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Effect of calcination temperature on phase transformation, structural and optical properties of sol–gel derived ZrO2 nanostructures
Physica E 66 (2015) 74–80
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Physica E journal homepage: www.elsevier.com/locate/physe
Effect of calcination temperature on phase transformation, structural and optical properties of sol–gel derived ZrO2 nanostructures Sachin Kumar, Snehasis Bhunia, Animesh K. Ojha n Department of Physics, Motilal Nehru National Institute of Technology, Allahabad, Allahabad 211004, India
H I G H L I G H T S
Shapes and sizes of ZrO2 nanostructures were modified with calcinations temperature. A rod like shape is observed for the ZrO2 calcined at 700 °C ZrO2. Tetragonal to monoclinic phase transition was observed at 600 °C. DFT calculations revealed that monoclinic phase is energetically more stable. The optical band gap is changed due to change in shape and size of ZrO2 nanostructures.
ar t ic l e i nf o
a b s t r a c t
Article history: Received 27 July 2014 Received in revised form 3 September 2014 Accepted 8 September 2014 Available online 8 October 2014
Zirconia (ZrO2) nanostructures of various sizes have been synthesized using sol–gel method followed by calcination of the samples from 500 to 700 °C. The calcined ZrO2 powder samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infra-red spectroscopy (FT-IR), UV–visible spectroscopy (UV–vis.), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). The phase transformation from tetragonal (t) to monoclinic (m) was observed. The average diameter of the ZrO2 nanostructures calcined at 500, 600 and 700 °C was calculated to be 8, 17 and 10 nm, respectively. The ZrO2 sample calcined at 500 °C with tetragonal phase shows a direct optical band gap of 5.1 eV. The value of optical band gap is decreased to 4.3 eV for the ZrO2 calcined at 600 °C, which contains both tetragonal (73%) and monoclinic (27%) phases. On further calcination at 700 °C, where the ZrO2 nanostructures have 36% tetragonal and 64% monoclinic phases, the optical band gap is calculated to be 4.8 eV. The enhancement in optical band gap for ZrO2 calcined at 700 °C may be due to the rod like shape of ZrO2 nanostructures. The tetragonal to monoclinic phase transformation was also confirmed by analyzing Raman spectroscopic data. The TG analysis revealed that the ZrO2 nanostructure with dominance of monoclinic phase is found to be more stable over the tetragonal phase. In order to confirm the phase stability of the two phases of ZrO2, single point energy is calculated corresponding to its monoclinic and tetragonal structures using density functional theory (DFT) calculations. The results obtained by theoretical calculations are in good agreement with the experimental findings. & Elsevier B.V. All rights reserved.
Keywords: ZrO2 nanostructures Sol–gel Optical properties DFT
1. Introduction Nanomaterials have sparked a great interest to the scientific community due the fact that it acquired unique chemical and physical properties at nano scale and these properties change drastically with their size and shape [1]. ZrO2 is one of the interesting oxide materials as it has wide range of optical applications in different fields such as; photonics, interferometry filter and for coating high power laser mirrors due to its hardness, n
Corresponding author. Fax: þ91 532 2545341. E-mail addresses: [email protected], [email protected] (A.K. Ojha).
http://dx.doi.org/10.1016/j.physe.2014.09.007 1386-9477/& Elsevier B.V. All rights reserved.
optical transparency and high refractive index [2]. It has excellent thermal application in thermal barrier coating [3]. The bulk form of ZrO2 has three polymorphs, cubic (c-ZrO2), tetragonal (t-ZrO2) and monoclinic (m-ZrO2) phases. At very high temperature, greater than 2370 °C, the ZrO2 has cubic phase [4]. The tetragonal phase of ZrO2 has been observed for temperature range, 2370–1170 °C [4]. However, at the temperature below to 1170 °C, ZrO2 exist in monoclinic phase [4]. On adding the metal oxide such as; MgO, CaO, and Y2O3, the transformation temperature of ZrO2 into different crystalline phases can be lowered [4]. In previous studies [5,6], it was found that the transformation from tetragonal to monoclinic phase causes a large volume expansion (4%) and
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extensive cracking in the material which leads to high fracture toughness. Various synthesis methods such as; co-precipitation, citrate technique, hydrothermal, electrodeposition, solvothermal and sol– gel [7] have been used to synthesize ZrO2 nanostructures. Among these synthesis methods, the sol–gel technique has received increasing attention by the research community in the recent years since it provides versatility and excellent control over the structure, shape and size and consequently it also provide a good control over the chemical, physical and optical properties of the synthesized materials. The sol–gel method of synthesis includes four steps: hydrolysis, polycondensation, drying, and thermal decomposition. The metallic precursor also plays a significant role for obtaining the crystalline phase by sol–gel method [8]. In case of ZrO2 nanostructures synthesized by sol–gel method, the crystal structure strongly depends on calcinations temperature which has a great influence on optical, structural and thermal properties of synthesized materials. In recent years, a large number of studies have been done to explore the effect of calcinations temperatures on structural, optical and thermal properties of ZrO2 nanostructures [9–15]. Liu et al. [10] had studied the annealing and doping effect on structure and optical properties of ZrO2 thin films. They focussed on the changes in refractive indices of the films with varying the calcinations temperature and doping of the rare earth elements. They found that the value of refractive index increases with doping of rare earth element in ZrO2 thin film. Very recently, Ashraf et al. [13] had studied annealing effect on photoluminescence properties of ZrO2 at nanoscale and submicron scale. They explained the photoluminescence behavior in visible and UV range of the ZrO2 nanostructures in terms of abundant oxygen vacancies as function of annealing temperatures. In another study Goharshadi et al. [14] had studied the effect of calcination temperature on vibrational, optical, and rheological properties of ZrO2 nanostructures. In view of the above studies on role of calcinations temperature on structural, thermal, electrical and optical properties of ZrO2 nanostructures, there is a lack of extensive investigations on size and shape dependent variation in optical band gap of ZrO2 nanostructures calcined at different temperatures. The calcination temperature modifies crystalline phases, shape and size of the ZrO2 nanostructures. The change in these parameters of the nanostructured ZrO2 may be responsible for modifications in various structural, optical and thermal properties of ZrO2 nanostructures. Therefore, it would be great interest and significance to investigate the role of crystalline phase, size and shape of the ZrO2 nanostructures on optical band gap and thermal stability. Thus, in the present work we have synthesized ZrO2 nanostructures using simple and low cost synthesis method and calcined them at 500, 600 and 700 °C to investigate the effect of crystalline phase, size and shape on its structural, optical, and thermal properties. XRD, TEM, RS, UV–vis., and TGA characterization techniques were employed to characterize the synthesized samples.
