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Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound
Author’s Accepted Manuscript Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound Hema Dutta, Anshuman Nandy, S.K. Pradhan www.elsevier.com/locate/jpcs
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S0022-3697(16)30055-5 http://dx.doi.org/10.1016/j.jpcs.2016.03.011 PCS7753
To appear in: Journal of Physical and Chemistry of Solids Received date: 8 October 2015 Revised date: 20 March 2016 Accepted date: 28 March 2016 Cite this article as: Hema Dutta, Anshuman Nandy and S.K. Pradhan, Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2016.03.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound Hema Dutta1, Anshuman Nandy2 and S. K. Pradhan2* 1
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Department of Physics, Vivekananda College, Burdwan-713103, West Bengal, India.
Department of Physics, The University of Burdwan, Burdwan-713104, West Bengal, India.
Abstract
A ZrO2 -TiO2 solid solution is obtained by high energy ball milling of equimolar mixture of monoclinic (m) ZrO2 and anatase (a) TiO2. Nanocrystalline orthorhombic ZrTiO4 compound is initiated from the nucleation of TiO2- ZrO2 solid solution with isostructural s-TiO2 (srilankite) base after 30 min of milling. After 12 hours of miliing, 95 mol% non- stoichiometric ZrTiO4 phase is formed. Post-annealing of 12h ball-milled powder mixture at 1073K for 1h in open air results in complete formation of stoichiometric ZrTiO4 compound. Microstructures of all powder mixtures milled for different durations have been characterized by Rietveld’s structure and microstructure refinement method using X-ray powder diffraction data. HRTEM images of 12h milled and annealed samples provide direct evidence of the results obtained from the Rietveld analysis. Optical bandgaps of ball milled and annealed ZrTiO4 compounds lie within the semiconducting region (~2.0eV) and increases with increase in milling time.
Keywords: Microstructure; X-Ray diffraction; TEM; Optical properties. *
Corresponding author, e-mail: [email protected]; Fax: +913422530452
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1. Introduction In recent years, zirconium titanate (ZrTiO4) compounds are extensively being used as dielectric resonators for microwave telecommunications [1- 3]. These materials are also used as structural ceramics because of their high fracture toughness, wear resistance and low thermal conductivity [4]. They have wide range of applications including high-temperature pigments [5], humidity sensors [6] and bi-functional catalysts [7, 8]. It is well established that in applications like catalysts, sensors and bioactive film materials, materials with high surface area, chemical purity and compositional homogeneity offers better performance. Generally, the mixed oxide method of preparing ZrTiO4 compound needs the heating of ZrO2 and TiO2 powder mixture above 1200–1600º C for a long duration [9]. This method was widely used in the last decade [1012]. In order to obtain fine-grained high quality stoichiometric ZrTiO4 powders at low processing temperatures, various chemical routes like hydrolysis of alkoxides [13], sol–gel [14], Pechini route[15] and co-precipitation [16] had been developed as alternatives to the conventional solid state reaction of mixed oxides [9, 17]. All of these techniques are intended to reduce the processing temperature of the compound even though these methods are more complicated than the mixed oxide route. In a previous work, Andreja et al. [18] reported formation of ZrTiO4 ceramics (only 5.3 wt %) from equimolar powder mixture of TiO2 and ZrO2 by high energy ball milling. But the authors failed to complete solid state reaction at room temperature and sintered 7h ball-milled powder in the temperature range 1100-1400ºC to obtain the final compound. Till now there is no report on detailed microstructure characterization of ZrTiO4 ceramics prepared by mechanochemical/ ball milling method at room temperature. Therefore, the main objectives of this work are (i) to explore a simple preparation route for ZrTiO4 powder via high 2
energy ball-milling technique at room temperature, (ii) microstructure characterization of the synthesized compound in terms of different kinds of lattice imperfections (iii) to study the phase transformation kinetics towards the production of ZrTiO4 ceramics during mechanical alloying and (iv) to study the effect of particle size on the optical band gap of ZrTiO4 compound. High energy ball-milling is a non-conventional way of producing nanocrystalline materials [19-22] and creates immense interest in the field of industrial research. High energy ball-milling introduces several mechanochemical phase transitions during synthesis of ceramic powder materials and the final product contains different phases with nano sized particles [2225]. We have adopted ball-milling method in the present work for preparation of nanocrystalline ZrTiO4 compound as an energy saving low cost method. To characterize prepared materials in terms of several microstructural defect parameters (change in lattice parameters, particle size, rms lattice strain, etc) XRD profiles of unmilled mixture and ball milled powders are analyzed employing Rietveld refinement technique [26-32]. This novel whole profile fitting method is one of the best methods for microstructure characterization and quantitative estimations of multiphase nanocrystalline material containing huge amount of lattice imperfections. Formation of ZrTiO4 phase by mechanical alloying is confirmed from (i) the analysis of XRD pattern of post-annealed (12h ball-milled, heat treated at 1073K for 1h) m-ZrO2 -a-TiO2 powder mixture containing reflection of orthorhombic ZrTiO4 phase only and (ii) study of HRTEM images of both 12 h as milled and annealed powders, which corroborates the results obtained from XRD pattern analysis employing the Rietveld method. The m-ZrO2 insulator turns into low band gap semiconductor through mixing of a-TiO2 at room temperature by mechanical alloying.
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To the best of our knowledge, the present work first time reports on preparation of nanocrystalline ZrTiO4 compound by mechanical alloying at room temperature and systematic study of phase formation through microstructure characterization.
2. Experimental Equimolar (1:1) mixture of elemental blend of monoclinic (m) ZrO2 and anatase (a) TiO2 powders was ball-milled in a high energy planetary ball mill (Model P5, Fritsch, GmbH, Germany) at room temperature using a vial of 80ml volume and 30 balls of 10 mm diameter, both made of harden chrome steel. The duration of milling varied from 5min to 12 h with intermediate pauses. The 12h ball-milled powder mixture was annealed at 1073K for 1h to complete the formation of ZrTiO4 phase by mechanical alloying. The X-ray powder diffraction patterns of the unmilled mixture, all ball-milled and postannealed powders were recorded in step scan (step size 0.02º 2θ, counting time 5 s) mode using Ni-filtered CuK radiation from a highly stabilized automated Philips X-ray generator (PW 1830) operated at 35 kV and 25 mA. The generator is coupled with a Philips X-ray powder diffractometer consisting of a PW 3710 mpd controller, PW 1050/37 goniometer and a proportional counter. For this experiment, 1º divergence slit, 0.2 mm receiving slit and 1º scatter slit were used. The step-scan data of the experimental samples were recorded for the entire angular range (15–80º 2θ) and stored in a PC, coupled with the diffractometer. Microstructure as well as selected area electron diffraction (SAED) patterns of both 12h milled and annealed powders were directly imaged using high resolution transmission electron microscopy (HRTEM) operated at 200 KV (Model JEOL JEM 2100). TEM specimens of powders were prepared by dissolving little bit of powder in doubly distilled acetone using 4
ultrasonic vibrator to avoid agglomeration of the fine particles. A small drop of the well dispersed solution was carefully placed on 3mm carbon coated grid and dried for several hours. UV-Vis spectra were recorded by Shimadzu UV-1800 spectrometer. The powder specimens were dispersed in alcohol and the optical absorbance value was recorded in the wavelength range with 1 nm step size. 3. Results and discussions 3. 1. Microstructure evolution by X-ray diffraction In the present study, we have adopted the Rietveld structure/microstructure refinement method [26-36] to analyze X-ray powder diffraction patterns of unmilled and all ball milled powders to obtain the refined structural and microstructural parameters, such as particle size and r.m.s. lattice strain using MAUD 2.26 [32] software with pseudo-Voigt profile fitting function considering asymmetry in the profile. To simulate the theoretical X-ray powder pattern following phases are incorporated in a single pattern (i) anatase (a- TiO2) (tetragonal; S. G.: I41/amd; ICDD PDF #21-1272) (ii) rutile (r- TiO2) (tetragonal; S. G.: P42/mmm; ICDD PDF #34-0180) (iii) srilankite (s- TiO2) (orthorhombic; S. G.: Pcab; ICDD PDF #23-1446) (iv) monoclinic (m)-ZrO2 (S. G.: P21/c ; ICDD PDF #37-1484) (v) cubic (c)-ZrO2(S. G.: Fm ̅ m; ICDD PDF#27-0997) and (vi) ZrTiO4 (orthorhombic; S.G.: Pbcn; ICDD PDF #34-0415). Fig. 1(a) illustrates an atomic model of ZrTiO4 structure and an enlarged portion representing its unit cell is shown in Fig. 1(b). In the orthorhombic unit cell, Ti atom can replace any Zr atom and thereby form a substitutional solid solution of ZrO2-TiO2 in large scale. Formation of Zr/Ti-O polyhedral structure with several (Zr/Ti)O6 octahedra is shown fig. 1(c) and an isolated (Zr/Ti)O6 octahedron with different (Zr/Ti)-O bond lengths and angles is shown
