The effect of size on structural and optical properties of microwave dielectric ZrTiO4 powders

JOURNAL OF SCIENCE OF HNUE Interdisciplinary Science, 2013, Vol. 58, No. 5, pp. 3-10 This paper is available online at THE EFFECT OF SIZE ON STRUCTURAL AND OPTICAL PROPERTIES OF MICROWAVE DIELECTRIC ZrTiO4 POWDERS La Qui Hoan1 and Nguyen Van Minh2 1College of Education, Thai Nguyen University, 2Faculty of Physics, Hanoi National University of Education Abstract. In the present work, we use various routes, such as sol-gel, hydrothermal and solid state reaction methods, to synthesize zirconium titanate (ZrTiO4) and investigate the crystal structure, particle size distribution, morphology and optical properties of the calcined powders. The main purpose of this study is to investigate the effect of size on the structural and optical properties of ZrTiO4 powders. X-ray diffraction (XRD) patterns, optical absorption and Raman scattering were applied as the probes for the evolution of crystalline size and distribution of ZrTiO4 powders. The small physical dimensions of ZrTiO4 crystals leads to a shift and broadening of the first-order Raman bands. The details of the evolution of the size and distribution of the particles on the 630 and 800 cm−1 Raman line shape (frequency, broadening and asymmetry) are presented. The XRD, scanning electron microscopy (SEM), absorption and Raman scattering results of ZrTiO4 powders suggests evidence of an effect at the grain size. Keywords: Effect, size, structural and optical properties, ZrTiO4 powders, XRD, SEM. 1. Introduction Recently zirconium titanate-based compositions have become extensively used as dielectric resonators in microwave telecommunications [1]. They also are of interest for a wide range of applications including catalysis, humidity sensors, high-temperature pigments and composites [2]. It is known that all forms of ZrTiO4 have the orthorhombic structure of α-PbO2 [3] and belong to the space group Pbcn. The stoichiometry of zirconium titanate is known to be an important factor for ensuring good properties [2, 4]. In order to obtain fine-grained, high quality and stoichiometric zirconium titanate Received April 17, 2013. Accepted June 4, 2013. Contact Nguyen Van Minh, e-mail address: minhsp@gmail.com 3 La Qui Hoan and Nguyen Van Minh powders at low processing temperatures, various chemical routes, for example, hydrolysis of alkoxides [5], sol–gel [6], thin films [7] and co-precipitation [8], have been developed as alternatives to the conventional solid state reaction of mixed oxides [9]. All of these techniques are aimed at reducing the temperature of preparation and improving the quality of the compound even though they are more involved and complicated than the mixed oxide route. Many properties of the materials are known to be anomalous when the grain diameter approaches nanometer size. In such materials, because of the quantum size and surface effects, the position, intensity and width of the peaks in the vibrational spectra and the band gap from the optical absorption spectra can be significantly different from those of the bulk phases. As these changes depend strongly on the microstructure of the nanomaterials, the optical absorption and vibrational spectra may play an important role in their characterization. However, very few studies have been done concerning these effects. In this study, we present the effect of particle size on the structural and optical properties of microwave dielectric ZrTiO4 powders. 2. Content 2.1. Experimental procedure The material was prepared using a modified sol-gel method in which the titanium alcoxide is hydrolyzed in the presence of zirconium oxychloride, as opposed to hydrolysing in pure acidic water. For a ceramic ZrTiO4 sample, the powder claimed by sol-gel route was moulded into a disk 10 mm in diameter and 4 mm thick and then sintered in the air at a temperature of 1200 oC for 8 h and cooled in a furnace. For the hydrothermal method, titanium (IV) isopropoxide (TTIP) and a zirconium (IV) isopropoxide isopropanol complex were used as precursors for TiO2 and ZnO2, respectively. TTIP and zirconium (IV) isopropoxide isopropanol (TiO2/ZnO2 = 2:1) were mixed separately (each in 100 mL of HNO3 solution), as reported earlier [11], and were continuously stirred for 30 min., 0.9 g of NaOH (0.2M NaOH solution) was added to the TTIP and zirconium (IV) isopropoxide isopropanol solution and the solution pH rose to 13. The autoclave was placed on a furnace and slowly heated to 180 oC (2 oC/min) and kept for another 10 h at the same temperature. The product was then allowed to cool to room temperature. Finally, the mixed oxides were washed three times with distilled water, then with ethanol and dried at 120 oC for 12 h. The dried mixed oxides were then calcined at a temperature of 500 oC in an electric furnace (2 oC/min) and, finally, ground to a fine powder using a mortar and pestle. Structural characterization was performed by means of X-ray diffraction using a D5005 diffractometer with Cu Kα radiation. The FE-SEM observation was carried out by using a S4800 (Hitachi) microscope. Raman measurements were performed in a back scattering geometry using a Jobin Yvon T 64000 triple spectrometer equipped with a 4 The effect of size on structural and optical properties of microwave dielectric ZrTiO4 powders cryogenic charge-coupled device (CCD) array detector, and the 514.5 nm line of an Ar ion laser. The absorption spectra were recorded by using a Jasco 670 UV-vis spectrometer. 