2. Experimental details 2.1. Synthesis ZrO2 nanostructures were synthesized using a very simple and low cost sol–gel method. ZrOCl2 8H2O and ammonia solution 25% (0.91) were purchased from SRL Pvt. Ltd., India and the chemicals have been used without further purification. An appropriate amount of ZrOCl2 8H2O was dissolved in distilled water and stirred vigorously for one hour. Thereafter, ammonia solution was added into the solution drop wise while stirring until the pH value of the solution is reached to 10–12. While adding the
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Fig. 1. Schematic representation of the synthesis steps used to synthesize ZrO2 nanostructures.
ammonia, the color of the solution turns out to be white and subsequently a white precipitate was formed. The mixed solution is kept on stirring till the gel type nature of the final solution is formed. Thereafter, the gel solution was dried at 100 °C and then grinded to make resulting powder. The powder thus obtained after grinding was then calcined at different temperatures 500, 600 and 700 °C for 3 h to obtain the final product. Conventional furnace was used for calcinations of ZrO2 powder samples obtained from the synthesis. The complete synthesis procedure along with calcinations temperatures of ZrO2 nanostructures is shown in Fig. 1. The ZrO2 powder obtained after calcinations is characterized by different experimental techniques. The structural, shape and size of the synthesized product were investigated using XRD and TEM measurements. Optical and thermal properties of the synthesized powder samples were studied by FT-IR, RS, UV–vis., and TGA measurements, respectively. The formation of final product during the synthesis process can be described by the following chemical reaction: ZrOCl2 8H2Oþ 2NH4OH ¼Zr(OH)4 þ2NH4Cl þ7H2O Zr(OH)4 ¼ZrO1.5 (OH)þ1.5H2O ZrO1.5(OH)¼ZrO2 þ 0.5H2O The crystallization of ZrO2 from amorphous Zr(OH)4 does not take place directly, but this transformation involved intermediate stages. In first stage, the loss of coordinated water and terminal hydroxo groups takes place. In next stage, oxolation of –OH functional group bridges to form embryonic oxide nuclei and in last stage, nuclei grow to form observable crystallites [16]. 2.2. Instrumentations The structural properties of synthesized powder samples were analyzed by XRD measurements using powder diffractometer (Bruker AXS D8) with Cu-Kα radiation (λ ¼1.5406 A°). The size and shape of the synthesized powder samples were studied by TEM measurements using TEM (Hitachi- H-8100). The FT-IR spectra of the synthesized samples have been recorded in the spectral range 500–4000 cm 1 with the help of Perkin-Elmer 1600 Fourier transform instrument using the KBr pellet technique. The absorption spectra of the samples are taken for the spectral range 200–800 nm using Perkin-Elmer Lambda 35 UV–visible spectrophotometer. The room temperature RS of the powder samples were recorded in the spectral range, 100–800 cm 1 using Thermo Scientific DXR-XI Raman Imaging Microscope. The 532 nm laser line of the Ar þ ions laser was used to illuminate the powder samples. Thermal properties of the synthesized samples are
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measured by thermo gravimetric analyzer measurements using (TA Instruments Q50).
Table 1 The calculated values of lattice perameter for tetragonal and monoclinic ZrO2 calcined at 500 and 700 °C. Parameters
Tetragonal (500 °C)
Monoclinic (700 °C)
Lattice constant (A°)
a¼ 3.6087 c ¼5.1468
a¼ 5.1444 b¼ 5.1870 c¼ 5.3090 β ¼80.75°
3. Results and discussion 3.1. XRD The XRD patterns of synthesized powder samples calcined at temperatures, 500, 600 and 700 °C are shown in Fig. 2. The diffraction peaks appeared in the XRD spectra of synthesized powder samples were indexed by JCPDS card no. 80-2155. The XRD spectra of powder sample calcined at 500 °C have five well resolved peaks at 2θ ¼30.2°, 35°, 50.4°, 60°, and 62.9° which are indexed as reflection from (101), (110), (112), (211) and (202) planes, respectively. These indexed peaks correspond to pure tetragonal phase of the ZrO2. However, the sample calcined at 600 °C has two more diffraction peaks in addition to the five peaks as we have observed in case of sample calcined at 500 °C. These two additional peaks are of weak intensity compared to the intensity of (101), (110), (112), (211) and (202) and indexed to the reflection from (111) and ( 111) planes of the monoclinic phase of ZrO2. It indicates that the sample calcined at 600 °C has mixture of both, tetragonal and monoclinic crystalline phases. The sample calcined at 700 °C has almost all the diffraction peaks corresponding to the monoclinic phase of ZrO2 along with the diffraction peaks of tetragonal phase. The intensity of diffraction peaks corresponding to monoclinic phase is relatively high than that of the intensity of XRD peaks corresponding to tetragonal phase. This implies that for the sample calcined at 700 °C, the monoclinic phase of ZrO2 dominants over the tetragonal phase. In order to have a qualitative estimate of the presence of these two phases of ZrO2 in the samples synthesized at 600 and 700 °C, we have used following relationship [17] to get the percentage of these two phases using the intensity of characteristic features of XRD peaks corresponding to monoclinic phase. The monoclinic content x m is calculated by the following equation [17]:
xm =
Im (−111) + Im (111) Im (−111) + Im(111) + It (101)
Here, Im(−111),Im( 111) and It ( 101) are the integrated intensities of the corresponding planes (the subscripts m and t denote the monoclinic and tetragonal phase, respectively). The samples
Fig. 2. XRD patterns of samples calcined at temperatures 500, 600, and 700 °C.
calcined at 600 and 700 °C are having monoclinic phase 27% and 64%, respectively. The lattice parameters for both, tetragonal and monoclinic phases of ZrO2 have been calculated using the following relation. The values thus calculated for ZrO2 calcined at 500, and 700 °C are compared and summarized in Table 1. For tetragonal [18]:
1 h2 + k 2 l2 = + 2 2 2 d a c For monoclinic [18]: k 2sin2β 1 1 ⎡ h2 = 2 ⎢ 2 + + d2 Sin β ⎣ a b2
=
1
⎡ h2 ⎢ +
2
2
k sin β
Sin2β ⎣ a2
b2
+
l2 c2 l2 c2
− −
2hl Cosβ ⎤ 1 ⎥⎦ d2 where ac
(a, b, c and β ) are
2hl Cosβ ⎤ ac ⎦⎥
the lattice parameters, d is interplanar spacing, (h, k, and l) are the Miller indices of the plane. The average nanocrystallite size for the samples calcined at 500 and 600 °C is calculated from the full width at half maxim (FWHM) of XRD peaks using Scherrer equation [19]:
Dhkl = 0.89λ /(β cosθ) where D is the crystallite size along (hkl) direction, β FWHM of the most intense diffraction peak, λ the wavelength of X-ray and θ, the Bragg angle. The average crystallite size was calculated to be 7, and 10 nm for the samples calcined at 500 and 600 °C, respectively. In tetragonal and cubic phase of ZrO2, the coordination number of Zr þ 4 cations is 8, whereas in monoclinic ZrO2, which is stable at lower temperatures, the coordination number is 7. The strong Zr– O covalent bond favours a seven-fold coordination number due to which monoclinic ZrO2 is found thermodynamically stable at lower temperatures [20]. The tetragonal phase in nanocrystalline ZrO2 can be stabilized at room temperature below a critical size of 10 nm (for an isolated, single, strain-free nanoparticle) or 30 nm (due to aggregation of ZrO2 nanocrystallites). In case of bulk ZrO2, the stability of tetragonal phase has been attributed to the presence of oxygen ion vacancies induced by doping tri-, tetra-, and penta-valent impurities in the ZrO2 lattice. In fact, the stabilization of tetragonal structure of ZrO2 nanocrystallite may be due to the generation of excess oxygen ion vacancies at nano sized [20]. In the present study, the crystallite size of ZrO2 vary from 7 nm for the sample calcined at 500 °C (t) to 10 nm for the sample calcined at 600 °C (t þm), which is consistent with the above discussions. These values are also in good agreement with that reported by Gravie et al. [21]. Surface energies also play a significant role in stabilizing the tetragonal phase as the particle size approaches to nano scale. In order to support the experimental results regarding the stabilities of two phases, we have calculated the single point energy of the monoclinic and tetragonal structures of ZrO2 using density functional theory (DFT). The single point energy of two phases of ZrO2 structures is calculated by using the local density approximation (LDA) and pseudo potential approximation using
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Gaussian09 package [22], which solves the Kohn–Sham equations using a plane-wave expansion for electronic charge density and wave functions. The metal cation Zr þ 4 have a nominal d° valence configuration which does not show strong valence electron effects. Therefore, it is expected that the structural calculations would be well described by employing the local density approximations (LDA) or the generalized gradient approximations (GGA). The single point energy for tetragonal and monoclinic structures has been calculated at B3LYP/ LANL2DZ level of theory. For calculating the single point energy, monoclinic and tetragonal structures of ZrO2 were made by Gauss View software using the experimental values of lattice parameters corresponding to tetragonal and monoclinic phases. Thus, the calculated value of single point energy could be used to explain the structural stability of these two phases. The theoretically calculated monoclinic and tetragonal structures of ZrO2 are shown in Fig. 3. The values of single point energy for tetragonal and monoclinic structures are calculated to be 1251.2271 and 1253.8386 Hartree, respectively. Thus, it is quite obvious that the single point energy for monoclinic structure of ZrO2 is greater than that of tetragonal structure. It implies that the monoclinic phase of ZrO2 is energetically more stable than that of tetragonal phase. The change of electron density around the binding sites of oxygen atoms may be one of the reasons for the larger value of single point energy for monoclinic structure. As a result, ZrO2 always intended to acquire monoclinic phase than that of the tetragonal phase.
