiO2 NP concentrations on the polarizability of the glass. Dimitrov et al. [33] acknowledged that glasses with higher refractive index possess large polarizability, high optical basicity, small metallization criterion and large third-order non-linear optical susceptibility. The observed decrease in electronic polarizability with the increase of TiO2 concentration may be due to the dual nature of ZnO that acts as glass modifier as well as former [28,34]. Besides, the lower polarizability improves the glass chemical durability. The influence of TiO2 NPs on the ionic packing density Vt is presented in Fig. 11. The ionic packing density is increased with the increase of TiO2 NP contents. This is due to the higher ionic radii of Ti atoms, which fits with greater probability to the free space of excess volume.
Fig. 6. Indirect and direct optical band gap energy versus TiO2 NP concentration.
Fig. 8. TiO2 NP concentration dependent glass refractive index.
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
181
Table 4 Physical properties of all samples. Sample code
ρ (g·cm−3)
Vm (cm3·mol−1)
αe (Å3)
N × 1020 (ion·cm−3)
Rm (cm3)
Vt
S1 S2 S3 S4 S5
3.15 3.22 3.23 3.40 3.42
32.88 31.93 31.67 29.89 29.57
7.57 7.48 7.42 7.00 6.93
0.00 1.83 3.80 6.04 8.14
19.09 18.87 18.71 17.67 17.48
0.54 0.55 0.56 0.59 0.60
Fig. 11. Ionic packing density against TiO2 NP concentration.
and 0.54–0.60, respectively. Incorporation of TiO2 NPs is demonstrated to be effective in modifying the absorption and physical properties of titania doped magnesium zinc phosphate glass.
Acknowledgments
Moreover, Ti ions fully filled most of the excess space volume and enhance the glass compactness.
The authors gratefully acknowledge the financial support from the Ministry of Education, Malaysia, and Universiti Teknologi Malaysia, through research Vot 4L032 (ERGS), 05H36 (GUP), 4F083 and 4F424 (FRGS).
4. Conclusion
References
Fig. 9. TiO2 NP concentration dependent glass density and molar volume.
A series of ZnO–MgO–P2O5 glasses with varying concentration of embedded TiO2 NPs are synthesized via melt-quenching technique. The TiO2 NP concentration dependent physical and absorption features are determined. Prepared samples are confirmed to be amorphous in nature with dispersed NPs of average size ≈5.78 nm. The indirect and direct optical band gaps are varied in the range of 3.56–3.32 eV and 3.71–3.55 eV, respectively. Urbach energy is decreased from 2.83–0.15 eV and the refractive index is increased from 2.27 to 2.32 with the increase of TiO2 NP contents. Furthermore, the physical parameters such as glass density, molar volume, molar refractivity, polarizability and ionic packing density are found to be in the range of 3.15– 3.42 g·cm−3, 32.88–29.57 cm3·mol−1, 19.09–17.48 cm3, 7.57–6.93 Å3,
Fig. 10. TiO2 NP concentration dependent electronic polarizability of the glasses.
[1] S. Khor, Z. Talib, W.M. Yunus, Optical properties of ternary zinc magnesium phosphate glasses, Ceram. Int. 38 (2012) 935–940. [2] C.E. Smith, R.K. Brow, The properties and structure of zinc magnesium phosphate glasses, J. Non-Cryst. Solids 390 (2014) 51–58. [3] S. Chawla, K. Jayanthi, H. Chander, D. Haranath, S. Halder, M. Kar, Synthesis and optical properties of ZnO/MgO nanocomposite, J. Alloys Compd. 