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in fig.1(d). It may be noted that the bond lengths as well as bond angles are not identical in all directions and as a result octahedra are tilted in ZrTiO4 lattice. The Marquardt least-squares procedure is adopted for minimization the difference between the observed (Io) and simulated (Ic) intensities of powder diffraction patterns and the minimization is carried out by using the reliability index parameter, Rwp (weighted residual error), RB (Bragg factor) and Rexp (expected error) [26-32]. This leads to the value of goodness of fit [28-30]: GoF = Rwp/Rexp Refinement continues till convergence is reached with the value of the quality factor, GoF approaching 1, which confirms the goodness of refinement. The Rietveld method was successfully applied for determination of the quantitative phase abundances of different kinds of nanocrystalline composite materials [37-40] and also adopted in the present case for quantitative estimation of different pertinent phases in the unmilled and all ball milled powders. The X-ray powder diffraction patterns of unmilled and ball milled (5, 15 and 30min, 1, 4, 8 and 12h) ZrO2 –TiO2 (1:1 molar ratio) powder mixture are shown in Fig. 2. The XRD powder pattern of unmilled sample is composed of only the reflections of starting m-ZrO2 and a-TiO2 phases with respective intensities according to their composition in the homogeneous mixture. The quantitative estimation and microstructural characterization of the individual phases have been carried out employing Rietveld’s whole X-ray profile fitting powder structure refinement method (MAUD 2.26) [26-32]. All the experimental patterns (Io) are fitted with the theoretically simulated and refined patterns (Ic) and some selected pattern are shown in Fig.3. From the almost linear residuals (Io-Ic), plotted at the bottom of individual patterns, it is clearly evident that all the
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reflections of all phases have been fitted quite well. The quality factors of fitting, GoF with very close value to 1 (1.07-1.48) also indicate the good quality of fitting [28-30]. Results of the Rietveld analysis are plotted in Figs.4-6 and precise analyses of these plots reveal the phase estimation and microstructure of the unmilled and all ball-milled TiO2-ZrO2 powder mixtures. Figs. 2 and 3 depict that in course of milling, intensities of a-TiO2 reflections decrease continuously with milling time. Phase identification indicates that a-TiO2 phase transforms to orthorhombic high pressure srilankite (s-TiO2) phase at the early stage of milling. At a glance, it is very difficult to notice the presence of high-pressure s-TiO2 phase in the ball milled XRD patterns because the characteristic diffraction peaks of this phase are very much broadened owing to high lattice strain value of nanometric s-TiO2 particles. In the previous works, Ren et al. [20], Begin-Colin et al. [23] and we [21] have also noticed the similar type of polymorphic phase transformation in ball milled a-TiO2 powder. It is also evident from Figs. 2 and 3 that intensities of both a-TiO2 and s-TiO2 decrease continuously with increasing milling time. It may be reasoned towards the initial transformation of a-TiO2 to s-TiO2 phase and then formation of mZrO2 – s-TiO2 solid solution. Critical observations of Figs.2 and 3 reveal that after 1h of milling, all reflections of a-TiO2 are completely disappeared in the XRD pattern, indicating complete phase transformation of a-TiO2 to s-TiO2. Noticeable change is observed at ~30º 2θ after 30 min of milling with an indication of appearance of strongest (111) reflection of ZrTiO4 compound. We also notice a few weak reflections of c- ZrO2 and r- TiO2 in the XRD pattern of all ballmilled powder before sintering. A noticeable change in the relative intensities of strongest ( ̅ 11) (r.i. 100%, 2θ~28.2º) and 2nd strongest (111) (r.i. 68%, 2θ~31.5º) reflections of m-ZrO2 phase has been observed after the formation of ZrTiO4 with increasing milling time. After 4 h of milling, intensities of these 7
reflections became apparently equal due to partially overlapping of strongest (111) reflection of ZrTiO4 phase with (111) reflection of m-ZrO2 phase. Continuous growth of ZrTiO4 phase changes the nature of the diffraction pattern considerably after 8 h of milling (Fig. 3). The (111) reflection of m-ZrO2 phase along with the partially overlapped (111) reflection of ZrTiO4 phase appears as strongest reflection during this milling time. Mechanical alloying up to 12h results in ZrTiO4 phase formation and the corresponding XRD patterns (Figs. 2 and 3) contains primarily the reflections of ZrTiO4 phase. These figures also illustrate that the ZrTiO4 phase gradually develops in expense of m-ZrO2-s-TiO2 solid solution with increasing milling time which agrees well with the mol fraction values obtained from Rietveld XRD pattern analysis (Fig. 4). Nature of variation in Fig. 4 depicts the mol fraction of a-TiO2 phase gradually decreases with the onset of milling and becomes nil after 1h. Formation of high pressure s-TiO2 phase at the same time clearly indicates a-TiO2 has been transformed to s-TiO2 phase within a short milling time. It is also evident that contents of both s-TiO2 and m-ZrO2 phases decrease continuously after nucleation of ZrTiO4 phase. Mol fraction of ZrTiO4 phase gradually increases with increasing milling time indicating m-ZrO2- s-TiO2 solid solution phase has been formed with progress of milling and instantaneously converted to ZrTiO4 phase. After 12h milling time ~95 mol% ZrTiO4 has been formed and final ball-milled powder mixture contains ZrTiO4 phase with a trace amount of m-ZrO2 and c-ZrO2 phases. Variations of lattice parameters of orthorhombic ZrTiO4 are depicted in Fig. 5(a). Nature of plots clearly indicate initially ‘c’ lattice parameter decreases with increasing milling time in between 30min to 1h milling and then remains almost invariant upto 12h of milling. Lattice parameter ‘a’ decreases and ‘b’ increases slowly with increasing milling time. It is also evident that (inset: Fig. 5(a)) (111) reflection has been shifted continuously towards higher angle which 8
indicates that ZrTiO4 lattice is contracted in the course of ball milling. It is observed that volume of the ZrTiO4 unit cell contracted from 0.135 nm3 (30min) to 0.129 nm3 (12h) with increasing milling time up to 12h (Fig. 5(b)). Particle sizes (coherently diffracting domain size) of the starting powders m-ZrO2 and aTiO2 are 34nm and 156nm respectively, measured by Rietveld method. Both particle size and lattice strain values of all these phases are found to be isotropic in nature and plotted in Fig. 6(a) and 6(b) respectively. Critical examination of Fig. 6(a) reveals that within 15min of milling, size of a-TiO2 particles rapidly reduces to ~60nm and at the same time transforms to s-TiO2 phase with nearly same size of a-TiO2 particles. Particle sizes of both s-TiO2 and m-ZrO2 decrease at a fast rate with increasing milling time up to 30 min and become nearly equal of each other. This equality in particle size values of s-TiO2 and m-ZrO2 play an important role to form the m-ZrO2s-TiO2 solid solution from which ZrTiO4 phase is nucleated. It is interesting to note that particle size of ZrTiO4 has a combined value (~27nm) of s-TiO2 and m-ZrO2 particle sizes at the time of formation. Particle size of ZrTiO4 decreases at a relatively slow rate up to 4h of milling and reduces to ~10nm after 12 h of milling. Nature of plot of particle sizes clearly indicate that ZrTiO4 phase is grown up from m-ZrO2- s-TiO2 solid solution. The variation of r.m.s. lattice strain value (Fig. 6(b)) depicts ZrTiO4 particles are nucleated with high lattice strain, that releases in a considerable amount with increasing milling time. This phenomenon is indicative of some kind of annealing effect [21]. The HRTEM and SEM images of 12h ball-milled sample are shown in Figs. 7. Fig. 7(a) gives direct evidence that the shape of ZrTiO4 particles (marked by white ring) are almost spherical in nature with a size of same order of magnitude (~10nm) as found from XRD analysis by Rietveld method. Fig. 7(b) represents the indexed selected area electron diffraction (SAED) 9
pattern and it reveals that ZrTiO4 has major phase content in 12h milled sample. From HRTEM image (fig. 7(c)) interplanar spacing of the planes in an isolated nanocrystalline particle is calculated and the (111) plane of orthorhombic ZrTiO4 with spacing 0.291 nm has been identified. It confirms the near full formation of ZrTiO4 phase in 12h milling. Fig. 7(d) represents another HRTEM image revealing high strain zone (marked by white polygon) of (111) plane. This high concentration of lattice strain results in peak broadening of ZrTiO4 reflections and the amount of lattice strain in the lattice has been measured by Rietveld refinement of XRD pattern and shown in Fig. 6(b). HRTEM micrograph of Fig. 7(e) illustrates the fast fourier transformed (FFT) pattern which also confirms the formation ZrTiO4 phase. The brightest spot corresponds to (111) plane of ZrTiO4. In the FFT pattern, the horizontal and vertical axes represent a* and b* axes respectively and all spots in the image are indexed accordingly. The FESEM micrographs of unmilled ZrO2-TiO2 powder mixture and 12 hour milled ZrTiO4 compound are shown in Fig. 7(f) and 7(g). It is evident that the average particle sizes obtained from these images are quite comparable as obtained from Rietveld alanysis. X-ray powder diffraction pattern of 12 h milled sample annealed at 1073 K along with the 12 h as-milled (inset, for comparison) sample are shown in Fig. 8. Comparing the intensity and line position of individual reflection of ball-milled and annealed samples it is clearly evident that annealing treatment results in release of lattice strain of ZrTiO4, as the XRD pattern of annealed powder shows sharp and well resolved reflections. This pattern contains only reflections of stoichiometric ZrTiO4 phase and confirms the formation of ZrTiO4 compound by mechanical alloying without any impurity phase either from the precursors or from the milling media.