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temperature, ZrO2 nanorods were observed of mean diameter 10 nm and mean length 32 nm. The shape and size of the nanostructures has a direct relationship with the crystal symmetry, growth method, and growth condition (on atmospheric conditions such as volume, pressure and temperature etc.). Mainly, two quantities (i) chemical potential and (ii) surface energy for the atoms at surfaces play an important role for structuring the shape and size of the nanomaterials. The shape and size of the nanostructured materials may be classified by calculating the average value of Gibbs free energy per unit area (interface) and it is defined as:
γ = Gs /A where Gs is considered as excess Gibbs free energy to form that surface for area A. For ideal interfaces involving crystalline materials, the surface energy should depend on the interface orientation. Every material exists in minimum stable energy configuration, and as increase of the thermal energy the nanostructures become unstable. In this case, the nanostructures try to attain the stable configuration of minimum energy at higher calcination temperature, which results into the change in the shape of nanostructures. At equilibrium, the crystal shape will be a perfect sphere if there is no anisotropy in the surface energies. However, anisotropies in the surface energies lead to different shape of the crystal. The equilibrium shape of crystal will have surface orientations with minimum surface energy for the planes with lower miller indices.
3.2. TEM
3.3. FT-IR
In order to determine the size and shape of the ZrO2 samples calcined at 500, 600, and 700 °C, the samples were further characterized using TEM measurements. Fig. 4 shows the TEM images of pure ZrO2 nanostructures calcined at different temperatures. It is clearly seen that in the sample calcined at 500 °C, most of the particles are spherical in nature with a narrow size distribution. The average particle size (diameter) is calculated by drawing the histogram and the average particle size turns out to be 8 nm. For the ZrO2 sample calcined at 600 °C, the average particle size is calculated to be 17 nm and the shapes of the particles are found to be spherical in nature. A small contribution of particles having size more than 20 nm is also found at this temperature which is greater than the critical diameter (18 nm) suggested by Chraska et al. [23] for the transformation of tetragonal to monoclinic phase. The presence of particles having the size greater than 20 nm signifies the monoclinic phase. The presence of monoclinic phase in the sample calcined at 600 °C is also confirmed by XRD result. For the sample calcined at 700 °C, the shapes of the nanostructures are found to be different than that of the samples calcined at 500 and 600 °C. At this
The FT-IR spectra of synthesized samples calcined at 500, 600, and 700 °C are shown in Fig. 5. By looking at the FT-IR spectra, it is clear that the ZrO2 nanostructures still contain water molecule since H2O and CO2 molecule have property to be chemisorbed easily on the ZrO2 surface when they exposed to the atmosphere. The broad peak at 3400 cm 1 is assigned to the O–H stretching vibration and the peak at 1632 cm 1 is associated to bending vibration of water molecules. The presence of these two bands shows that the hydroxyl groups are associated to the surface of ZrO2 in larger amount for all samples calcined at 500, 600, and 700 °C [7]. The presence of higher amount of surface hydroxyl group may be the indication for the potential application of the compound in the field of photocatalytic activity. In addition to these two bands, a weak band corresponding to the adsorption of gas-phase CO2 is visible at 2365 cm 1.
Fig. 3. Structure for monoclinic and tetragonal phases of ZrO2 nanostructures.
3.4. TGA Thermogravimetric analysis curve of ZrO2 nanostructures synthesized by sol–gel method is shown in Fig. 6. A considerable weight loss (2%) can be observed for the tetragonal ZrO2 calcined at 500 °C in the range of 40–100 °C. This weight loss can be attributed to the removal of water present on the surface of ZrO2 nanostructures. The second weight loss at 200 °C can be attributed to removal of chemically and physically attach water molecules at the surface of ZrO2 nanostructures. While the third weight loss at high temperature range may be attributed to the transformation of ZrO2 from tetragonal to monoclinic phase. It could be due to the fact that during the calcinations process, the crystal density, enthalpy, Gibbs free energy changes due to the variation in vacant sites, oxygen vacancies and defects present in the sample resulting into the weight loss during transformation [24]. The transformation of tetragonal to monoclinic phase is also confirmed by XRD data. TG analysis shows the poor thermal stability of tetragonal ZrO2. The sample calcined at 600 °C shows better thermal stability as compared to sample calcined at 500 °C. This
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Fig. 4. TEM micrographs along with histogram of ZrO2 nanostructures calcined at temperatures 500, 600, and 700 °C.
Fig. 6. TGA curves of ZrO2 nanostructures calcined at 500, 600, and 700 °C. Fig. 5. FT-IR spectra of ZrO2 nanostructures calcined at 500, 600, and 700 °C.