459 (2008) 457–460. [4] R.B. Rao, D.K. Rao, N. Veeraiah, The role of titanium ions on structural, dielectric and optical properties of Li2O–MgO–B2O3 glass system, Mater. Chem. Phys. 87 (2004) 357–369. [5] N. Bayal, P. Jeevanandam, Synthesis of TiO2–MgO mixed metal oxide nanoparticles via a sol–gel method and studies on their optical properties, Ceram. Int. 40 (2014) 15463–15477. [6] X. Zhao, Q. Zhao, J. Yu, B. Liu, Development of multifunctional photoactive selfcleaning glasses, J. Non-Cryst. Solids 354 (2008) 1424–1430. [7] J. Wang, C. Lu, J. Xiong, Self-cleaning and depollution of fiber reinforced cement materials modified by neutral TiO2/SiO2 hydrosol photoactive coatings Appl, Surf. Sci. 298 (2014) 19–25. [8] N. Shimoji, T. Hashimoto, H. Nasu, K. Kamiya, Non-linear optical properties of Li2O–TiO2–P2O5 glasses, J. Non-Cryst. Solids 324 (2003) 50–57. [9] A. Shaim, M. Et-Tabirou, Role of titanium in sodium titanophosphate glasses and a model of structural units, Mater. Chem. Phys. 80 (2003) 63–67. [10] R.K. Brow, Review: the structure of simple phosphate glasses J, Non-Cryst. Solids 263 (2000) 1–28. [11] M.S. Reddy, M.R. Reddy, M. Nagarjuna, P.R. Rao, Influence of titanium ions on spectroscopic and dielectric properties of PbO–Bi2O3–As2O3 glasses, IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2009. [12] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229. [13] T. Suhasini, J.S. Kumar, T. Sasikala, K. Jang, H.S. Lee, M. Jayasimhadri, J.H. Jeong, S.S. Yi, L.R. Moorthy, Absorption and fluorescence properties of Sm3+ ions in fluoride containing phosphate glasses, Opt. Mater. 31 (2009) 1167–1172. [14] E. Davis, N. Mott, Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors, Philos. Mag. 22 (1970) 0903–0922. [15] R. Shannon, C.T. Prewiit, Effective ionic radii in oxides and fluorides, Acta Crystallogr. B 25 (1969) 925–946. [16] N. Zarifah, M. Halimah, M. Hashim, B. Azmi, W. Daud, Magnetic behaviour of (Fe2O3) × (TeO2)1 − x glass system due to iron oxide, Chalcogenide Lett. 7 (2010) 565–571. [17] R. Dhanalakshmi, A. Pandikumar, R. Ramaraj, Functionalized silicate supported TiO2–ZnO nanocomposite film and its application in simultaneous photocatalytic degradation of toxic molecules, Materials Science Forum, Trans Tech Publ., 2013.
182
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
[18] M. Yu, Z. Wang, H. Liu, S. Xie, J. Wu, H. Jiang, J. Zhang, L. Li, J. Li, Laundering durability of photocatalyzed self-cleaning cotton fabric with TiO2 nanoparticles covalently immobilized, ACS Appl. Mater. Interfaces 5 (2013) 3697–3703. [19] L. Yao, J. He, Recent progress in antireflection and self-cleaning technology—from surface engineering to functional surfaces, Prog. Mater. Sci. 61 (2014) 94–143. [20] N. Yusoff, M. Sahar, S. Ghoshal, Sm3+: Ag NPs assisted modification in absorption features of magnesium tellurite glass, J. Mol. Struct. 1079 (2015) 167–172. [21] S. Pourteimoor, M.E. Azim-Araghi, Optical studies of TiO2–lead phthalocyanine nanocomposite thin films prepared by electron beam evaporation, Mater. Sci. Semicond. Process. 18 (2014) 97–104. [22] M. Abdel-Baki, F. El-Diasty, Optical properties of oxide glasses containing transition metals: case of titanium-and chromium-containing glasses, Curr. Opinion Solid State Mater. Sci. 10 (2006) 217–229. [23] G.N. Raju, Y. Gandhi, N.S. Rao, N. Veeraiah, Study on the influence of TiO2 on the insulating strength of ZnO–ZnF2–B2O3 glasses by means of dielectric properties, Solid State Commun. 139 (2006) 64–69. [24] Y.S.M. Alajerami, S. Hashim, W.M.S.W. Hassan, A.T. Ramli, The effect of titanium oxide on the optical properties of lithium potassium borate glass, J. Mol. Struct. 1026 (2012) 159–167. [25] N.N. Rao, I. Kityk, V.R. Kumar, C.S. Rao, M. Piasecki, P. Bragiel, N. Veeraiah, Dc field induced optical effects in ZnF2–PbO–TeO2: TiO2 glass ceramics, Ceram. Int. 38 (2012) 2551–2562.