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HRTEM image obtained from the 12 h ball-milled powder annealed at 1073 K are presented in Fig. 9. The indexed SAED pattern (Fig. 9(a)) resembles well with its XRD pattern (Fig. 8). From Fig. 9(b) interplanar spacing of the planes of annealed ZrTiO4 particle (~11nm particle size) is calculated and the respective values 0.291 and 0.247 nm are attributed to (111) and (200) planes of orthorhombic ZrTiO4 phase. It may also be noted that the lattice planes are perfectly parallel in the focused zone and also in some other areas (not shown). It indicates that the lattice strain accumulated in the ZrTiO4 nanoparticles during ball milling has been released significantly during annealing at 1073K. 3.2. Optical Bandgap: The optical bandgap is determined for the 8 h and 12 h ball-milled ZrTiO4 compounds by Ultraviolet-Visible (UV-Vis) spectroscopy. ZrTiO4 shows light absorption due to a metal legend charge transfer between Ti4+-O2-. The wavelength dependent absorbance spectra of 8 h milled sample are shown in the Fig. 10(a). The compound exhibits 30% absorbance value at band gap energy which is 1.82 eV. The absorption peak is appeared at 680nm and thus the bandgap is in visible range of spectrum. For the determination of accurate band-gap value, Tauc formula is used in the present study. [41]. αhγ =β (hγ- Eg)n where α is the absorbance, hγ is the photon energy, Eg is the band gap energy. β= 1(constant) and n is a constant whose value is 0.5 for direct band gap semiconductor. The Tauc plot for the 8 h milled sample is shown in the Fig. 10(b). The calculated band gap from the intercept of the slope is 1.81 eV. As the 8 h milled compound is composed of 87 mol% ZrTiO4 phase, so the optical absorbance is exhibited mostly by ZrTiO4 phase. The bandgap value ascertains that this material shows semiconducting properties. 11
Fig. 10(c) shows the absorbance spectrum of 12 h milled compound. Here the maximum absorbance is found at 573.18nm wavelength which is again in the visible wavelength region. The absorption edge shifted towards lower wavelength which indicates a blue shift in bandgap energy. The corresponding bandgap value is shown in the Fig. 10(c) and also calculated from Tauc formula. The Tauc plot and the calculated band gap is shown in Fig. 10(d). The optical bandgap value for 12 h milled compound is found to be 2.18 eV. This sample is composed of 95 mol% ZrTiO4 phase, so the optical absorbance is almost only due to ZrTiO4 compound. Bandgap value increases from 1.81 to 2.18 eV due to decrease in particle size value from 15.6 nm to 10.9 nm with increase in milling time. The optical bandgap is also determined for ZrTiO4 compound annealed at 1073K. The absorbance spectrum is shown in Fig. 10(e) and Tauc plot is shown in Fig. 10(f). A sharper absorption peak for the annealed compound indicates that the particles are monodispersed. Decrease in band gap energy from 2.18eV to 2.1eV after annealing the 12h milled compound is due to small increase in particle size from 10.9nm to 11.5nm due to annealing. The 1073K annealed compound is well crystalline, almost strain free with 11.5nm particle size and its optical bandgap is 2.1eV. Thus this material can be suitably used as a semiconductor in various applications. The optical bandgap study also reveals that the bandgap energy of semiconducting ZrTiO4 nanocrystals can be fine tuned by controlling the particle size alone. 4. Conclusion: The orthorhombic ZrTiO4 phase has been prepared by mechanical alloying the equimolar composition of m-ZrO2 and a-TiO2 powder mixture at room temperature. ZrTiO4 compound has been initiated from the isostructural s-TiO2 based TiO2–ZrO2 solid-solution after 30 min of milling. 12 h ball-milling produces ZrTiO4 compound with a major phase content (~95 mol%) 12
with some minor phases. When 12 h ball-milled compound is annealed at 1073 K for 1h, stoichiometric single ZrTiO4 phase has been formed. A detailed microstructural analysis confirms the gradual formation of ZrTiO4 compound. HRTEM study gives direct evidence of the formation and structure of ZrTiO4 nanoparticles and release of lattice strain from the milled powder after annealing. Optical property study reveals that the compound shows optical absorption in visible wavelength region and band gap of ZrTiO4 is around 2.1 eV which lies in semiconducting region. Optical bandgap of ZrTiO4 compound increases due to reduction in particle size due to ball milling. Annealing of ZrTiO4 ball milled compound at 1073K results in growth of strain free 11.5nm nanoparticles with 2.1 eV optical bandgap which can be used as a semiconductor in different applications.
Acknowledgement: The authors wish to thank to the University Grants Commission (UGC), India for granting CASI programme under the thrust area ‘‘Condensed Matter Physics including Laser applications’’ to the Dept. of Physics, The University of Burdwan and S.K.P. also thankful to UGC for granting the Major Research Project [F. No. 41-845/2012(SR)] under the financial assistance of which the work has been carried out.
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[34] J.D. Jorgensen, Z. Hu, S. Teslic, D.N. Argyriou and V. Short, Pressure-induced cubic-toorthorhombic phase transition in ZrW2O8, Phys. Rev. B 59 (1999) 215. [35] S. Yamazaki and H. Toraya, Rietveld refinement of site-occupancy parameters of Mg2xMnxSiO4
using a new weight function in least-squares fitting, J. Appl. Crystallogr. 32 (1999)
51-59. [36] H. Dutta, M. Sinha, Y.C. Lee and S.K. Pradhan, Microstructure characterization and phase transformation kinetics of ball-mill prepared nanocrystalline Mg–Zn-ferrite by Rietveld's analysis and electron microscopy, Mat. Chem. Phys. 105 (2007) 31-37. [37] H. Toraya, Estimation of statistical uncertainties in quantitative phase analysis using the Rietveld method and the whole-powder-pattern decomposition method, J. Appl. Crystallogr. 33 (2000) 1324-1328. [38] H. Dutta, S.K. Manik and S.K. Pradhan, Phase transformation kinetic study and microstructure characterization of ball milled m-ZrO2-10 mol% a-TiO2 by Rietveld method, J. Appl. Crystallogr. 36 (2003) 260–268. [39] S.K. Manik, H. Dutta and S.K. Pradhan, Microstructure characterization and phase transformation kinetics of polymorphic transformed ball milled a-TiO 2–10 mol% m-ZrO 2 mixture by Rietveld method, Mat. Chem. Phys. 82 (2003) 848-859. [40] A. Gajovic, N. Tomasic, I. Djerdj, D.S. Su and K. Furic, Influence of mechanochemical processing to luminescence properties in Y2O3 powder, J. Alloys Compds. 456 (2008) 313–319. [41] J. C. Tauc, Optical Properties of Solids. Amsterdam: North-Holland; 1972.
17
a
b
c
d
Fig. 1(a) Atomic model of ZrTiO4. (b) ZrTiO4 unit cell. (c) (Zr/Ti)O6 octahedral arrangement in ZrTiO4 lattice. (d) an isolated ZrTiO4 octahedron with different (Zr/Ti)-O bond lengths and bond angles.