may be due to the presence of monoclinic phase content along with the tetragonal phase. The sample calcined at 700 °C shows one step weight loss ( 0.5%) due to the dehydration. From XRD analysis, it is clear that the monoclinic phase dominates in the sample calcined at 700 °C, which reveals the better stability of monoclinic phase over the tetragonal phase of ZrO2 nanostructures. 3.5. Raman Raman spectroscopy is a non-destructive tool to analyze the vibrational and structural properties of the materials. It can also be used to detect small domains of different polymorphous of the materials. It is predicted by group analysis [25] that the tetragonal
phase of ZrO2 should have 6 (A1g þ2B1g þ3 Eg) vibrational modes and monoclinic phase of ZrO2 should have 18 (9Ag þ9Bg) vibrational modes. However, none of the previous experimental studies have reported the presence of 18 vibrational modes as determined by the group analysis. Experimentally, 14 vibrational modes are identified easily in monoclinic ZrO2 for both Ag and Bg polarization conditions [26]. One vibrational mode is represented as superposition of (Agþ Bg) species. Thus, total 15 vibrational modes are found and used to describe the ZrO2 nanostructures. Fig. 7 shows the Raman spectra of ZrO2 nanostructures calcined at 500, 600 and 700 °C. In the sample calcined at 500 °C, 6 signature peaks of tetragonal phase are observed. These peaks may be attributed to B1g at ( 156 cm 1), Eg at ( 277 cm 1), B1 g at ( 325 cm 1), Eg at ( 469 cm 1), A1 g at ( 617 cm 1) and Eg at ( 655 cm 1). For ZrO2 calcined at 600 °C, new signature peaks at 193, 390 and 485 cm 1 corresponding to monoclinic phase is appeared and
S. Kumar et al. / Physica E 66 (2015) 74–80
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tetragonal ZrO2 takes place along one of the three axes. Due to this fact, theoretically predicted peaks for monoclinic ZrO2 (18 modes) are found to be more than that of the peaks calculated for tetragonal ZrO2 (6 modes). The analysis of Raman spectra confirms the presence of mixed phases in the samples calcined at 600 and 700 °C and monoclinic phase dominates over the tetragonal phase at higher temperature. The results obtained by analysing the Raman spectra are in good agreement with those obtained by the XRD. 3.6. UV–visible
Fig. 7. Raman spectra of the ZrO2 samples calcined at 500, 600, and 700 °C.
they become prominent in the sample calcined at 700 °C. For monoclinic phase of ZrO2 (P21/c) calcined at 700 °C, 15 vibrational modes are identified with overlapping of some peaks (193 with 203 cm 1 and 342 with 357 cm 1). The observed peak positions match nicely with the reported Raman spectra for monoclinic ZrO2 [27,28]. The peaks observed at 156 and 277 cm 1 in the sample calcined at 700 °C correspond to tetragonal phase and the strong peaks at 193, 232, 390 and 485, cm 1 correspond to monoclinic phase of ZrO2. In sample calcined at 700 °C, the other peaks (at 469 and 655 cm 1) corresponding to tetragonal phase are disappeared completely. The eigen vectors corresponding these two vibrational modes (at 469 and 655 cm 1) are represented in Fig. 8. The Raman bands obtained by theoretical calculations also confirm the same structural information as we have obtained experimentally. For tetragonal ZrO2 calcined at 500 °C, all Zr atoms are symmetrically moving inward and outward direction about oxygen atom at frequency 469 (Eg) cm 1 contributed due to short range potential between Zr–O. In case of monoclinic ZrO2 calcined at 700 °C, the short range potential between O–Zr and O–O contribute to the modes of vibration at 193 (Ag) cm 1, 232 (Bg) cm 1, 390 (Bg) and 485 (Ag) cm 1. From the theoretical calculation, it is also confirmed that the variation in polarizability components takes place along all the three axes for monoclinic ZrO2. While the variation in polarizability component for
Fig. 8. Eigen vectors corresponding to 469 and 655 cm 1 vibrational modes of tetragonal ZrO2.
ZrO2 is an active and typical photon absorber and photocatalyst among wide band gap metal oxides [29]. The UV–vis. absorption spectra of the ZrO2 samples calcined at 500, 600, and 700 °C recorded for the spectral range 200–800 nm are shown in inset of Fig. 9. The spectra show a week absorption band at 232 nm. It may be due to the transition of electron from valence band to conduction band. The electronic configuration of Zr þ 4 ion is d0, it means that no characteristic features corresponding to d-d transition could be found in the visible region. The absorption peak is appeared due to O 2-Zr þ 4 charge transfer transition corresponding to excitation of electron from valence band to conduction band. The UV–vis. data are used to estimate the optical band gap of ZrO2 nanostructures calcined at 500, 600, and 700 °C by extrapolating the linear portion of (ahν)2 versus hν plots to intercept the photon energy axis as shown in Fig. 9. The value of optical band gap for the ZrO2 sample calcined at 500 °C is calculated to be 5.1 eV and for the sample calcined at 600 °C is calculated to be 4.3 eV. The decrease in optical band gap for the sample calcined at 600 °C may be due to the increase of particle size of ZrO2 due to presence of defects such as oxygen vacancies. The oxygen vacancies change the electronic levels occurs between the valance and conduction band of ZrO2 [29]. The relation between particle size and optical band gap due to three dimensional confinements is predicted by the following simplest model based on the effective mass [30]:
Eg = Ebulk +
h2 ⎛ 1 1 ⎞ 1.8 e2 ⎜ + ⁎ ⎟⎟ − 2 ⎜ m⁎ π ε 0 εR m 4 8R ⎝ e h ⎠
where Ebulk is the band gap of bulk semiconductor, r is the radius of the particle, me⁎ is the electron effective mass, mh⁎ is the hole
Fig. 9. Plots of (ahν)2 versus photon energy of the ZrO2 nanostructures calcined at 500, 600, and 700 °C, inset shows absorption spectra of the same.
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effective masses, ε is the dielectric constant of semiconductor and e is the electronic charge. In this equation, the second term represents the kinetic energy and the third term represents the Coulomb energy. This equation reveals that as the dimension of particle decreases, the energy increases. Quantum confinement in semiconductors produces electrons and hole as a pair in a bound state, known as exciton. The exciton has a finite size in the crystal expressed as Bohr exciton radius which gives the approximate dimension of the semiconductor crystal for quantum confinement effect [31]. If the size of crystal is smaller than the size of exciton, the charge carriers become spatially confined. Liang et al. [32] reported that the optical band gap of a nanorod depends on the width and length of the rod. They presented a qualitative description of band gap for quantum rods using the concepts of quantum confinement and band mixing. They reported that the emission peak positions are more sensitive to width than the length of nano rod. In our case, when the sample is calcined at 700 °C, the ZrO2 nanorods of mean diameter (width) 10 nm are obtained and the band gap value is calculated to be 4.8 eV. The increase in value of band gap compared to the ZrO2 sample calcined at 600 and 700 °C may be due the decrease of mean diameter (17–10 nm) of ZrO2 nanorods than that of the size of ZrO2 nanostructures calcined at 600 °C and 700 °C. This type of variation in optical band gap has also been observed by Chetri et al. [33] in SnO2 nanoparticles. They explained the increase in band gap by Burstein Moss effect. According to their explanation the increase in concentration of oxygen vacancies produce the donor levels which may leads to the increase in electrons near the conduction band edge. 1
OO ⇆ 2 O2 ↑ + VO • • + 2e−1 The defects are conveniently written in Kroger–Vink notation.
4. Conclusion ZrO2 nanostructures calcined at 500, 600, and 700 °C have been prepared by sol–gel method. The structural analysis reveals tetragonal to monoclinic phase transition with increasing the calcinations temperature. TEM analysis incorporates the finding of XRD result and reveals that the particle size also plays a significant role to stabilize the phase of ZrO2. UV–vis. spectra reveal a weak absorption band due to electronic transition from valance to conduction band. The optical band gap reveals a strong dependence on shape and size of the ZrO2 nanostructures. Raman Spectra reveal six characteristic features for tetragonal ZrO2 calcined at 500 °C and mix phases in the samples calcined at 600 and 700 °C. In the sample calcined at 700 °C, 15 Raman active modes were observed out of 18 vibrational modes of monoclinic ZrO2. TG analysis reveals the better thermal stability of monoclinic phase over the tetragonal phase of ZrO2.