[26] T. Satyanarayana, I. Kityk, K. Ozga, M. Piasecki, P. Bragiel, M. Brik, V.R. Kumar, A. Reshak, N. Veeraiah, Role of titanium valence states in optical and electronic features of PbO–Sb2O3–B2O3: TiO2 glass alloys, J. Alloys Compd. 482 (2009) 283–297. [27] W. Stambouli, H. Elhouichet, M. Ferid, Study of thermal, structural and optical properties of tellurite glass with different TiO2 composition, J. Mol. Struct. 1028 (2012) 39–43. [28] W. Widanarto, M. Sahar, S. Ghoshal, R. Arifin, M. Rohani, K. Hamzah, M. Jandra, Natural Fe3O4 nanoparticles embedded zinc–tellurite glasses: polarizability and optical properties, Mater. Chem. Phys. 138 (2013) 174–178. [29] W. Widanarto, M. Sahar, S. Ghoshal, R. Arifin, M. Rohani, K. Hamzah, Effect of natural Fe3O4 nanoparticles on structural and optical properties of Er3+ doped tellurite glass, J. Magn. Magn. Mater. 326 (2013) 123–128. [30] J.-C. Sabadel, P. Armand, P.-E. Lippens, D. Cachau-Herreillat, E. Philippot, Mössbauer and XANES of TeO2–BaO–TiO2 glasses, J. Non-Cryst. Solids 244 (1999) 143–150. [31] B. Raghavaiah, C. Laxmikanth, N. Veeraiah, Spectroscopic studies of titanium ions in PbO–Sb2O3–As2O3 glass system, Opt. Commun. 235 (2004) 341–349. [32] Y.B. Saddeek, Elastic properties of Gd3+-doped tellurovanadate glasses using pulseecho technique, Mater. Chem. Phys. 91 (2005) 146–153. [33] V. Dimitrov, T. Komatsu, Classification of oxide glasses: a polarizability approach, J. Solid State Chem. 178 (2005) 831–846. [34] M. Azlan, M. Halimah, S. Shafinas, W. Daud, Polarizability and optical basicity of Er3+ ions doped tellurite based glasses, Chalcogenide Lett. 11 (2014) 319–335.
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Physical and absorption properties of titanium nanoparticles incorporated into zinc magnesium phosphate glass S.F. Ismail, M.R. Sahar ⁎, S.K. Ghoshal Physics Department, Advanced Optical Materials Research Group, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
a r t i c l e
i n f o
Article history: Received 7 July 2015 Received in revised form 27 November 2015 Accepted 30 November 2015 Available online 02 December 2015 Keywords: Amorphous state Optical band gap Physical properties Titanium nanoparticles
a b s t r a c t We report the influences of Titania (TiO2) nanoparticles (NPs) on the physical and optical properties of melt quench synthesized zinc magnesium phosphate glasses. Five glass samples with composition (42 − x)P2O5– 50ZnO–8MgO–xTiO2, where x = 0, 1, 2, 3, 4 mol% are prepared and characterized. XRD pattern verified the amorphous nature of all samples. TEM images manifested the growth of Ti NPs of average size ≈ 5.78 nm. TiO2 NP concentration dependent variation in the physical properties including glass density, molar volume, molar refractivity, electronic polarizability and ionic packing density are determined. The values of glass refractive indices, density and ionic packing density are increased with the increase of TiO2 NP contents. Conversely, the Urbach energy, direct and indirect optical band gap are found to decrease with the increase of TiO2 NP concentration. These glass compositions may be potential for various solid state devices including laser. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Certainly, phosphate glasses are prospective over conventional silicate and borate systems due to their unique physical properties such as high temperatures expansion coefficient, low melting and transition temperatures, lower indices of refraction and prominent ultra-violet transmission. Recently, intensive research on ZnO–P2O5, MgO–P2O5 and ZnO–MgO–P2O5 glasses displaying their superior structural, physical, thermal and optical properties made them potential for diverse device applications [1–3]. Khor and co-workers [1] examined the physical and optical properties of simultaneous admixture of MgO and ZnO into the phosphate compositions and demonstrated their strong influenced on overall properties. Lately, TiO2 NPs received special attention due to its several advantages properties and potential benefits including low cost, chemical stability, non-toxicity and, high reactivity under UV light irradiation [4]. Moreover, TiO2 is a wide band gap (≈3.2 eV) semiconductor [5,6]. Generally, TiO2 NPs are widely used for photovoltaic cells, electro-chromic materials, photo-catalysis, self-cleaning glass and waste water treatment [5,7]. The reconstruction of the TiO2 occurs upon prolonged heating, where the titanium ions usually exist in Ti4 + state. These Ti4+ ions form TiO4, TiO6 and rarely TiO5 (comprising of trigonal bipyramids) structural units in the glass network [8,9]. However, some reports acknowledged the existence of Ti3+ valence state in the glass matrices [10,11]. According to Diebold [12], slightly defective TiO2 surface reveals weak feature due to the presence of small amounts of Ti3+ defects when heated to 1000 K. ⁎ Corresponding author. E-mail address: [email protected] (M.R. Sahar).
http://dx.doi.org/10.1016/j.matchar.2015.11.030 1044-5803/© 2015 Elsevier Inc. All rights reserved.