18
10000
Intensity (arb. units)
8000
6000 0min
5min 4000
15min 30min 1h
2000 4h
8h 12h 0 Srilankite M- ZrO2 ZrTiO4
Anatase Rutile
C- ZrO2
20
30
40
50
60
70
80
2 (degree)
Fig. 2. X-ray powder diffraction patterns of unmilled and ball-milled m-ZrO2-50 mol% a-TiO2 powder mixtures. The peak positions of different phases are shown as small bars at the bottom of the figure.
19
8000
o I 0 6000
Ic
0 min
Intensity (arb. units)
Io-Ic
30 min 4000
Io-Ic
1h 2000
Io-Ic
4h
Io-Ic
12h
Io-Ic
0
Anatase Rutile
20
30
40 50 2degree)
60
70
Srilankite M- ZrO2 ZrTiO4 C- ZrO2
80
Fig. 3. Typical Rietveld output of X-ray powder diffraction patterns of ball-milled m-ZrO2 -aTiO2 powder mixtures. Experimental data points are shown as hollow circles, while refined simulated patterns are shown as continuous lines. The difference between the experimental data (Io) and the fitted simulated pattern (Ic) is shown as a continuous line (Io - Ic) under each diffraction pattern.
20
1.0
0.8
Mol fraction
a-TiO2 m-ZrO2 0.6
c-ZrO2 s-TiO2 r-TiO2
0.4
ZrTiO4 0.2
0.0 0
2
4
6
8
10
12
Milling Time(h)
Fig. 4.Variations of mol fractions of different phases obtained by ball milling of m-ZrO2 -a-TiO2 mixture powders with increasing milling time.
21
Intensity (arb. units)
a b c
0.64
Lattice parameter (nm)
2500
0.60
2000
30
35
40
ZrTiO4
1500
1000
45
0.136
a
30min
0.135
Unit Cell Volume
1h
0.134
4h
Volume (nm3)
ZrTiO4
0.68
8h
500 12h
2 (degree)
0.56
0.133
b
0.132 0.131 0.130
0.52
0.129 0
0.48
2
4
6
8
10
12
Milling time (h)
0
2
4
6
8
10
12
Milling time (h)
Fig. 5. (a) Variations of lattice parameters of ZrTiO4 compound with increasing milling time. Inset: Shifting of (111) peak towards higher angle in XRD pattern of m-ZrO2 -a-TiO2 mixture powders. (b) Variation in unit cell volume of ZrTiO4 phase.
22
160
100
m-ZrO2
c-ZrO2
s-TiO2
r-TiO2
ZrTiO4
80 60 30
R.M.S strain x103
Particle size (nm)
a 80
a-TiO2
60
40
20
b
a-TiO2 m-ZrO2
25
c-ZrO2
20
s-TiO2
15
ZrTiO4
r-TiO2
10 5 0
0 0
2
4
6
8
10
0
12
2
4
6
8
10
12
Milling time(h)
Milling Time (h)
Fig. 6. Variations of (a) particle size (b) r.m.s strain of different phases obtained by ball milling m-ZrO2 -a-TiO2 mixture powders with increasing milling time.
23
b
a
c
d
e
f
g
Fig. 7. HRTEM of 12h ball milled equimolar m-ZrO2 -a-TiO2 mixture powders (a) transmission micrograph showing spherical ZrTiO4 particles, (b) indexed selected area electron diffraction (SAED) pattern, (c) micrograph containing (111) planes in a nanocrystalline particle, (d) micrograph indicating high strain zone of (111) plane of ZrTiO4, (e) micrograph of fast fourier transformed (FFT) pattern, (f) FESEM micrograph of unmilled powder mixture and (g) FESEM micrograph of 12 hour milled ZrTiO4 powder.
24
(111)
900
ZrTiO4
800
12h ball-milled
700
1500
600 500
30
40
50
60
70
(114) (241)
(022)
(020) (002)
20
(113) (222) (023) (132)
(121)
500
(202) (221)
400
1000
(110)
Intensity (arb. unit)
2000
12 h ball-milled+ annealed(1073K)
0
ZrTiO4
20
40
2 (degree)
60
80
Fig. 8. XRD pattern of 12 h milled m-ZrO2 -a-TiO2 mixture powders annealed at 1073 K. Inset: XRD pattern of 12 h ball-milled powder sample.
25
b
a
Fig. 9. HRTEM micrographs of 12 h milled m-ZrO2 -a-TiO2 mixture powders annealed at 1073 K, (a) indexed selected area electron diffraction (SAED) pattern (b) micrograph containing (111) and (200) planes.
26
0.31
0.8
ZrTiO4, 8 h milled
Absorbance
h
0.30
0.6
a
h
Absorbance
0.29 0.28 0.27
b 0.4
0.26
680 nm 0.25
(Eg=1.82 eV)
0.24 300
400
500
600
700
800
900
1.81 eV
0.2
1000
1.2
Wavelength (nm)
1.6
2.4
2.8
3.2
heV)
0.268
1.2
ZrTiO4, 12 h milled
h
Absorbance
1.0
h
0.264
Absorbance
2.0
0.260
c
0.8
d 0.6
0.256 0.4
573.18 nm 0.252
2.18 eV
(Eg=2.163 eV)
400
500
600
700
800
0.2 1.5
900
2.0
Wavelength (nm)
4.0
h
Absorbance
0.25
h
Absorbance
3.5
0.30
e
0.005
f
0.20
0.15
584nm (Eg=2.123 ev) 0.000 300
3.0
heV)
ZrTiO4, 12 h milled, 1073K annealed 0.010
2.5
400
500
0.10
2.10 ev
0.05 600
700
800
1.0
Wavelength (nm)
1.5
2.0
2.5
3.0
heV)
3.5
4.0
4.5
Fig. 10. (a) UV-Vis absorbance spectra and respective (b) Tauc plot of m-ZrO2-50 mol% a-TiO2 powder mixtures milled for 8 h. (c) UV-Vis absorbance spectra and respective (d) Tauc plot of m-ZrO2-50 mol% a-TiO2 powder mixtures milled for 12 h. (e) UV-Vis absorbance spectra and respective (f) Tauc plot of 12 h milled m-ZrO2 -a-TiO2 mixture powders annealed at 1073 K.
27
Highlights: 12 h ball-milling produces orthorhombic ZrTiO4 compound at room temperature. A detailed microstructural analysis is carried out. HRTEM study confirms the formation of ZrTiO4 nanoparticles. Optical bandgap of ZrTiO4 is found to be in semiconducting region. The bandgap of 1073K annealed semiconducting ZrTiO4 nanoparticles is 2.1eV.
28
Graphical Abstract
X-ray diffraction pattern and atomic structure of ZrTiO4 compound:
(111)
900
ZrTiO4
800
12h ball-milled
700
1500
600 500
(022)
30
40
50
60
70
(114) (241)
(121)
20
(113) (222) (023) (132)
(020) (002)
(110)
500
(202) (221)
400
1000
12 h ball-milled+ annealed(1073K)
0
ZrTiO4
20
40
2 (degree)
60
80
Optical bandgap of semiconducting ZrTiO4 compound:
0.30
ZrTiO4, 12 h milled, 1073K annealed
h
Absorbance
0.25
h
0.010
Absorbance
Intensity (arb. unit)
2000
0.15
0.005
0.10
584nm (Eg=2.123 ev) 0.000 300
0.20
400
500
2.10 ev
0.05 600
700
Wavelength (nm)
800
1.0
1.5
2.0
2.5
3.0
heV)
3.5
4.0
4.5
PII: DOI: Reference:
S0022-3697(16)30055-5 http://dx.doi.org/10.1016/j.jpcs.2016.03.011 PCS7753
To appear in: Journal of Physical and Chemistry of Solids Received date: 8 October 2015 Revised date: 20 March 2016 Accepted date: 28 March 2016 Cite this article as: Hema Dutta, Anshuman Nandy and S.K. Pradhan, Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2016.03.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound Hema Dutta1, Anshuman Nandy2 and S. K. Pradhan2* 1
2
Department of Physics, Vivekananda College, Burdwan-713103, West Bengal, India.
Department of Physics, The University of Burdwan, Burdwan-713104, West Bengal, India.