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Physica E journal homepage: www.elsevier.com/locate/physe
Effect of calcination temperature on phase transformation, structural and optical properties of sol–gel derived ZrO2 nanostructures Sachin Kumar, Snehasis Bhunia, Animesh K. Ojha n Department of Physics, Motilal Nehru National Institute of Technology, Allahabad, Allahabad 211004, India
H I G H L I G H T S
Shapes and sizes of ZrO2 nanostructures were modified with calcinations temperature. A rod like shape is observed for the ZrO2 calcined at 700 °C ZrO2. Tetragonal to monoclinic phase transition was observed at 600 °C. DFT calculations revealed that monoclinic phase is energetically more stable. The optical band gap is changed due to change in shape and size of ZrO2 nanostructures.
ar t ic l e i nf o
a b s t r a c t
Article history: Received 27 July 2014 Received in revised form 3 September 2014 Accepted 8 September 2014 Available online 8 October 2014
Zirconia (ZrO2) nanostructures of various sizes have been synthesized using sol–gel method followed by calcination of the samples from 500 to 700 °C. The calcined ZrO2 powder samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infra-red spectroscopy (FT-IR), UV–visible spectroscopy (UV–vis.), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). The phase transformation from tetragonal (t) to monoclinic (m) was observed. The average diameter of the ZrO2 nanostructures calcined at 500, 600 and 700 °C was calculated to be 8, 17 and 10 nm, respectively. The ZrO2 sample calcined at 500 °C with tetragonal phase shows a direct optical band gap of 5.1 eV. The value of optical band gap is decreased to 4.3 eV for the ZrO2 calcined at 600 °C, which contains both tetragonal (73%) and monoclinic (27%) phases. On further calcination at 700 °C, where the ZrO2 nanostructures have 36% tetragonal and 64% monoclinic phases, the optical band gap is calculated to be 4.8 eV. The enhancement in optical band gap for ZrO2 calcined at 700 °C may be due to the rod like shape of ZrO2 nanostructures. The tetragonal to monoclinic phase transformation was also confirmed by analyzing Raman spectroscopic data. The TG analysis revealed that the ZrO2 nanostructure with dominance of monoclinic phase is found to be more stable over the tetragonal phase. In order to confirm the phase stability of the two phases of ZrO2, single point energy is calculated corresponding to its monoclinic and tetragonal structures using density functional theory (DFT) calculations. The results obtained by theoretical calculations are in good agreement with the experimental findings. & Elsevier B.V. All rights reserved.
Keywords: ZrO2 nanostructures Sol–gel Optical properties DFT
1. Introduction Nanomaterials have sparked a great interest to the scientific community due the fact that it acquired unique chemical and physical properties at nano scale and these properties change drastically with their size and shape [1]. ZrO2 is one of the interesting oxide materials as it has wide range of optical applications in different fields such as; photonics, interferometry filter and for coating high power laser mirrors due to its hardness, n
Corresponding author. Fax: þ91 532 2545341. E-mail addresses: [email protected], [email protected] (A.K. Ojha).
http://dx.doi.org/10.1016/j.physe.2014.09.007 1386-9477/& Elsevier B.V. All rights reserved.
optical transparency and high refractive index [2]. It has excellent thermal application in thermal barrier coating [3]. The bulk form of ZrO2 has three polymorphs, cubic (c-ZrO2), tetragonal (t-ZrO2) and monoclinic (m-ZrO2) phases. At very high temperature, greater than 2370 °C, the ZrO2 has cubic phase [4]. The tetragonal phase of ZrO2 has been observed for temperature range, 2370–1170 °C [4]. However, at the temperature below to 1170 °C, ZrO2 exist in monoclinic phase [4]. On adding the metal oxide such as; MgO, CaO, and Y2O3, the transformation temperature of ZrO2 into different crystalline phases can be lowered [4]. In previous studies [5,6], it was found that the transformation from tetragonal to monoclinic phase causes a large volume expansion (4%) and
S. Kumar et al. / Physica E 66 (2015) 74–80
extensive cracking in the material which leads to high fracture toughness. Various synthesis methods such as; co-precipitation, citrate technique, hydrothermal, electrodeposition, solvothermal and sol– gel [7] have been used to synthesize ZrO2 nanostructures. Among these synthesis methods, the sol–gel technique has received increasing attention by the research community in the recent years since it provides versatility and excellent control over the structure, shape and size and consequently it also provide a good control over the chemical, physical and optical properties of the synthesized materials. The sol–gel method of synthesis includes four steps: hydrolysis, polycondensation, drying, and thermal decomposition. The metallic precursor also plays a significant role for obtaining the crystalline phase by sol–gel method [8]. In case of ZrO2 nanostructures synthesized by sol–gel method, the crystal structure strongly depends on calcinations temperature which has a great influence on optical, structural and thermal properties of synthesized materials. In recent years, a large number of studies have been done to explore the effect of calcinations temperatures on structural, optical and thermal properties of ZrO2 nanostructures [9–15]. Liu et al. [10] had studied the annealing and doping effect on structure and optical properties of ZrO2 thin films. They focussed on the changes in refractive indices of the films with varying the calcinations temperature and doping of the rare earth elements. They found that the value of refractive index increases with doping of rare earth element in ZrO2 thin film. Very recently, Ashraf et al. [13] had studied annealing effect on photoluminescence properties of ZrO2 at nanoscale and submicron scale. They explained the photoluminescence behavior in visible and UV range of the ZrO2 nanostructures in terms of abundant oxygen vacancies as function of annealing temperatures. In another study Goharshadi et al. [14] had studied the effect of calcination temperature on vibrational, optical, and rheological properties of ZrO2 nanostructures. In view of the above studies on role of calcinations temperature on structural, thermal, electrical and optical properties of ZrO2 nanostructures, there is a lack of extensive investigations on size and shape dependent variation in optical band gap of ZrO2 nanostructures calcined at different temperatures. The calcination temperature modifies crystalline phases, shape and size of the ZrO2 nanostructures. The change in these parameters of the nanostructured ZrO2 may be responsible for modifications in various structural, optical and thermal properties of ZrO2 nanostructures. Therefore, it would be great interest and significance to investigate the role of crystalline phase, size and shape of the ZrO2 nanostructures on optical band gap and thermal stability. Thus, in the present work we have synthesized ZrO2 nanostructures using simple and low cost synthesis method and calcined them at 500, 600 and 700 °C to investigate the effect of crystalline phase, size and shape on its structural, optical, and thermal properties. XRD, TEM, RS, UV–vis., and TGA characterization techniques were employed to characterize the synthesized samples.
