Solid state synthesis and structural characterization of zinc titanates

Accepted Manuscript Solid State Synthesis and Structural Characterization of Zinc Titanates Sarra Ayed, Helmi Abdelkefi, Hamadi Khemakhem, Adel Matoussi PII:

S0925-8388(16)30865-9

DOI:

10.1016/j.jallcom.2016.03.244

Reference:

JALCOM 37132

To appear in:

Journal of Alloys and Compounds

Received Date: 11 January 2016 Revised Date:

19 March 2016

Accepted Date: 28 March 2016

Please cite this article as: S. Ayed, H. Abdelkefi, H. Khemakhem, A. Matoussi, Solid State Synthesis and Structural Characterization of Zinc Titanates, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.03.244. 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 proof before it is published in its final 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.

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Solid State Synthesis and Structural Characterization of Zinc Titanates Sarra Ayeda,*, Helmi Abdelkefib, Hamadi Khemakhemb and Adel Matoussia a

Laboratory of Composite Ceramic and Polymer Materials, Scientific Faculty of Sfax, Tunisia.

b

Laboratory of Ferroelectric Materials, Scientific Faculty of Sfax, Tunisia.

E-mail:[email protected]

Abstract:

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* Corresponding author: Phone number: +21693365002

Zinc titanate composite materials were synthesized via solid state sintering process using high-purity metal oxide powders (purity~99.99%). The titanium incorporation into ZnO

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matrix was investigated by X-ray diffraction which revealed the coexistence of spinel Zn2TiO4 and hexagonal ZnTiO3 with the ZnO wurtzite structures. No reflection peaks of rutile

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TiO2 phase were detected. The IR spectroscopy and Raman scattering spectroscopy were used to characterize the structural and chemical properties of the ZnO/TiO2 composites. The IR bands and vibrational modes of all crystalline phases were detected. The effect of TiO2 doping rates (x = 3, 5 and 7 wt %) on bands shifting, Raman intensity and structural quality was discussed.

1. Introduction:

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Keywords: Zinc titanates, sintering, IR spectroscopy, Raman spectroscopy.

Zinc titanates are interesting and technologically important materials that attracted great deals of research. They have been extensively investigated for many applications such as

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catalytic sorbents for the desulfurization of hot coal gases, white color pigments [1] and most importantly as dielectric materials for microwave devices [2-4]. As suggested by Levy [5-6],

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there are five compounds in ZnO/TiO2 system. However, only three compounds were confirmed to exist at high temperatures Zn2TiO4, ZnTiO3 and Zn2Ti3O8. Among these compounds, ilmenite-type hexagonal ZnTiO3 has been reported to possess superior electrical properties. It has a rhombohedra structure which can be decomposed into Zn2TiO4 and rutile TiO2 at T ≥ 945 °C during the solid state reaction process. So, it is considered to be a metastable phase above this temperature [7]. Synthesis and characterization of ZnO/TiO2 system have been reported by many authors [7-11]. Zinc titanates still catch the attention of the researchers. M.R. Vaezi and al. [10] synthesized Zn2TiO4/ ZnTiO3 via CBD (Chemical Bath Deposition) method and reported the temperature effect on morphologies and compositions of the compounds. They found that 1

ACCEPTED MANUSCRIPT the appropriate temperature of synthesis ZnO/ Zn2TiO4/ ZnTiO3 nanocomposite powders is in the range between 25 °C and 55 °C. Lei Hou and al. [11] have obtained ilmenite ZnTiO3 using sol gel process. The thermal behavior and phase transformation of the gels revealed that complete crystallization of hexagonal ZnTiO3 is obtained at about 800 °C. P. Vlazan and al. [12] have prepared TiO2/ZnO core-shell nanoparticles by hydrothermal method in two stages.

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They observed the agglomeration of nanoparticles, the decrease of the resistivity and the increase of the electrical conductivity with temperature. Yamaguchi and al. [13] elucidated that Zn2Ti3O8 is a low temperature form of ZnTiO3. Zn2TiO4 can be easily obtained by conventional solid state reaction between 2ZnO and 1TiO2. However, pure ZnTiO3

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preparation from a mixture of 1ZnO and 1TiO2 has not been successful due to its decomposition at about 945 °C. A. Stoyanova and al. [14] proved that the synthesized ZnO/TiO2 nanocomposites via nonhydrolitic method can be good inorganic antimicrobial

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agents. B.C. Yadav and al. [15] prepared nanocristalline zinc titanate via simple physicochemical process and demonstrated the sensing behavior of ZnTiO3 to liquefied petroleum gas. A. Shalaby and al. [16] applied a sol gel method due to obtain a nanocomposite material containing several active phases and possessing a powerful bactericidal effect. Actually, there are several methods to prepare zinc titanates ceramic

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materials but we chose to adopt conventional solid state reaction in our research paper because it is simpler to operate and uses cheap and easily available oxides as starting materials. In addition to this method of synthesis, we picked low TiO2 concentrations in order to avoid the segregation of rutile TiO2 secondary phase as occurred in our previous team work

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with high MgO concentrations [17].