Abstract
A ZrO2 -TiO2 solid solution is obtained by high energy ball milling of equimolar mixture of monoclinic (m) ZrO2 and anatase (a) TiO2. Nanocrystalline orthorhombic ZrTiO4 compound is initiated from the nucleation of TiO2- ZrO2 solid solution with isostructural s-TiO2 (srilankite) base after 30 min of milling. After 12 hours of miliing, 95 mol% non- stoichiometric ZrTiO4 phase is formed. Post-annealing of 12h ball-milled powder mixture at 1073K for 1h in open air results in complete formation of stoichiometric ZrTiO4 compound. Microstructures of all powder mixtures milled for different durations have been characterized by Rietveld’s structure and microstructure refinement method using X-ray powder diffraction data. HRTEM images of 12h milled and annealed samples provide direct evidence of the results obtained from the Rietveld analysis. Optical bandgaps of ball milled and annealed ZrTiO4 compounds lie within the semiconducting region (~2.0eV) and increases with increase in milling time.
Keywords: Microstructure; X-Ray diffraction; TEM; Optical properties. *
Corresponding author, e-mail: [email protected]; Fax: +913422530452
1
1. Introduction In recent years, zirconium titanate (ZrTiO4) compounds are extensively being used as dielectric resonators for microwave telecommunications [1- 3]. These materials are also used as structural ceramics because of their high fracture toughness, wear resistance and low thermal conductivity [4]. They have wide range of applications including high-temperature pigments [5], humidity sensors [6] and bi-functional catalysts [7, 8]. It is well established that in applications like catalysts, sensors and bioactive film materials, materials with high surface area, chemical purity and compositional homogeneity offers better performance. Generally, the mixed oxide method of preparing ZrTiO4 compound needs the heating of ZrO2 and TiO2 powder mixture above 1200–1600º C for a long duration [9]. This method was widely used in the last decade [1012]. In order to obtain fine-grained high quality stoichiometric ZrTiO4 powders at low processing temperatures, various chemical routes like hydrolysis of alkoxides [13], sol–gel [14], Pechini route[15] and co-precipitation [16] had been developed as alternatives to the conventional solid state reaction of mixed oxides [9, 17]. All of these techniques are intended to reduce the processing temperature of the compound even though these methods are more complicated than the mixed oxide route. In a previous work, Andreja et al. [18] reported formation of ZrTiO4 ceramics (only 5.3 wt %) from equimolar powder mixture of TiO2 and ZrO2 by high energy ball milling. But the authors failed to complete solid state reaction at room temperature and sintered 7h ball-milled powder in the temperature range 1100-1400ºC to obtain the final compound. Till now there is no report on detailed microstructure characterization of ZrTiO4 ceramics prepared by mechanochemical/ ball milling method at room temperature. Therefore, the main objectives of this work are (i) to explore a simple preparation route for ZrTiO4 powder via high 2
energy ball-milling technique at room temperature, (ii) microstructure characterization of the synthesized compound in terms of different kinds of lattice imperfections (iii) to study the phase transformation kinetics towards the production of ZrTiO4 ceramics during mechanical alloying and (iv) to study the effect of particle size on the optical band gap of ZrTiO4 compound. High energy ball-milling is a non-conventional way of producing nanocrystalline materials [19-22] and creates immense interest in the field of industrial research. High energy ball-milling introduces several mechanochemical phase transitions during synthesis of ceramic powder materials and the final product contains different phases with nano sized particles [2225]. We have adopted ball-milling method in the present work for preparation of nanocrystalline ZrTiO4 compound as an energy saving low cost method. To characterize prepared materials in terms of several microstructural defect parameters (change in lattice parameters, particle size, rms lattice strain, etc) XRD profiles of unmilled mixture and ball milled powders are analyzed employing Rietveld refinement technique [26-32]. This novel whole profile fitting method is one of the best methods for microstructure characterization and quantitative estimations of multiphase nanocrystalline material containing huge amount of lattice imperfections. Formation of ZrTiO4 phase by mechanical alloying is confirmed from (i) the analysis of XRD pattern of post-annealed (12h ball-milled, heat treated at 1073K for 1h) m-ZrO2 -a-TiO2 powder mixture containing reflection of orthorhombic ZrTiO4 phase only and (ii) study of HRTEM images of both 12 h as milled and annealed powders, which corroborates the results obtained from XRD pattern analysis employing the Rietveld method. The m-ZrO2 insulator turns into low band gap semiconductor through mixing of a-TiO2 at room temperature by mechanical alloying.
3
To the best of our knowledge, the present work first time reports on preparation of nanocrystalline ZrTiO4 compound by mechanical alloying at room temperature and systematic study of phase formation through microstructure characterization.
2. Experimental Equimolar (1:1) mixture of elemental blend of monoclinic (m) ZrO2 and anatase (a) TiO2 powders was ball-milled in a high energy planetary ball mill (Model P5, Fritsch, GmbH, Germany) at room temperature using a vial of 80ml volume and 30 balls of 10 mm diameter, both made of harden chrome steel. The duration of milling varied from 5min to 12 h with intermediate pauses. The 12h ball-milled powder mixture was annealed at 1073K for 1h to complete the formation of ZrTiO4 phase by mechanical alloying. The X-ray powder diffraction patterns of the unmilled mixture, all ball-milled and postannealed powders were recorded in step scan (step size 0.02º 2θ, counting time 5 s) mode using Ni-filtered CuK radiation from a highly stabilized automated Philips X-ray generator (PW 1830) operated at 35 kV and 25 mA. The generator is coupled with a Philips X-ray powder diffractometer consisting of a PW 3710 mpd controller, PW 1050/37 goniometer and a proportional counter. For this experiment, 1º divergence slit, 0.2 mm receiving slit and 1º scatter slit were used. The step-scan data of the experimental samples were recorded for the entire angular range (15–80º 2θ) and stored in a PC, coupled with the diffractometer. Microstructure as well as selected area electron diffraction (SAED) patterns of both 12h milled and annealed powders were directly imaged using high resolution transmission electron microscopy (HRTEM) operated at 200 KV (Model JEOL JEM 2100). TEM specimens of powders were prepared by dissolving little bit of powder in doubly distilled acetone using 4
ultrasonic vibrator to avoid agglomeration of the fine particles. A small drop of the well dispersed solution was carefully placed on 3mm carbon coated grid and dried for several hours. UV-Vis spectra were recorded by Shimadzu UV-1800 spectrometer. The powder specimens were dispersed in alcohol and the optical absorbance value was recorded in the wavelength range with 1 nm step size. 3. Results and discussions 3. 1. Microstructure evolution by X-ray diffraction In the present study, we have adopted the Rietveld structure/microstructure refinement method [26-36] to analyze X-ray powder diffraction patterns of unmilled and all ball milled powders to obtain the refined structural and microstructural parameters, such as particle size and r.m.s. lattice strain using MAUD 2.26 [32] software with pseudo-Voigt profile fitting function considering asymmetry in the profile. To simulate the theoretical X-ray powder pattern following phases are incorporated in a single pattern (i) anatase (a- TiO2) (tetragonal; S. G.: I41/amd; ICDD PDF #21-1272) (ii) rutile (r- TiO2) (tetragonal; S. G.: P42/mmm; ICDD PDF #34-0180) (iii) srilankite (s- TiO2) (orthorhombic; S. G.: Pcab; ICDD PDF #23-1446) (iv) monoclinic (m)-ZrO2 (S. G.: P21/c ; ICDD PDF #37-1484) (v) cubic (c)-ZrO2(S. G.: Fm ̅ m; ICDD PDF#27-0997) and (vi) ZrTiO4 (orthorhombic; S.G.: Pbcn; ICDD PDF #34-0415). Fig. 1(a) illustrates an atomic model of ZrTiO4 structure and an enlarged portion representing its unit cell is shown in Fig. 1(b). In the orthorhombic unit cell, Ti atom can replace any Zr atom and thereby form a substitutional solid solution of ZrO2-TiO2 in large scale. Formation of Zr/Ti-O polyhedral structure with several (Zr/Ti)O6 octahedra is shown fig. 1(c) and an isolated (Zr/Ti)O6 octahedron with different (Zr/Ti)-O bond lengths and angles is shown
5