2. Experimental details 2.1. Synthesis ZrO2 nanostructures were synthesized using a very simple and low cost sol–gel method. ZrOCl2 8H2O and ammonia solution 25% (0.91) were purchased from SRL Pvt. Ltd., India and the chemicals have been used without further purification. An appropriate amount of ZrOCl2 8H2O was dissolved in distilled water and stirred vigorously for one hour. Thereafter, ammonia solution was added into the solution drop wise while stirring until the pH value of the solution is reached to 10–12. While adding the
75
Fig. 1. Schematic representation of the synthesis steps used to synthesize ZrO2 nanostructures.
ammonia, the color of the solution turns out to be white and subsequently a white precipitate was formed. The mixed solution is kept on stirring till the gel type nature of the final solution is formed. Thereafter, the gel solution was dried at 100 °C and then grinded to make resulting powder. The powder thus obtained after grinding was then calcined at different temperatures 500, 600 and 700 °C for 3 h to obtain the final product. Conventional furnace was used for calcinations of ZrO2 powder samples obtained from the synthesis. The complete synthesis procedure along with calcinations temperatures of ZrO2 nanostructures is shown in Fig. 1. The ZrO2 powder obtained after calcinations is characterized by different experimental techniques. The structural, shape and size of the synthesized product were investigated using XRD and TEM measurements. Optical and thermal properties of the synthesized powder samples were studied by FT-IR, RS, UV–vis., and TGA measurements, respectively. The formation of final product during the synthesis process can be described by the following chemical reaction: ZrOCl2 8H2Oþ 2NH4OH ¼Zr(OH)4 þ2NH4Cl þ7H2O Zr(OH)4 ¼ZrO1.5 (OH)þ1.5H2O ZrO1.5(OH)¼ZrO2 þ 0.5H2O The crystallization of ZrO2 from amorphous Zr(OH)4 does not take place directly, but this transformation involved intermediate stages. In first stage, the loss of coordinated water and terminal hydroxo groups takes place. In next stage, oxolation of –OH functional group bridges to form embryonic oxide nuclei and in last stage, nuclei grow to form observable crystallites [16]. 2.2. Instrumentations The structural properties of synthesized powder samples were analyzed by XRD measurements using powder diffractometer (Bruker AXS D8) with Cu-Kα radiation (λ ¼1.5406 A°). The size and shape of the synthesized powder samples were studied by TEM measurements using TEM (Hitachi- H-8100). The FT-IR spectra of the synthesized samples have been recorded in the spectral range 500–4000 cm 1 with the help of Perkin-Elmer 1600 Fourier transform instrument using the KBr pellet technique. The absorption spectra of the samples are taken for the spectral range 200–800 nm using Perkin-Elmer Lambda 35 UV–visible spectrophotometer. The room temperature RS of the powder samples were recorded in the spectral range, 100–800 cm 1 using Thermo Scientific DXR-XI Raman Imaging Microscope. The 532 nm laser line of the Ar þ ions laser was used to illuminate the powder samples. Thermal properties of the synthesized samples are
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measured by thermo gravimetric analyzer measurements using (TA Instruments Q50).
Table 1 The calculated values of lattice perameter for tetragonal and monoclinic ZrO2 calcined at 500 and 700 °C. Parameters
Tetragonal (500 °C)
Monoclinic (700 °C)
Lattice constant (A°)
a¼ 3.6087 c ¼5.1468
a¼ 5.1444 b¼ 5.1870 c¼ 5.3090 β ¼80.75°
3. Results and discussion 3.1. XRD The XRD patterns of synthesized powder samples calcined at temperatures, 500, 600 and 700 °C are shown in Fig. 2. The diffraction peaks appeared in the XRD spectra of synthesized powder samples were indexed by JCPDS card no. 80-2155. The XRD spectra of powder sample calcined at 500 °C have five well resolved peaks at 2θ ¼30.2°, 35°, 50.4°, 60°, and 62.9° which are indexed as reflection from (101), (110), (112), (211) and (202) planes, respectively. These indexed peaks correspond to pure tetragonal phase of the ZrO2. However, the sample calcined at 600 °C has two more diffraction peaks in addition to the five peaks as we have observed in case of sample calcined at 500 °C. These two additional peaks are of weak intensity compared to the intensity of (101), (110), (112), (211) and (202) and indexed to the reflection from (111) and ( 111) planes of the monoclinic phase of ZrO2. It indicates that the sample calcined at 600 °C has mixture of both, tetragonal and monoclinic crystalline phases. The sample calcined at 700 °C has almost all the diffraction peaks corresponding to the monoclinic phase of ZrO2 along with the diffraction peaks of tetragonal phase. The intensity of diffraction peaks corresponding to monoclinic phase is relatively high than that of the intensity of XRD peaks corresponding to tetragonal phase. This implies that for the sample calcined at 700 °C, the monoclinic phase of ZrO2 dominants over the tetragonal phase. In order to have a qualitative estimate of the presence of these two phases of ZrO2 in the samples synthesized at 600 and 700 °C, we have used following relationship [17] to get the percentage of these two phases using the intensity of characteristic features of XRD peaks corresponding to monoclinic phase. The monoclinic content x m is calculated by the following equation [17]:
xm =
Im (−111) + Im (111) Im (−111) + Im(111) + It (101)
Here, Im(−111),Im( 111) and It ( 101) are the integrated intensities of the corresponding planes (the subscripts m and t denote the monoclinic and tetragonal phase, respectively). The samples
Fig. 2. XRD patterns of samples calcined at temperatures 500, 600, and 700 °C.
calcined at 600 and 700 °C are having monoclinic phase 27% and 64%, respectively. The lattice parameters for both, tetragonal and monoclinic phases of ZrO2 have been calculated using the following relation. The values thus calculated for ZrO2 calcined at 500, and 700 °C are compared and summarized in Table 1. For tetragonal [18]:
1 h2 + k 2 l2 = + 2 2 2 d a c For monoclinic [18]: k 2sin2β 1 1 ⎡ h2 = 2 ⎢ 2 + + d2 Sin β ⎣ a b2
=
1
⎡ h2 ⎢ +
2
2
k sin β
Sin2β ⎣ a2
b2
+
l2 c2 l2 c2
− −
2hl Cosβ ⎤ 1 ⎥⎦ d2 where ac
(a, b, c and β ) are
2hl Cosβ ⎤ ac ⎦⎥
the lattice parameters, d is interplanar spacing, (h, k, and l) are the Miller indices of the plane. The average nanocrystallite size for the samples calcined at 500 and 600 °C is calculated from the full width at half maxim (FWHM) of XRD peaks using Scherrer equation [19]:
Dhkl = 0.89λ /(β cosθ) where D is the crystallite size along (hkl) direction, β FWHM of the most intense diffraction peak, λ the wavelength of X-ray and θ, the Bragg angle. The average crystallite size was calculated to be 7, and 10 nm for the samples calcined at 500 and 600 °C, respectively. In tetragonal and cubic phase of ZrO2, the coordination number of Zr þ 4 cations is 8, whereas in monoclinic ZrO2, which is stable at lower temperatures, the coordination number is 7. The strong Zr– O covalent bond favours a seven-fold coordination number due to which monoclinic ZrO2 is found thermodynamically stable at lower temperatures [20]. The tetragonal phase in nanocrystalline ZrO2 can be stabilized at room temperature below a critical size of 10 nm (for an isolated, single, strain-free nanoparticle) or 30 nm (due to aggregation of ZrO2 nanocrystallites). In case of bulk ZrO2, the stability of tetragonal phase has been attributed to the presence of oxygen ion vacancies induced by doping tri-, tetra-, and penta-valent impurities in the ZrO2 lattice. In fact, the stabilization of tetragonal structure of ZrO2 nanocrystallite may be due to the generation of excess oxygen ion vacancies at nano sized [20]. In the present study, the crystallite size of ZrO2 vary from 7 nm for the sample calcined at 500 °C (t) to 10 nm for the sample calcined at 600 °C (t þm), which is consistent with the above discussions. These values are also in good agreement with that reported by Gravie et al. [21]. Surface energies also play a significant role in stabilizing the tetragonal phase as the particle size approaches to nano scale. In order to support the experimental results regarding the stabilities of two phases, we have calculated the single point energy of the monoclinic and tetragonal structures of ZrO2 using density functional theory (DFT). The single point energy of two phases of ZrO2 structures is calculated by using the local density approximation (LDA) and pseudo potential approximation using
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Gaussian09 package [22], which solves the Kohn–Sham equations using a plane-wave expansion for electronic charge density and wave functions. The metal cation Zr þ 4 have a nominal d° valence configuration which does not show strong valence electron effects. Therefore, it is expected that the structural calculations would be well described by employing the local density approximations (LDA) or the generalized gradient approximations (GGA). The single point energy for tetragonal and monoclinic structures has been calculated at B3LYP/ LANL2DZ level of theory. For calculating the single point energy, monoclinic and tetragonal structures of ZrO2 were made by Gauss View software using the experimental values of lattice parameters corresponding to tetragonal and monoclinic phases. Thus, the calculated value of single point energy could be used to explain the structural stability of these two phases. The theoretically calculated monoclinic and tetragonal structures of ZrO2 are shown in Fig. 3. The values of single point energy for tetragonal and monoclinic structures are calculated to be 1251.2271 and 1253.8386 Hartree, respectively. Thus, it is quite obvious that the single point energy for monoclinic structure of ZrO2 is greater than that of tetragonal structure. It implies that the monoclinic phase of ZrO2 is energetically more stable than that of tetragonal phase. The change of electron density around the binding sites of oxygen atoms may be one of the reasons for the larger value of single point energy for monoclinic structure. As a result, ZrO2 always intended to acquire monoclinic phase than that of the tetragonal phase.