The goal of this study is to examine the effects of varying concentrations of TiO2 NPs on the physical and optical properties of zinc magnesium phosphate amorphous matrix. Ti NP mediated emergent physical and optical properties of ZnO–MgO–P2O5 glass system are inspected. Results are analyzed, compared and discussed.
2. Experimental The nominal glass compositions with their codes are listed in Table 1. About 20 g batch with analytical grade glass constituents of P2O5 (purity 98.94%), ZnO (purity 99%), MgO (99%) and TiO2 (purity 99.7%) in powder form are well-mixed and melted in alumina crucibles at 1100 °C for 2 h in an electrical furnace so that a homogeneously mixed melt is obtained. The melt is then transferred to an annealing furnace and poured on the brass mold before being annealed at 400 °C for 3 h to reduce the mechanical stress that causes embrittlement [13]. The vitreous/amorphous state of the prepared samples is examined by X-ray diffraction (XRD) measurement on a Siemen Diffractometer that used Cu-Kα (λ = 1.54 Å) as radiation source. Relatively fine glass powder is placed in a sample holder and scanned from 2θ = 10–80° with a step of 0.50° at running voltage of 30 kV and current 20 mA. The presence of NPs in the glass matrix is detected using a transmission electron microscope (TEM, Philips CM12), which operated at 200 kV with Dock version 3.2 image analyzer. Specimens for TEM are prepared by dispersing the powder in acetone via ultrasonic bath. The solution is then placed onto copper grid and allowed to dry before got ready for characterization. TEM images are analyzed to determine the NP sizes, shapes, and distribution in the glass matrix. In addition, the Energy Dispersive of X-Ray (EDX) analysis is used to analyze the sample composition.
178
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
Table 1 Chemical compositions of the synthesized glasses with codes. Chemical composition (mol.%) Sample code S1 S2 S3 S4 S5
P2O5
MgO
ZnO
TiO2
42 41 40 39 38
8 8 8 8 8
50 50 50 50 50
0 1 2 3 4
Room temperature UV–Vis–NIR measurements in the wavelength range of 200–700 nm (Shimadzu 3101PC UV–Vis–NIR scanning spectrophotometer) are carried out on well-polished (up to 1200 μm grid size using diamond paste), highly transparent and shiny glass samples of thickness 2 mm. The absorption spectra are further exploited to evaluate optical band gaps, Urbach energies, and refractive indices of all prepared samples. The photon energy (hv) dependent optical absorption coefficient, α(ν) near the absorption band edge yields, αðvÞ ¼
2:303A d
ð1Þ
where A is the absorbance and d is the thickness of the sample. Following Davis and Mott [14], for amorphous and semiconducting materials the values of α(v) as a function of photon energy can be calculated from, αðvÞ ¼
n C hv−Eopt hv
ð2Þ
where C is a constant, Eopt is the optical band gap energy with n = 2 for indirect transition and n = 1/2 for direct transition. The Urbach energy (ΔE) that characterizes the extent of the exponential tail of the absorption edge can be written as hv αðvÞ ¼ B exp ΔE
ð3Þ
where B is a constant. The Urbach energy measures the width of the band tails of the localized states. The refractive index (n) is related to the indirect optical band gap energy EIopt via sffiffiffiffiffiffiffiffi EIopt n2 −1 : ¼ 1− 2 20 n þ2
ð4Þ
Fig. 2. (a) TEM image of S5; (b) size distribution of TiO2 NPs in S5.