in fig.1(d). It may be noted that the bond lengths as well as bond angles are not identical in all directions and as a result octahedra are tilted in ZrTiO4 lattice. The Marquardt least-squares procedure is adopted for minimization the difference between the observed (Io) and simulated (Ic) intensities of powder diffraction patterns and the minimization is carried out by using the reliability index parameter, Rwp (weighted residual error), RB (Bragg factor) and Rexp (expected error) [26-32]. This leads to the value of goodness of fit [28-30]: GoF = Rwp/Rexp Refinement continues till convergence is reached with the value of the quality factor, GoF approaching 1, which confirms the goodness of refinement. The Rietveld method was successfully applied for determination of the quantitative phase abundances of different kinds of nanocrystalline composite materials [37-40] and also adopted in the present case for quantitative estimation of different pertinent phases in the unmilled and all ball milled powders. The X-ray powder diffraction patterns of unmilled and ball milled (5, 15 and 30min, 1, 4, 8 and 12h) ZrO2 –TiO2 (1:1 molar ratio) powder mixture are shown in Fig. 2. The XRD powder pattern of unmilled sample is composed of only the reflections of starting m-ZrO2 and a-TiO2 phases with respective intensities according to their composition in the homogeneous mixture. The quantitative estimation and microstructural characterization of the individual phases have been carried out employing Rietveld’s whole X-ray profile fitting powder structure refinement method (MAUD 2.26) [26-32]. All the experimental patterns (Io) are fitted with the theoretically simulated and refined patterns (Ic) and some selected pattern are shown in Fig.3. From the almost linear residuals (Io-Ic), plotted at the bottom of individual patterns, it is clearly evident that all the
6
reflections of all phases have been fitted quite well. The quality factors of fitting, GoF with very close value to 1 (1.07-1.48) also indicate the good quality of fitting [28-30]. Results of the Rietveld analysis are plotted in Figs.4-6 and precise analyses of these plots reveal the phase estimation and microstructure of the unmilled and all ball-milled TiO2-ZrO2 powder mixtures. Figs. 2 and 3 depict that in course of milling, intensities of a-TiO2 reflections decrease continuously with milling time. Phase identification indicates that a-TiO2 phase transforms to orthorhombic high pressure srilankite (s-TiO2) phase at the early stage of milling. At a glance, it is very difficult to notice the presence of high-pressure s-TiO2 phase in the ball milled XRD patterns because the characteristic diffraction peaks of this phase are very much broadened owing to high lattice strain value of nanometric s-TiO2 particles. In the previous works, Ren et al. [20], Begin-Colin et al. [23] and we [21] have also noticed the similar type of polymorphic phase transformation in ball milled a-TiO2 powder. It is also evident from Figs. 2 and 3 that intensities of both a-TiO2 and s-TiO2 decrease continuously with increasing milling time. It may be reasoned towards the initial transformation of a-TiO2 to s-TiO2 phase and then formation of mZrO2 – s-TiO2 solid solution. Critical observations of Figs.2 and 3 reveal that after 1h of milling, all reflections of a-TiO2 are completely disappeared in the XRD pattern, indicating complete phase transformation of a-TiO2 to s-TiO2. Noticeable change is observed at ~30º 2θ after 30 min of milling with an indication of appearance of strongest (111) reflection of ZrTiO4 compound. We also notice a few weak reflections of c- ZrO2 and r- TiO2 in the XRD pattern of all ballmilled powder before sintering. A noticeable change in the relative intensities of strongest ( ̅ 11) (r.i. 100%, 2θ~28.2º) and 2nd strongest (111) (r.i. 68%, 2θ~31.5º) reflections of m-ZrO2 phase has been observed after the formation of ZrTiO4 with increasing milling time. After 4 h of milling, intensities of these 7
reflections became apparently equal due to partially overlapping of strongest (111) reflection of ZrTiO4 phase with (111) reflection of m-ZrO2 phase. Continuous growth of ZrTiO4 phase changes the nature of the diffraction pattern considerably after 8 h of milling (Fig. 3). The (111) reflection of m-ZrO2 phase along with the partially overlapped (111) reflection of ZrTiO4 phase appears as strongest reflection during this milling time. Mechanical alloying up to 12h results in ZrTiO4 phase formation and the corresponding XRD patterns (Figs. 2 and 3) contains primarily the reflections of ZrTiO4 phase. These figures also illustrate that the ZrTiO4 phase gradually develops in expense of m-ZrO2-s-TiO2 solid solution with increasing milling time which agrees well with the mol fraction values obtained from Rietveld XRD pattern analysis (Fig. 4). Nature of variation in Fig. 4 depicts the mol fraction of a-TiO2 phase gradually decreases with the onset of milling and becomes nil after 1h. Formation of high pressure s-TiO2 phase at the same time clearly indicates a-TiO2 has been transformed to s-TiO2 phase within a short milling time. It is also evident that contents of both s-TiO2 and m-ZrO2 phases decrease continuously after nucleation of ZrTiO4 phase. Mol fraction of ZrTiO4 phase gradually increases with increasing milling time indicating m-ZrO2- s-TiO2 solid solution phase has been formed with progress of milling and instantaneously converted to ZrTiO4 phase. After 12h milling time ~95 mol% ZrTiO4 has been formed and final ball-milled powder mixture contains ZrTiO4 phase with a trace amount of m-ZrO2 and c-ZrO2 phases. Variations of lattice parameters of orthorhombic ZrTiO4 are depicted in Fig. 5(a). Nature of plots clearly indicate initially ‘c’ lattice parameter decreases with increasing milling time in between 30min to 1h milling and then remains almost invariant upto 12h of milling. Lattice parameter ‘a’ decreases and ‘b’ increases slowly with increasing milling time. It is also evident that (inset: Fig. 5(a)) (111) reflection has been shifted continuously towards higher angle which 8
indicates that ZrTiO4 lattice is contracted in the course of ball milling. It is observed that volume of the ZrTiO4 unit cell contracted from 0.135 nm3 (30min) to 0.129 nm3 (12h) with increasing milling time up to 12h (Fig. 5(b)). Particle sizes (coherently diffracting domain size) of the starting powders m-ZrO2 and aTiO2 are 34nm and 156nm respectively, measured by Rietveld method. Both particle size and lattice strain values of all these phases are found to be isotropic in nature and plotted in Fig. 6(a) and 6(b) respectively. Critical examination of Fig. 6(a) reveals that within 15min of milling, size of a-TiO2 particles rapidly reduces to ~60nm and at the same time transforms to s-TiO2 phase with nearly same size of a-TiO2 particles. Particle sizes of both s-TiO2 and m-ZrO2 decrease at a fast rate with increasing milling time up to 30 min and become nearly equal of each other. This equality in particle size values of s-TiO2 and m-ZrO2 play an important role to form the m-ZrO2s-TiO2 solid solution from which ZrTiO4 phase is nucleated. It is interesting to note that particle size of ZrTiO4 has a combined value (~27nm) of s-TiO2 and m-ZrO2 particle sizes at the time of formation. Particle size of ZrTiO4 decreases at a relatively slow rate up to 4h of milling and reduces to ~10nm after 12 h of milling. Nature of plot of particle sizes clearly indicate that ZrTiO4 phase is grown up from m-ZrO2- s-TiO2 solid solution. The variation of r.m.s. lattice strain value (Fig. 6(b)) depicts ZrTiO4 particles are nucleated with high lattice strain, that releases in a considerable amount with increasing milling time. This phenomenon is indicative of some kind of annealing effect [21]. The HRTEM and SEM images of 12h ball-milled sample are shown in Figs. 7. Fig. 7(a) gives direct evidence that the shape of ZrTiO4 particles (marked by white ring) are almost spherical in nature with a size of same order of magnitude (~10nm) as found from XRD analysis by Rietveld method. Fig. 7(b) represents the indexed selected area electron diffraction (SAED) 9
pattern and it reveals that ZrTiO4 has major phase content in 12h milled sample. From HRTEM image (fig. 7(c)) interplanar spacing of the planes in an isolated nanocrystalline particle is calculated and the (111) plane of orthorhombic ZrTiO4 with spacing 0.291 nm has been identified. It confirms the near full formation of ZrTiO4 phase in 12h milling. Fig. 7(d) represents another HRTEM image revealing high strain zone (marked by white polygon) of (111) plane. This high concentration of lattice strain results in peak broadening of ZrTiO4 reflections and the amount of lattice strain in the lattice has been measured by Rietveld refinement of XRD pattern and shown in Fig. 6(b). HRTEM micrograph of Fig. 7(e) illustrates the fast fourier transformed (FFT) pattern which also confirms the formation ZrTiO4 phase. The brightest spot corresponds to (111) plane of ZrTiO4. In the FFT pattern, the horizontal and vertical axes represent a* and b* axes respectively and all spots in the image are indexed accordingly. The FESEM micrographs of unmilled ZrO2-TiO2 powder mixture and 12 hour milled ZrTiO4 compound are shown in Fig. 7(f) and 7(g). It is evident that the average particle sizes obtained from these images are quite comparable as obtained from Rietveld alanysis. X-ray powder diffraction pattern of 12 h milled sample annealed at 1073 K along with the 12 h as-milled (inset, for comparison) sample are shown in Fig. 8. Comparing the intensity and line position of individual reflection of ball-milled and annealed samples it is clearly evident that annealing treatment results in release of lattice strain of ZrTiO4, as the XRD pattern of annealed powder shows sharp and well resolved reflections. This pattern contains only reflections of stoichiometric ZrTiO4 phase and confirms the formation of ZrTiO4 compound by mechanical alloying without any impurity phase either from the precursors or from the milling media.