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temperature, ZrO2 nanorods were observed of mean diameter 10 nm and mean length 32 nm. The shape and size of the nanostructures has a direct relationship with the crystal symmetry, growth method, and growth condition (on atmospheric conditions such as volume, pressure and temperature etc.). Mainly, two quantities (i) chemical potential and (ii) surface energy for the atoms at surfaces play an important role for structuring the shape and size of the nanomaterials. The shape and size of the nanostructured materials may be classified by calculating the average value of Gibbs free energy per unit area (interface) and it is defined as:
γ = Gs /A where Gs is considered as excess Gibbs free energy to form that surface for area A. For ideal interfaces involving crystalline materials, the surface energy should depend on the interface orientation. Every material exists in minimum stable energy configuration, and as increase of the thermal energy the nanostructures become unstable. In this case, the nanostructures try to attain the stable configuration of minimum energy at higher calcination temperature, which results into the change in the shape of nanostructures. At equilibrium, the crystal shape will be a perfect sphere if there is no anisotropy in the surface energies. However, anisotropies in the surface energies lead to different shape of the crystal. The equilibrium shape of crystal will have surface orientations with minimum surface energy for the planes with lower miller indices.
3.2. TEM
3.3. FT-IR
In order to determine the size and shape of the ZrO2 samples calcined at 500, 600, and 700 °C, the samples were further characterized using TEM measurements. Fig. 4 shows the TEM images of pure ZrO2 nanostructures calcined at different temperatures. It is clearly seen that in the sample calcined at 500 °C, most of the particles are spherical in nature with a narrow size distribution. The average particle size (diameter) is calculated by drawing the histogram and the average particle size turns out to be 8 nm. For the ZrO2 sample calcined at 600 °C, the average particle size is calculated to be 17 nm and the shapes of the particles are found to be spherical in nature. A small contribution of particles having size more than 20 nm is also found at this temperature which is greater than the critical diameter (18 nm) suggested by Chraska et al. [23] for the transformation of tetragonal to monoclinic phase. The presence of particles having the size greater than 20 nm signifies the monoclinic phase. The presence of monoclinic phase in the sample calcined at 600 °C is also confirmed by XRD result. For the sample calcined at 700 °C, the shapes of the nanostructures are found to be different than that of the samples calcined at 500 and 600 °C. At this
The FT-IR spectra of synthesized samples calcined at 500, 600, and 700 °C are shown in Fig. 5. By looking at the FT-IR spectra, it is clear that the ZrO2 nanostructures still contain water molecule since H2O and CO2 molecule have property to be chemisorbed easily on the ZrO2 surface when they exposed to the atmosphere. The broad peak at 3400 cm 1 is assigned to the O–H stretching vibration and the peak at 1632 cm 1 is associated to bending vibration of water molecules. The presence of these two bands shows that the hydroxyl groups are associated to the surface of ZrO2 in larger amount for all samples calcined at 500, 600, and 700 °C [7]. The presence of higher amount of surface hydroxyl group may be the indication for the potential application of the compound in the field of photocatalytic activity. In addition to these two bands, a weak band corresponding to the adsorption of gas-phase CO2 is visible at 2365 cm 1.
Fig. 3. Structure for monoclinic and tetragonal phases of ZrO2 nanostructures.
3.4. TGA Thermogravimetric analysis curve of ZrO2 nanostructures synthesized by sol–gel method is shown in Fig. 6. A considerable weight loss (2%) can be observed for the tetragonal ZrO2 calcined at 500 °C in the range of 40–100 °C. This weight loss can be attributed to the removal of water present on the surface of ZrO2 nanostructures. The second weight loss at 200 °C can be attributed to removal of chemically and physically attach water molecules at the surface of ZrO2 nanostructures. While the third weight loss at high temperature range may be attributed to the transformation of ZrO2 from tetragonal to monoclinic phase. It could be due to the fact that during the calcinations process, the crystal density, enthalpy, Gibbs free energy changes due to the variation in vacant sites, oxygen vacancies and defects present in the sample resulting into the weight loss during transformation [24]. The transformation of tetragonal to monoclinic phase is also confirmed by XRD data. TG analysis shows the poor thermal stability of tetragonal ZrO2. The sample calcined at 600 °C shows better thermal stability as compared to sample calcined at 500 °C. This
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Fig. 4. TEM micrographs along with histogram of ZrO2 nanostructures calcined at temperatures 500, 600, and 700 °C.
Fig. 6. TGA curves of ZrO2 nanostructures calcined at 500, 600, and 700 °C. Fig. 5. FT-IR spectra of ZrO2 nanostructures calcined at 500, 600, and 700 °C.