2.2. Results and discussions The shape and size of the ZrTiO4 particles were examined from SEM micrographs as shown in Figure 1. From the SEM images of the particles, it can be seen that the variation of microstructure and size depends on the route of synthesization. The particle sizes can be estimated from the SEM micrographs and are seen to be in the range of 8 - 300 nm. The particle sizes of the ZrTiO4 are 300, 30 and 8 nm, corresponding to samples prepared by solid state reaction, sol-gel and hydrothermal methods, respectively. In general, the particles are agglomerated and basically irregular in shape, with a substantial variation in particle size and morphology. The ZrTiO4 grains, synthesized using the hydrothermal method, are composed of homogeneous nanocrystalline particles with a grain size of less than 10 nm (see Figure 1c). The shape of the ZrTiO4 particles is spherical and they have an average size of 7 - 8 nm. Figure 1. SEM images of ZrTiO4 samples synthesized by (a): solid state reaction, (b): sol-gel and (c) hydrothermal routes The same spherical particle shape can be obtained using the sol-gel route, however, the average particle size is larger than when using the hydrothermal method. For the ceramic sample, the grains were very large at about 300 nm. The phase structure and crystallinity of ZrTiO4 were examined using XRD analysis. 5 La Qui Hoan and Nguyen Van Minh The XRD patterns of ZrTiO4, preparing by different routes, are shown in Figure 2. The strongest reflections apparent in the majority of the XRD patterns indicate the formation of the ZrTiO4 phase. In our experiment, the XRD patterns indicate the formation of an α-PbO2-type structure with an orthorhombic symmetry zirconium titanate phase, which could be matched with ICDD file No. 34 - 415. The orthorhombic unit cell was found to have the following dimensions: a = 5.00, b = 5.43 and c = 4.76 A˚ for the ceramic sample and a = 4.82, b = 5.48 and c = 4.83 A˚ for the sol-gel and hydrothermal samples. These values are in good agreement with those reported by Zhang et al. [12]. It can be seen from the XRD presented in Figure 2 that the particles show characteristic peaks of the zirconium titanate crystalline phase, the most important of which is the peak at 2θ = 30.43o. The peaks become broader as particle size decreases in agreement with the Scherer formula. From half width of the peak of 30.43o, using the Scherrer formula, the received particle sizes were around 12 nm, 26 nm, 320 nm for hydrothermal, sol gel and ceramic routes, respectively. The obtained particle size is consistent with the mean size observed by SEM experiments. The structural size effect has been previously reported by Wang and Herron [13] in small thiophenolate-caped CdS clusters. It has been found that the lattice constant decreases by ≈ 3%, respectively, to the bulk solid value. By comparison with our results, we see that the lattice deformation could lead to a decrease of the lattice constant. Figure 2. XRD patterns of ZrTiO4 synthesized by (a): solid state reaction, (b): sol-gel and (c) hydrothermal routes The absorption spectra of the samples are shown in Figure 3. It was known that UV-visible diffuse reflectance spectroscopy is a useful spectroscopic technique that probes the electronic structure and domain size of transition metal oxides. The position of 6 The effect of size on structural and optical properties of microwave dielectric ZrTiO4 powders the absorption edge is sensitive to the bonding between metal oxide polyhedra. According to Wood and Tauc [14], the band gap in the high-energy region of the absorbance spectra are related to the absorbance and photon energy by hvα = (hv −Eoptg )2 where, α is absorbance, h is Plank’s constant, v is the frequency and Eoptg is the optical band gap. The band gap is obtained by fitting the linear region of curves as illustrated in Figure 3. From this formula and experimental data we can calculate the band-gap energy of pure ZrTiO4 at close to 3.08 eV, 3.30 eV and 3.43 eV corresponding to ceramic, sol-gel and hydrothermal powders, respectively. The largest particles at 300 nm exhibit the spectrum characteristic of bulk solid zirconium titanate, which is different from the spectra of the two remaining powders. Poty R. de Lucena et al. [15] concluded that disordered ZrTiO4 presents a non-saturated absorption tail in the 2.0 - 4.0 eV energy range that broadens the absorbance curve of oxides. Compared to that, there is no tail formation in our data, suggesting the high order in present samples. Figure 3. Optical absorption spectra of ZrTiO4 synthesized by (a): solid state reaction, (b): sol-gel and (c) hydrothermal routes. The band gap is obtained by fitting the linear region of curves (dotted line) The reasons for the band gap blue shift using effective mass approximation (EMA) have been discussed in a number of publications [13]. Generally, the EMA works well for relatively large particle sizes in the weak confinement domain, and it begins to disagree with the experiment in the strong confinement domain of sizes. Therefore, the blue shift in this case can originate from a confinement effect due to decreased