10
HRTEM image obtained from the 12 h ball-milled powder annealed at 1073 K are presented in Fig. 9. The indexed SAED pattern (Fig. 9(a)) resembles well with its XRD pattern (Fig. 8). From Fig. 9(b) interplanar spacing of the planes of annealed ZrTiO4 particle (~11nm particle size) is calculated and the respective values 0.291 and 0.247 nm are attributed to (111) and (200) planes of orthorhombic ZrTiO4 phase. It may also be noted that the lattice planes are perfectly parallel in the focused zone and also in some other areas (not shown). It indicates that the lattice strain accumulated in the ZrTiO4 nanoparticles during ball milling has been released significantly during annealing at 1073K. 3.2. Optical Bandgap: The optical bandgap is determined for the 8 h and 12 h ball-milled ZrTiO4 compounds by Ultraviolet-Visible (UV-Vis) spectroscopy. ZrTiO4 shows light absorption due to a metal legend charge transfer between Ti4+-O2-. The wavelength dependent absorbance spectra of 8 h milled sample are shown in the Fig. 10(a). The compound exhibits 30% absorbance value at band gap energy which is 1.82 eV. The absorption peak is appeared at 680nm and thus the bandgap is in visible range of spectrum. For the determination of accurate band-gap value, Tauc formula is used in the present study. [41]. αhγ =β (hγ- Eg)n where α is the absorbance, hγ is the photon energy, Eg is the band gap energy. β= 1(constant) and n is a constant whose value is 0.5 for direct band gap semiconductor. The Tauc plot for the 8 h milled sample is shown in the Fig. 10(b). The calculated band gap from the intercept of the slope is 1.81 eV. As the 8 h milled compound is composed of 87 mol% ZrTiO4 phase, so the optical absorbance is exhibited mostly by ZrTiO4 phase. The bandgap value ascertains that this material shows semiconducting properties. 11
Fig. 10(c) shows the absorbance spectrum of 12 h milled compound. Here the maximum absorbance is found at 573.18nm wavelength which is again in the visible wavelength region. The absorption edge shifted towards lower wavelength which indicates a blue shift in bandgap energy. The corresponding bandgap value is shown in the Fig. 10(c) and also calculated from Tauc formula. The Tauc plot and the calculated band gap is shown in Fig. 10(d). The optical bandgap value for 12 h milled compound is found to be 2.18 eV. This sample is composed of 95 mol% ZrTiO4 phase, so the optical absorbance is almost only due to ZrTiO4 compound. Bandgap value increases from 1.81 to 2.18 eV due to decrease in particle size value from 15.6 nm to 10.9 nm with increase in milling time. The optical bandgap is also determined for ZrTiO4 compound annealed at 1073K. The absorbance spectrum is shown in Fig. 10(e) and Tauc plot is shown in Fig. 10(f). A sharper absorption peak for the annealed compound indicates that the particles are monodispersed. Decrease in band gap energy from 2.18eV to 2.1eV after annealing the 12h milled compound is due to small increase in particle size from 10.9nm to 11.5nm due to annealing. The 1073K annealed compound is well crystalline, almost strain free with 11.5nm particle size and its optical bandgap is 2.1eV. Thus this material can be suitably used as a semiconductor in various applications. The optical bandgap study also reveals that the bandgap energy of semiconducting ZrTiO4 nanocrystals can be fine tuned by controlling the particle size alone. 4. Conclusion: The orthorhombic ZrTiO4 phase has been prepared by mechanical alloying the equimolar composition of m-ZrO2 and a-TiO2 powder mixture at room temperature. ZrTiO4 compound has been initiated from the isostructural s-TiO2 based TiO2–ZrO2 solid-solution after 30 min of milling. 12 h ball-milling produces ZrTiO4 compound with a major phase content (~95 mol%) 12
with some minor phases. When 12 h ball-milled compound is annealed at 1073 K for 1h, stoichiometric single ZrTiO4 phase has been formed. A detailed microstructural analysis confirms the gradual formation of ZrTiO4 compound. HRTEM study gives direct evidence of the formation and structure of ZrTiO4 nanoparticles and release of lattice strain from the milled powder after annealing. Optical property study reveals that the compound shows optical absorption in visible wavelength region and band gap of ZrTiO4 is around 2.1 eV which lies in semiconducting region. Optical bandgap of ZrTiO4 compound increases due to reduction in particle size due to ball milling. Annealing of ZrTiO4 ball milled compound at 1073K results in growth of strain free 11.5nm nanoparticles with 2.1 eV optical bandgap which can be used as a semiconductor in different applications.
Acknowledgement: The authors wish to thank to the University Grants Commission (UGC), India for granting CASI programme under the thrust area ‘‘Condensed Matter Physics including Laser applications’’ to the Dept. of Physics, The University of Burdwan and S.K.P. also thankful to UGC for granting the Major Research Project [F. No. 41-845/2012(SR)] under the financial assistance of which the work has been carried out.
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[14] J.A. Navio, F.J. Marchena, M. Macias, P.J. Sanchez-Soto and P. Pichat, Formation of zirconium titanate powder from a sol-gel prepared reactive precursor, J. Mater. Sci. 27 (1992) 2463–2467. [15] A. Bianco, M. Paci and R. Freer, Zirconium titanate: from polymeric precursors to bulk ceramics, J. Eur. Ceram. Soc. 18 (1998) 1235–1243. [16] M. Daturi, A. Cremona, F. Milella, G. Busca and E. Vogna, Characterization of zirconiatitania powders prepared by coprecipitation, J. Eur. Ceram. Soc. 18 (1998) 1079–1087. [17] M. Dondi, F. Matteucci and G. Cruciani, Zirconium titanate ceramic pigments: Crystal structure, optical spectroscopy and technological properties, J. solid state chem. 179 (2006) 233246. [18] A. Gajović, I. Djerdj, K. Furić, R. Schlögl and D. S. Su, Preparation of nanostructured ZrTiO4 by solid state reaction in equimolar mixture of TiO2 and ZrO2, Cryst. Res. Technol. 41 (2006) 1076-1081. [19] N.J. Welham, T. Kerr and P.E. Willis, Ambient-Temperature Mechanochemical Formation of Titanium Nitride-Alumina Composites from TiO2 and FeTiO3, J. Am. Ceram. Soc. 82 (1999) 2332-2336. [20] R. Ren, Z. Yang and L.L. Shaw, Polymorphic transformation and powder characteristics of TiO2 during high energy milling, J. Mater. Sci. 35 (2000) 6015-6026. [21] H. Dutta, P. Sahu, S.K. Pradhan and M. De, Microstructure characterization of polymorphic transformed ball-milled anatase TiO2 by Rietveld method, Mater. Chem. Phys. 77 (2003) 153164. [22] H. Dutta and S.K. Pradhan, Microstructure characterization of high energy ball-milled nanocrystalline V2O5 by Rietveld analysis, Mater. Chem. Phys. 77 (2003) 868-877.
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[23] S. Begin-Colin, T. Girot, G. Le Caer and A. Mocellin, Kinetics and Mechanisms of Phase Transformations Induced by Ball-Milling in Anatase TiO2, J. Solid State Chem. 149 (2000) 4148. [24] S. Sen, M.L. Ram, S. Roy and B.K. Sarkar, The structural transformation of anatase TiO2 by high-energy vibrational ball milling, J. Mater. Res. 14 (1999) 841-848. [25] M. Sinha, H. Dutta and S.K. Pradhan, Phase stability of nanocrystalline Mg–Zn ferrite at elevated temperatures, Jpn. J. Appl. Phys. 47 (2008) 8667. [26] H.M. Rietveld, Line profiles of neutron powder-diffraction peaks for structure refinement, Acta Crystallogr. 22 (1967) 151-152. [27] H.M. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65-71. [28] R.A. Young and D.B. Wiles, Profile shape functions in Rietveld refinements, J. Appl. Crystallogr. 15 (1982) 430-438. [29] L. Lutterotti, P. Scardi and P. Maistrelli, LSI-a computer program for simultaneous refinement of material structure and microstructure, J. Appl. Crystallogr. 25 (1992) 459-462. [30] R.A. Young, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press/IUCr, 1996, 1-38. [31] S.K. Manik and S.K. Pradhan, X-ray microstructure characterization of ball-milled nanocrystalline microwave dielectric CaZrO3 by Rietveld method, J. Appl. Crystallogr. 38 (2005) 291-298. [32] L. Lutterotti, MAUD version 2.26. http://www.ing.unitn.it/_/luttero/maud, 2002. [33] H. Toraya, Weighting scheme for the minimization function in Rietveld refinement, J. Appl. Crystallogr. 31 (1998) 333-343.