may be due to the presence of monoclinic phase content along with the tetragonal phase. The sample calcined at 700 °C shows one step weight loss ( 0.5%) due to the dehydration. From XRD analysis, it is clear that the monoclinic phase dominates in the sample calcined at 700 °C, which reveals the better stability of monoclinic phase over the tetragonal phase of ZrO2 nanostructures. 3.5. Raman Raman spectroscopy is a non-destructive tool to analyze the vibrational and structural properties of the materials. It can also be used to detect small domains of different polymorphous of the materials. It is predicted by group analysis [25] that the tetragonal
phase of ZrO2 should have 6 (A1g þ2B1g þ3 Eg) vibrational modes and monoclinic phase of ZrO2 should have 18 (9Ag þ9Bg) vibrational modes. However, none of the previous experimental studies have reported the presence of 18 vibrational modes as determined by the group analysis. Experimentally, 14 vibrational modes are identified easily in monoclinic ZrO2 for both Ag and Bg polarization conditions [26]. One vibrational mode is represented as superposition of (Agþ Bg) species. Thus, total 15 vibrational modes are found and used to describe the ZrO2 nanostructures. Fig. 7 shows the Raman spectra of ZrO2 nanostructures calcined at 500, 600 and 700 °C. In the sample calcined at 500 °C, 6 signature peaks of tetragonal phase are observed. These peaks may be attributed to B1g at ( 156 cm 1), Eg at ( 277 cm 1), B1 g at ( 325 cm 1), Eg at ( 469 cm 1), A1 g at ( 617 cm 1) and Eg at ( 655 cm 1). For ZrO2 calcined at 600 °C, new signature peaks at 193, 390 and 485 cm 1 corresponding to monoclinic phase is appeared and
S. Kumar et al. / Physica E 66 (2015) 74–80
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tetragonal ZrO2 takes place along one of the three axes. Due to this fact, theoretically predicted peaks for monoclinic ZrO2 (18 modes) are found to be more than that of the peaks calculated for tetragonal ZrO2 (6 modes). The analysis of Raman spectra confirms the presence of mixed phases in the samples calcined at 600 and 700 °C and monoclinic phase dominates over the tetragonal phase at higher temperature. The results obtained by analysing the Raman spectra are in good agreement with those obtained by the XRD. 3.6. UV–visible
Fig. 7. Raman spectra of the ZrO2 samples calcined at 500, 600, and 700 °C.
they become prominent in the sample calcined at 700 °C. For monoclinic phase of ZrO2 (P21/c) calcined at 700 °C, 15 vibrational modes are identified with overlapping of some peaks (193 with 203 cm 1 and 342 with 357 cm 1). The observed peak positions match nicely with the reported Raman spectra for monoclinic ZrO2 [27,28]. The peaks observed at 156 and 277 cm 1 in the sample calcined at 700 °C correspond to tetragonal phase and the strong peaks at 193, 232, 390 and 485, cm 1 correspond to monoclinic phase of ZrO2. In sample calcined at 700 °C, the other peaks (at 469 and 655 cm 1) corresponding to tetragonal phase are disappeared completely. The eigen vectors corresponding these two vibrational modes (at 469 and 655 cm 1) are represented in Fig. 8. The Raman bands obtained by theoretical calculations also confirm the same structural information as we have obtained experimentally. For tetragonal ZrO2 calcined at 500 °C, all Zr atoms are symmetrically moving inward and outward direction about oxygen atom at frequency 469 (Eg) cm 1 contributed due to short range potential between Zr–O. In case of monoclinic ZrO2 calcined at 700 °C, the short range potential between O–Zr and O–O contribute to the modes of vibration at 193 (Ag) cm 1, 232 (Bg) cm 1, 390 (Bg) and 485 (Ag) cm 1. From the theoretical calculation, it is also confirmed that the variation in polarizability components takes place along all the three axes for monoclinic ZrO2. While the variation in polarizability component for
Fig. 8. Eigen vectors corresponding to 469 and 655 cm 1 vibrational modes of tetragonal ZrO2.
ZrO2 is an active and typical photon absorber and photocatalyst among wide band gap metal oxides [29]. The UV–vis. absorption spectra of the ZrO2 samples calcined at 500, 600, and 700 °C recorded for the spectral range 200–800 nm are shown in inset of Fig. 9. The spectra show a week absorption band at 232 nm. It may be due to the transition of electron from valence band to conduction band. The electronic configuration of Zr þ 4 ion is d0, it means that no characteristic features corresponding to d-d transition could be found in the visible region. The absorption peak is appeared due to O 2-Zr þ 4 charge transfer transition corresponding to excitation of electron from valence band to conduction band. The UV–vis. data are used to estimate the optical band gap of ZrO2 nanostructures calcined at 500, 600, and 700 °C by extrapolating the linear portion of (ahν)2 versus hν plots to intercept the photon energy axis as shown in Fig. 9. The value of optical band gap for the ZrO2 sample calcined at 500 °C is calculated to be 5.1 eV and for the sample calcined at 600 °C is calculated to be 4.3 eV. The decrease in optical band gap for the sample calcined at 600 °C may be due to the increase of particle size of ZrO2 due to presence of defects such as oxygen vacancies. The oxygen vacancies change the electronic levels occurs between the valance and conduction band of ZrO2 [29]. The relation between particle size and optical band gap due to three dimensional confinements is predicted by the following simplest model based on the effective mass [30]:
Eg = Ebulk +
h2 ⎛ 1 1 ⎞ 1.8 e2 ⎜ + ⁎ ⎟⎟ − 2 ⎜ m⁎ π ε 0 εR m 4 8R ⎝ e h ⎠
where Ebulk is the band gap of bulk semiconductor, r is the radius of the particle, me⁎ is the electron effective mass, mh⁎ is the hole
Fig. 9. Plots of (ahν)2 versus photon energy of the ZrO2 nanostructures calcined at 500, 600, and 700 °C, inset shows absorption spectra of the same.
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effective masses, ε is the dielectric constant of semiconductor and e is the electronic charge. In this equation, the second term represents the kinetic energy and the third term represents the Coulomb energy. This equation reveals that as the dimension of particle decreases, the energy increases. Quantum confinement in semiconductors produces electrons and hole as a pair in a bound state, known as exciton. The exciton has a finite size in the crystal expressed as Bohr exciton radius which gives the approximate dimension of the semiconductor crystal for quantum confinement effect [31]. If the size of crystal is smaller than the size of exciton, the charge carriers become spatially confined. Liang et al. [32] reported that the optical band gap of a nanorod depends on the width and length of the rod. They presented a qualitative description of band gap for quantum rods using the concepts of quantum confinement and band mixing. They reported that the emission peak positions are more sensitive to width than the length of nano rod. In our case, when the sample is calcined at 700 °C, the ZrO2 nanorods of mean diameter (width) 10 nm are obtained and the band gap value is calculated to be 4.8 eV. The increase in value of band gap compared to the ZrO2 sample calcined at 600 and 700 °C may be due the decrease of mean diameter (17–10 nm) of ZrO2 nanorods than that of the size of ZrO2 nanostructures calcined at 600 °C and 700 °C. This type of variation in optical band gap has also been observed by Chetri et al. [33] in SnO2 nanoparticles. They explained the increase in band gap by Burstein Moss effect. According to their explanation the increase in concentration of oxygen vacancies produce the donor levels which may leads to the increase in electrons near the conduction band edge. 1
OO ⇆ 2 O2 ↑ + VO • • + 2e−1 The defects are conveniently written in Kroger–Vink notation.
4. Conclusion ZrO2 nanostructures calcined at 500, 600, and 700 °C have been prepared by sol–gel method. The structural analysis reveals tetragonal to monoclinic phase transition with increasing the calcinations temperature. TEM analysis incorporates the finding of XRD result and reveals that the particle size also plays a significant role to stabilize the phase of ZrO2. UV–vis. spectra reveal a weak absorption band due to electronic transition from valance to conduction band. The optical band gap reveals a strong dependence on shape and size of the ZrO2 nanostructures. Raman Spectra reveal six characteristic features for tetragonal ZrO2 calcined at 500 °C and mix phases in the samples calcined at 600 and 700 °C. In the sample calcined at 700 °C, 15 Raman active modes were observed out of 18 vibrational modes of monoclinic ZrO2. TG analysis reveals the better thermal stability of monoclinic phase over the tetragonal phase of ZrO2.
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