size. 7 La Qui Hoan and Nguyen Van Minh Figure 4 presents the Raman spectra for ZrTiO4 powders synthesized by various routes. In a study of ZrTiO4 catalytic properties, Reddy et al. [16] described some characteristic Raman peaks of the ZrTiO4 crystalline phase. The representation for the Raman active normal modes in ZrTiO4 with an α-PbO2 structure can be written using the following representation: Γ = 4Ag + 5B1g + 4B2g + 5B3g Figure 4. Raman spectra of ZrTiO4 was synthesized by (a): solid state reaction, (b): sol-gel and (c) hydrothermal routes The observed peak positions were compared with values reported in the literature for orthorhombic ZrTiO4 [15, 18-20] and are displayed in Table 1. As shown in the Raman band at or near 800 cm−1 (Figure 4), both the degree of the line broadening and the asymmetry in the line shape continuously change with the variation in particle size. This observation, together with the argument made in this section, suggests that the mode at 800 cm−1 reflects a continuous variation in the degree of cation positional ordering. We now focus our attention on the observed asymmetry in the Raman line shape. A phonon confinement concept has been used to explain the broad and asymmetric line shapes observed in Si and GaAs-based semiconductors [17]. This asymmetry is not expected in a single-crystal having a perfect translational symmetry. In this case, the phonons propagate as plane waves without any hindrance and only the Brillouin zone-center modes are Raman-active because of the conservation of crystal momentum. According to the uncertainty principle, however, the introduction of defects that limit the spatial correlation of phonons then gives rise to a relaxation of the q = 0 selection rule. In case of incommensurately ordered ZrTiO4, the origin of the phonon confinement can be attributed to a faulty boundary that induces an incommensurate (IC) structure and breaks the long-range translational symmetry. 8 The effect of size on structural and optical properties of microwave dielectric ZrTiO4 powders Figure 5. The intensity ratio of peaks at 800 and 600 cm−1 depends on particle size Table 1. Raman identified normal modes in crystalline ZrTiO4 [18] [19] [20] [15] This study - - - 85 - 131 124 - 131 135 160 154 - 160 161 - 258 260 - - 273 269 290 276 279 327 331 320 333 335 - 394 - - - 407 415 400 411 411 - 537 - - 538 579 590 580 566 - 626 - 603 612 637 646 640 637 648 - - - 768 - 792 795 800 802 800 3. Conclusion We have obtained pure ZrTiO4 phases by using sol gel, hydrothermal and solid state reaction routes. The particles were found to have sizes varying from 8, 20 to 300 nm for the hydrothermal, sol gel and ceramic route, respectively. The change in grain size resulted in some changes in cell parameters as well as optical band gap and Raman spectra. We have demonstrated that the analysis of the shift in absorption edge based on the phonon confinement model is a promising new approach to analyzing the role of nano-scale in nanomaterials. 9 La Qui Hoan and Nguyen Van Minh Acknowledgments: This work was supported by the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam and the Research Foundation – Flanders (FWO) of Belgium (Cod FWO.2011.23). REFERENCES [1] M. Leoni, M. Viviani, G. Battilana, A.M. Fiorello, M. Viticoli, 2001. J. Eur. Ceram. Soc., 21, pp. 1739-1741. [2] K. Tanabe, 1970. Kodansha, Tokyo, Academic Press. New York, London. [3] R.E. Newnham, 1967. J. Am. Ceram. Soc., 50, p. 216. [4] A.J. Moulson, J.M., 1990. Chapman & Hall, New York. [5] S. Hirano, T. Hayashi, A. Hattori, 1991. Soc. 74, pp. 1320-1324. [6] V. dos Santos, M. Zeni, J.M. Hohemberger and C.P. Bergmann, 2010. Rev. Adv. Mater. Sci., 24 , pp. 44-47. [7] D. Pamu, K. Sudheendran, M. Ghanashyam Krishna, K.C. James Raju, 2010. 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Among the various candidate solutions that have been proposed thus far, the Peccei-Quinn mechanism is the most attractive candidate as a solution of the strong CP problem where the CP-violating phase θ (θ 6 10−9) is explained by the existence of a new pseudo-scalar field called the axion [8]. At present, axion mass is constrained by laboratory [5], astrophysical and cosmological considerations [12, 13] to between 10−6 eV and 10−3 eV. If the axion has a mass near the low limit of order 10−5 eV, it is a good candidate for the dark matter of the universe. In addition, an axino (the fermionic partner of the axion) naturally appears in SUSY models [4] which acquires a mass from three-loop Feynman diagrams in a typical range of between a few eV to a maximum of 1 keV [14]. Candidates for dark matter can appear in different models, such as the 3-3-1 models [7] or in supersymmetric and superstring theories [2]. Light particles with a two photon interaction can be transformed into photons in an external electric or magnetic field by an effect first discussed by Primakoff [9]. This effect is the basis of Sikivie’s methods for the detection of axions in a resonant cavity [10]. Various terrestrial experiments to detect invisible axions by making use of their coupling to photons have been proposed [6] and results from such experiments have appeared recently [3]. The experiment CAST [1] at CERN searches Received January 15, 2013. Accepted May 24, 2013. Contact Le Nhu Thuc, e-mail address: thucln@hnue.edu.vn 11