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17
a
b
c
d
Fig. 1(a) Atomic model of ZrTiO4. (b) ZrTiO4 unit cell. (c) (Zr/Ti)O6 octahedral arrangement in ZrTiO4 lattice. (d) an isolated ZrTiO4 octahedron with different (Zr/Ti)-O bond lengths and bond angles.
18
10000
Intensity (arb. units)
8000
6000 0min
5min 4000
15min 30min 1h
2000 4h
8h 12h 0 Srilankite M- ZrO2 ZrTiO4
Anatase Rutile
C- ZrO2
20
30
40
50
60
70
80
2 (degree)
Fig. 2. X-ray powder diffraction patterns of unmilled and ball-milled m-ZrO2-50 mol% a-TiO2 powder mixtures. The peak positions of different phases are shown as small bars at the bottom of the figure.
19
8000
o I 0 6000
Ic
0 min
Intensity (arb. units)
Io-Ic
30 min 4000
Io-Ic
1h 2000
Io-Ic
4h
Io-Ic
12h
Io-Ic
0
Anatase Rutile
20
30
40 50 2degree)
60
70
Srilankite M- ZrO2 ZrTiO4 C- ZrO2
80
Fig. 3. Typical Rietveld output of X-ray powder diffraction patterns of ball-milled m-ZrO2 -aTiO2 powder mixtures. Experimental data points are shown as hollow circles, while refined simulated patterns are shown as continuous lines. The difference between the experimental data (Io) and the fitted simulated pattern (Ic) is shown as a continuous line (Io - Ic) under each diffraction pattern.
20
1.0
0.8
Mol fraction
a-TiO2 m-ZrO2 0.6
c-ZrO2 s-TiO2 r-TiO2
0.4
ZrTiO4 0.2
0.0 0
2
4
6
8
10
12
Milling Time(h)
Fig. 4.Variations of mol fractions of different phases obtained by ball milling of m-ZrO2 -a-TiO2 mixture powders with increasing milling time.
21
Intensity (arb. units)
a b c
0.64
Lattice parameter (nm)
2500
0.60
2000
30
35
40
ZrTiO4
1500
1000
45
0.136
a
30min
0.135
Unit Cell Volume
1h
0.134
4h
Volume (nm3)
ZrTiO4
0.68
8h
500 12h
2 (degree)
0.56
0.133
b
0.132 0.131 0.130
0.52
0.129 0
0.48
2
4
6
8
10
12
Milling time (h)
0
2
4
6
8
10
12
Milling time (h)
Fig. 5. (a) Variations of lattice parameters of ZrTiO4 compound with increasing milling time. Inset: Shifting of (111) peak towards higher angle in XRD pattern of m-ZrO2 -a-TiO2 mixture powders. (b) Variation in unit cell volume of ZrTiO4 phase.
22
160
100
m-ZrO2
c-ZrO2
s-TiO2
r-TiO2
ZrTiO4
80 60 30
R.M.S strain x103
Particle size (nm)
a 80
a-TiO2
60
40
20
b
a-TiO2 m-ZrO2
25
c-ZrO2
20
s-TiO2
15
ZrTiO4
r-TiO2
10 5 0
0 0
2
4
6
8
10
0
12
2
4
6
8
10
12
Milling time(h)
Milling Time (h)
Fig. 6. Variations of (a) particle size (b) r.m.s strain of different phases obtained by ball milling m-ZrO2 -a-TiO2 mixture powders with increasing milling time.
23
b
a
c
d
e
f
g
Fig. 7. HRTEM of 12h ball milled equimolar m-ZrO2 -a-TiO2 mixture powders (a) transmission micrograph showing spherical ZrTiO4 particles, (b) indexed selected area electron diffraction (SAED) pattern, (c) micrograph containing (111) planes in a nanocrystalline particle, (d) micrograph indicating high strain zone of (111) plane of ZrTiO4, (e) micrograph of fast fourier transformed (FFT) pattern, (f) FESEM micrograph of unmilled powder mixture and (g) FESEM micrograph of 12 hour milled ZrTiO4 powder.
24
(111)
900
ZrTiO4
800
12h ball-milled
700
1500
600 500
30
40
50
60
70
(114) (241)
(022)
(020) (002)
20
(113) (222) (023) (132)
(121)
500
(202) (221)
400
1000
(110)
Intensity (arb. unit)
2000
12 h ball-milled+ annealed(1073K)
0
ZrTiO4
20
40
2 (degree)
60
80
Fig. 8. XRD pattern of 12 h milled m-ZrO2 -a-TiO2 mixture powders annealed at 1073 K. Inset: XRD pattern of 12 h ball-milled powder sample.
25
b
a
Fig. 9. HRTEM micrographs of 12 h milled m-ZrO2 -a-TiO2 mixture powders annealed at 1073 K, (a) indexed selected area electron diffraction (SAED) pattern (b) micrograph containing (111) and (200) planes.
26
0.31
0.8
ZrTiO4, 8 h milled
Absorbance
h
0.30
0.6
a
h
Absorbance
0.29 0.28 0.27
b 0.4
0.26
680 nm 0.25
(Eg=1.82 eV)
0.24 300
400
500
600
700
800
900
1.81 eV
0.2
1000
1.2
Wavelength (nm)
1.6
2.4
2.8
3.2
heV)
0.268
1.2
ZrTiO4, 12 h milled
h
Absorbance
1.0
h
0.264
Absorbance
2.0
0.260
c
0.8
d 0.6
0.256 0.4
573.18 nm 0.252
2.18 eV
(Eg=2.163 eV)
400
500
600
700
800
0.2 1.5
900
2.0
Wavelength (nm)
4.0
h
Absorbance
0.25
h
Absorbance
3.5
0.30
e
0.005
f
0.20
0.15
584nm (Eg=2.123 ev) 0.000 300
3.0
heV)
ZrTiO4, 12 h milled, 1073K annealed 0.010
2.5
400
500
0.10
2.10 ev
0.05 600
700
800
1.0
Wavelength (nm)
1.5
2.0
2.5
3.0
heV)
3.5
4.0
4.5
Fig. 10. (a) UV-Vis absorbance spectra and respective (b) Tauc plot of m-ZrO2-50 mol% a-TiO2 powder mixtures milled for 8 h. (c) UV-Vis absorbance spectra and respective (d) Tauc plot of m-ZrO2-50 mol% a-TiO2 powder mixtures milled for 12 h. (e) UV-Vis absorbance spectra and respective (f) Tauc plot of 12 h milled m-ZrO2 -a-TiO2 mixture powders annealed at 1073 K.
27
Highlights: 12 h ball-milling produces orthorhombic ZrTiO4 compound at room temperature. A detailed microstructural analysis is carried out. HRTEM study confirms the formation of ZrTiO4 nanoparticles. Optical bandgap of ZrTiO4 is found to be in semiconducting region. The bandgap of 1073K annealed semiconducting ZrTiO4 nanoparticles is 2.1eV.
28
Graphical Abstract
X-ray diffraction pattern and atomic structure of ZrTiO4 compound:
(111)
900
ZrTiO4
800
12h ball-milled
700
1500
600 500
(022)
30
40
50
60
70
(114) (241)
(121)
20
(113) (222) (023) (132)
(020) (002)
(110)
500
(202) (221)
400
1000
12 h ball-milled+ annealed(1073K)
0
ZrTiO4
20
40
2 (degree)
60
80
Optical bandgap of semiconducting ZrTiO4 compound:
0.30
ZrTiO4, 12 h milled, 1073K annealed
h
Absorbance
0.25
h
0.010
Absorbance
Intensity (arb. unit)
2000
0.15
0.005
0.10
584nm (Eg=2.123 ev) 0.000 300
0.20
400
500
2.10 ev
0.05 600
700
Wavelength (nm)
800
1.0
1.5
2.0
2.5
3.0
heV)
3.5
4.0
4.5