The density (ρ in g/cm3) of glass samples is determined via Archimedes method with toluene (insoluble in water and does not react with glass samples) as immersion liquid. The estimated error in density measurement is ≈ ± 0.001 g/cm3. The sample weight in the air and within the immersion liquid is measured using a digital balance. Glass density is calculated using, ρ ¼ ρT
WA WA −WL
ð5Þ
where ρT is the density of toluene (0.8669 g/cm3), WA and WL are the weight of the sample in the air and liquid respectively. The molar volume (Vm in cm3/mol) is calculated from Vm ¼
X ni mi ρ
ð6Þ
where ni and mi denote the molar fraction and molecular weight of ith component, respectively. The molar refractivity (Rm), the electronic polarizability (αe) and the ionic packing density (Vt) are determined from the value of molar volume using the following relations: Rm ¼
n2 −1 ðV m Þ n2 þ 2
ð7Þ
3 Rm 4πNa
ð8Þ
X V t ¼ ð4=3πÞ ðr 3i ni Þ=V m
ð9Þ
αm ¼
i
Fig. 1. Typical XRD pattern for S1, S3 and S5 glass samples.
N ¼ np Na ρ =M
ð10Þ
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
179
Fig. 3. EDX spectrum of S5 glass sample.
where ri is the ionic radii of the ith component, np is the mol% of TiO2, Na is the Avogadro's number and M is total molecular weight. Ionic radius of Shannon and Prewitt [15] is used for Vt calculation. 3. Results and discussions The XRD pattern of the prepared samples (Fig. 1) being devoid of any sharp Bragg peaks shows lack of crystalline order. Furthermore, the presence of a broad hump in the range of 20°–30° confirms their amorphous nature [16]. Conversely, the existences of NPs hardly detected by XRD due to their fairly low concentration compared to that of host and modifier contents. The typical TEM image of the sample as shown in Fig. 2(a) clearly reveals the presence of spherical and non-spherical shaped TiO2 NPs (black spots) in the glass matrix. The NP size distribution is found to be Gaussian with the average diameter ≈5.78 ± 0.1 nm (Fig. 2(b)). Fig. 3 shows the EDX spectrum for S5 and it confirmed the existence of Ti NPs as observed in the TEM image. The analysis for the composition has been done and the result summarized in Table 2. As can see that nominal and actual result for sample S5 are in good conformity. The room temperature UV–Vis–NIR spectra of the samples in the range of 200–700 nm as displayed in Fig. 4 clearly manifest the strong absorption from TiO2 NPs with characteristic UV wavelength [17]. The intensity of the surface plasmon resonance (SPR) peaks evidenced in between 208 and 324 nm are enhanced with the increase of TiO2 NP concentration accompanied by red shifts. This prominent absorption is ascribed to the transition from the valence to the conduction band of TiO2 NPs. It is worth noting that TiO2 being an ideal photo-catalyst in decomposing organic contamination are suitable for environmental applications. It is well known that TiO2 has strong oxidation power under UV illumination. Additionally, the surface of titanium nanoparticles embedded zinc magnesium phosphate glass becomes hydrophilic or hydrophobic with a water contact angle less or more than 90°, making the surface easily self-cleaned [18,19]. These two characteristics of TiO2 glass are responsible for achieving outstanding self-cleaning property when exposed outdoor. Using the absorption edges, optical band gap energy for indirect and direct transition (Table 3) are evaluated via Tauc plot [20–23]. From Fig. 5(a) and (b) the value of Eopt for indirect and direct transition are obtained by extrapolating the linear part of the curve to the x-axis = 0. Fig. 6 shows the reduction in Eopt values for indirect 3.56–3.32 eV and direct 3.71–3.55 eV with the increase of TiO2 NP contents. This reduction in Eopt is ascribed to the increasing number of
non-bridging oxygen (NBO). Consequently, the electrons localization states in the bang gap are minimized [20,24]. The increase in the degree of localization of electrons causes an enhancement of the donor centers in the glass network. The emergence of higher concentration of these donor centers reduced the optical band gap and shifts the absorption edge gradually towards higher wavelength [25,26]. Therefore, more electrons can easily be transferred from the valence band to the conduction band. In the amorphous materials, the lack of crystalline long-range order and the presence of disorder dominate the phonon assisted processes associated with the tailing of the density of states into the normally forbidden energy band gap [21,22,27]. The Urbach energy (Table 3) being a measure of the width of band tails of localized states is used to characterize the degree of disorder in amorphous and crystalline systems. The Urbach energy as depicted Fig. 7 is found to decrease with the increase of TiO2 NP contents, which is in good agreement with earlier observation [28,29]. This decrease is primarily due to the reduction of the width of the localized density of states in the forbidden gap. In fact, the density of electron with higher energy is reduced because of the enhanced electronic transition from the valence band to the conduction band. The glasses with the smaller Urbach energy would have greater tendency to minimize the static disorder in the glass structure. As illustrated in Fig. 8, the refractive index displayed an increase with increasing TiO2 NP concentration. The higher values of glass refractive index reflect smaller band gap and more compact or regular network structure. This is due to the generation of less defect and strengthening of covalency. The enhancement in the refractive index with the increase of TiO2 NP contents is related to the formation of
Table 2 Nominal/actual composition of 38P2O5–50ZnO–8MgO–4TiO2 (S5).
Nominal Actual
P2O5
ZnO
MgO
TiO2
38 50.38
50 40.68
8 6.40
4 2.58
Fig. 4. Absorbance spectra all glasses.
180
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
Table 3 Indirect and direct optical band gap energy, Urbach energy and refractive index of studies samples. Sample code
EIopt (eV)
ED opt (eV)
ΔE (eV)
n
S1 S2 S3 S4 S5
3.56 3.40 3.38 3.35 3.32
3.71 3.60 3.59 3.56 3.55
2.83 0.23 0.19 0.16 0.15
2.27 2.29 2.30 2.31 2.32
Fig. 7. Urbach energy of glasses versus TiO2 concentration.
Fig. 5. A typical Tau plot for (a) indirect and (b) direct optical band gap for S2, S3, S4 and S5.
more Ti4+ ions in the glass networks [27,30]. Consequently, the increase in the number NBO tends to make the glass more compact. Furthermore, the empty d-shell of Ti4+ metal ions contributes to large increase of the linear and non-linear indices of the glasses. Table 4 enlists the TiO2 NP concentration dependent various physical parameters. Fig. 9 depicts the variation of density and molar volume as a function of TiO2 NP concentration. The observed inverse relationship between density and molar volume is also reported by Raghavaiah et al. and Satyanarayana et al. [26,31]. The increase in glass density with the increase of TiO2 NP contents modifies the glass network rigidity. This is majorly due to the larger relative masses and ionic radius of titanium ions than the phosphate ions in the glass host [20,32]. According to Shannon, ionic radii for Ti4+ and P5+ ions are 0.605 and 0.29 Å, respectively [15]. The decrease in free volume and the inter-atomic distances also increases the rigidity of the glass. Fig. 10 shows the effects of TiO2 NP concentrations on the polarizability of the glass. Dimitrov et al. [33] acknowledged that glasses with higher refractive index possess large polarizability, high optical basicity, small metallization criterion and large third-order non-linear optical susceptibility. The observed decrease in electronic polarizability with the increase of TiO2 concentration may be due to the dual nature of ZnO that acts as glass modifier as well as former [28,34]. Besides, the lower polarizability improves the glass chemical durability. The influence of TiO2 NPs on the ionic packing density Vt is presented in Fig. 11. The ionic packing density is increased with the increase of TiO2 NP contents. This is due to the higher ionic radii of Ti atoms, which fits with greater probability to the free space of excess volume.
Fig. 6. Indirect and direct optical band gap energy versus TiO2 NP concentration.
Fig. 8. TiO2 NP concentration dependent glass refractive index.
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
181
Table 4 Physical properties of all samples. Sample code
ρ (g·cm−3)
Vm (cm3·mol−1)
αe (Å3)
N × 1020 (ion·cm−3)
Rm (cm3)
Vt
S1 S2 S3 S4 S5
3.15 3.22 3.23 3.40 3.42
32.88 31.93 31.67 29.89 29.57
7.57 7.48 7.42 7.00 6.93
0.00 1.83 3.80 6.04 8.14
19.09 18.87 18.71 17.67 17.48
0.54 0.55 0.56 0.59 0.60
Fig. 11. Ionic packing density against TiO2 NP concentration.
and 0.54–0.60, respectively. Incorporation of TiO2 NPs is demonstrated to be effective in modifying the absorption and physical properties of titania doped magnesium zinc phosphate glass.
Acknowledgments
Moreover, Ti ions fully filled most of the excess space volume and enhance the glass compactness.
The authors gratefully acknowledge the financial support from the Ministry of Education, Malaysia, and Universiti Teknologi Malaysia, through research Vot 4L032 (ERGS), 05H36 (GUP), 4F083 and 4F424 (FRGS).
4. Conclusion
References
Fig. 9. TiO2 NP concentration dependent glass density and molar volume.
A series of ZnO–MgO–P2O5 glasses with varying concentration of embedded TiO2 NPs are synthesized via melt-quenching technique. The TiO2 NP concentration dependent physical and absorption features are determined. Prepared samples are confirmed to be amorphous in nature with dispersed NPs of average size ≈5.78 nm. The indirect and direct optical band gaps are varied in the range of 3.56–3.32 eV and 3.71–3.55 eV, respectively. Urbach energy is decreased from 2.83–0.15 eV and the refractive index is increased from 2.27 to 2.32 with the increase of TiO2 NP contents. Furthermore, the physical parameters such as glass density, molar volume, molar refractivity, polarizability and ionic packing density are found to be in the range of 3.15– 3.42 g·cm−3, 32.88–29.57 cm3·mol−1, 19.09–17.48 cm3, 7.57–6.93 Å3,
Fig. 10. TiO2 NP concentration dependent electronic polarizability of the glasses.
[1] S. Khor, Z. Talib, W.M. Yunus, Optical properties of ternary zinc magnesium phosphate glasses, Ceram. Int. 38 (2012) 935–940. [2] C.E. Smith, R.K. Brow, The properties and structure of zinc magnesium phosphate glasses, J. Non-Cryst. Solids 390 (2014) 51–58. [3] S. Chawla, K. Jayanthi, H. Chander, D. Haranath, S. Halder, M. Kar, Synthesis and optical properties of ZnO/MgO nanocomposite, J. Alloys Compd. 459 (2008) 457–460. [4] R.B. Rao, D.K. Rao, N. Veeraiah, The role of titanium ions on structural, dielectric and optical properties of Li2O–MgO–B2O3 glass system, Mater. Chem. Phys. 87 (2004) 357–369. [5] N. Bayal, P. Jeevanandam, Synthesis of TiO2–MgO mixed metal oxide nanoparticles via a sol–gel method and studies on their optical properties, Ceram. Int. 40 (2014) 15463–15477. [6] X. Zhao, Q. Zhao, J. Yu, B. Liu, Development of multifunctional photoactive selfcleaning glasses, J. Non-Cryst. Solids 354 (2008) 1424–1430. [7] J. Wang, C. Lu, J. Xiong, Self-cleaning and depollution of fiber reinforced cement materials modified by neutral TiO2/SiO2 hydrosol photoactive coatings Appl, Surf. Sci. 298 (2014) 19–25. [8] N. Shimoji, T. Hashimoto, H. Nasu, K. Kamiya, Non-linear optical properties of Li2O–TiO2–P2O5 glasses, J. Non-Cryst. Solids 324 (2003) 50–57. [9] A. Shaim, M. Et-Tabirou, Role of titanium in sodium titanophosphate glasses and a model of structural units, Mater. Chem. Phys. 80 (2003) 63–67. [10] R.K. Brow, Review: the structure of simple phosphate glasses J, Non-Cryst. Solids 263 (2000) 1–28. [11] M.S. Reddy, M.R. Reddy, M. Nagarjuna, P.R. Rao, Influence of titanium ions on spectroscopic and dielectric properties of PbO–Bi2O3–As2O3 glasses, IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2009. [12] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229. [13] T. Suhasini, J.S. Kumar, T. Sasikala, K. Jang, H.S. Lee, M. Jayasimhadri, J.H. Jeong, S.S. Yi, L.R. Moorthy, Absorption and fluorescence properties of Sm3+ ions in fluoride containing phosphate glasses, Opt. Mater. 31 (2009) 1167–1172. [14] E. Davis, N. Mott, Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors, Philos. Mag. 22 (1970) 0903–0922. [15] R. Shannon, C.T. Prewiit, Effective ionic radii in oxides and fluorides, Acta Crystallogr. B 25 (1969) 925–946. [16] N. Zarifah, M. Halimah, M. Hashim, B. Azmi, W. Daud, Magnetic behaviour of (Fe2O3) × (TeO2)1 − x glass system due to iron oxide, Chalcogenide Lett. 7 (2010) 565–571. [17] R. Dhanalakshmi, A. Pandikumar, R. Ramaraj, Functionalized silicate supported TiO2–ZnO nanocomposite film and its application in simultaneous photocatalytic degradation of toxic molecules, Materials Science Forum, Trans Tech Publ., 2013.
182
S.F. Ismail et al. / Materials Characterization 111 (2016) 177–182
[18] M. Yu, Z. Wang, H. Liu, S. Xie, J. Wu, H. Jiang, J. Zhang, L. Li, J. Li, Laundering durability of photocatalyzed self-cleaning cotton fabric with TiO2 nanoparticles covalently immobilized, ACS Appl. Mater. Interfaces 5 (2013) 3697–3703. [19] L. Yao, J. He, Recent progress in antireflection and self-cleaning technology—from surface engineering to functional surfaces, Prog. Mater. Sci. 61 (2014) 94–143. [20] N. Yusoff, M. Sahar, S. Ghoshal, Sm3+: Ag NPs assisted modification in absorption features of magnesium tellurite glass, J. Mol. Struct. 1079 (2015) 167–172. [21] S. Pourteimoor, M.E. Azim-Araghi, Optical studies of TiO2–lead phthalocyanine nanocomposite thin films prepared by electron beam evaporation, Mater. Sci. Semicond. Process. 18 (2014) 97–104. [22] M. Abdel-Baki, F. El-Diasty, Optical properties of oxide glasses containing transition metals: case of titanium-and chromium-containing glasses, Curr. Opinion Solid State Mater. Sci. 10 (2006) 217–229. [23] G.N. Raju, Y. Gandhi, N.S. Rao, N. Veeraiah, Study on the influence of TiO2 on the insulating strength of ZnO–ZnF2–B2O3 glasses by means of dielectric properties, Solid State Commun. 139 (2006) 64–69. [24] Y.S.M. Alajerami, S. Hashim, W.M.S.W. Hassan, A.T. Ramli, The effect of titanium oxide on the optical properties of lithium potassium borate glass, J. Mol. Struct. 1026 (2012) 159–167. [25] N.N. Rao, I. Kityk, V.R. Kumar, C.S. Rao, M. Piasecki, P. Bragiel, N. Veeraiah, Dc field induced optical effects in ZnF2–PbO–TeO2: TiO2 glass ceramics, Ceram. Int. 38 (2012) 2551–2562.
[26] T. Satyanarayana, I. Kityk, K. Ozga, M. Piasecki, P. Bragiel, M. Brik, V.R. Kumar, A. Reshak, N. Veeraiah, Role of titanium valence states in optical and electronic features of PbO–Sb2O3–B2O3: TiO2 glass alloys, J. Alloys Compd. 482 (2009) 283–297. [27] W. Stambouli, H. Elhouichet, M. Ferid, Study of thermal, structural and optical properties of tellurite glass with different TiO2 composition, J. Mol. Struct. 1028 (2012) 39–43. [28] W. Widanarto, M. Sahar, S. Ghoshal, R. Arifin, M. Rohani, K. Hamzah, M. Jandra, Natural Fe3O4 nanoparticles embedded zinc–tellurite glasses: polarizability and optical properties, Mater. Chem. Phys. 138 (2013) 174–178. [29] W. Widanarto, M. Sahar, S. Ghoshal, R. Arifin, M. Rohani, K. Hamzah, Effect of natural Fe3O4 nanoparticles on structural and optical properties of Er3+ doped tellurite glass, J. Magn. Magn. Mater. 326 (2013) 123–128. [30] J.-C. Sabadel, P. Armand, P.-E. Lippens, D. Cachau-Herreillat, E. Philippot, Mössbauer and XANES of TeO2–BaO–TiO2 glasses, J. Non-Cryst. Solids 244 (1999) 143–150. [31] B. Raghavaiah, C. Laxmikanth, N. Veeraiah, Spectroscopic studies of titanium ions in PbO–Sb2O3–As2O3 glass system, Opt. Commun. 235 (2004) 341–349. [32] Y.B. Saddeek, Elastic properties of Gd3+-doped tellurovanadate glasses using pulseecho technique, Mater. Chem. Phys. 91 (2005) 146–153. [33] V. Dimitrov, T. Komatsu, Classification of oxide glasses: a polarizability approach, J. Solid State Chem. 178 (2005) 831–846. [34] M. Azlan, M. Halimah, S. Shafinas, W. Daud, Polarizability and optical basicity of Er3+ ions doped tellurite based glasses, Chalcogenide Lett. 11 (2014) 319–335.