The aim of the current work is to investigate the effect of the TiO2 incorporation into ZnO matrix via XRD analysis, infra red and Raman spectroscopy measurements, using high-

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purity raw materials.

2. Experimental details: The zinc titanates materials were prepared by using a conventional solid state sintering

process using pure ZnO and TiO2 powders (purity ~99.99%) as the starting materials. The chosen weight amounts of TiO2 were (x = 3, 5 and 7 wt %). The mixed components were homogenously milled in an agate and then calcined in air at 300°C for 3 hours. The mixed powders were pressed into pellets (of 1 mm in thickness and 8 mm in diameter) and sintered at high temperature 900°C for 24 hours with heating rate of 20°C/min. Pellets were then cooled slowly to room temperature. The Optical measurements were examined in our 2

ACCEPTED MANUSCRIPT previous study in the wavelength range 200-1000 nm via UV-VIS-NIR spectrophotometer (SHIMADZU UV-3101PC) and were compared with some literature data as shown in Table 1 [14,15,18]. Powder X-ray diffraction (XRD) studies were carried out in the scan range 2θ =25°- 60° using Bruker axs D8 advance diffractometer with Cu-Kα radiation wavelength of 0.15406 nm. IR spectroscopy was performed using Perkin Elmer spectrometer ranging from

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400 to 2500 cm-1. Raman spectra were executed by Micro-Raman Horiba HR 800 in the wave number range 50-2000 cm-1. All measurements were taken at room temperature. 3. Results and discussion: 3.1 XRD analysis:

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Fig.1 shows the XRD patterns of doped ZnO powder with different TiO2 concentrations. ZnO peaks are typically identified using JCPDS card N° 36-1451. The

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prominent diffraction peaks attributed to hexagonal wurtzite ZnO are located at 2θ = 31.82°, 34.48°, 36.31°, 47.59° and 56.60° which are respectively assigned to (100), (002), (101), (102) and (110) plane reflections. The TiO2 incorporation in ZnO matrix is proved by the appearance of some peaks referred to zinc titanates ceramic materials such as spinel Zn2TiO4 and ilmenite-type hexagonal ZnTiO3. Nevertheless, no reflection peaks of TiO2 structure are detected. It shows at 2θ = 29.88°, 36.72° and 42.74°, the Zn2TiO4 peaks attributed

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respectively to (112), (202) and (220) planes according to JCPDS card N° 19-1483. The hexagonal ZnTiO3 peaks are situated at 2θ = 35.16°, 53.00° and 56.66° assigned to (110), (116) and (018) respectively (JCPDS card N° 26-1500). The ionic crystal radii of Zn2+ and Ti4+ are respectively 0.75 and 0.61 A° which stimulate essentially the slight blue shift

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followed after that by a red shift for TiO2 doping rate x= 7 wt.% and the notable intensity changes of zinc titanates peaks as observed in the inset. These changes are most likely caused

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by the existence of uniform and non-uniform strains in doped ZnO [19]. In our case, the alloying is more enhanced with increasing TiO2 doping concentrations as demonstrated in our previous investigations [18], also, the intensities of ZnTiO3 peaks still rising until an amount x= 5wt% and decreasing after that. The TiO2 effect can be viewed also in the structural parameters such as lattice constants

(“a” and “c”), grain size and lattice stress calculated respectively from the equations below:

1 d (2hkl )

4 h 2 + k 2 + hk l2 = ( )+ 2 3 a2 c

(1)

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Kλ β hkl cos θ

(2)

σ =Yhkl ε

(3)

Where dhkl is the distance between adjacent planes in the Miller indices (hkl), D is the grain

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size, λ is the wavelength of the used X-ray radiation, k is the constant equal to 0.9, βhkl is the full width at half maximum (FWHM) of the diffraction peak, θ is its Bragg diffraction angle, Yhkl is Young’s modulus, ε is the strain constant and σ is the lattice stress. For a hexagonal structure, Young’s modulus is calculated as ~ 127 GPa [20].

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Fig.2.a) illustrates the decrease of lattice parameters just before the TiO2 doping rate 5 wt%. This decrease should be engendering lattice strain and stress in the bulk of synthesized

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ZnO/TiO2 ternaries. Indeed, it is seen in Fig.2.b the diminishing of lattice stress which can be generated most likely from the existence of impurities, defects and lattice distortion from 50.8 to -38.1 MPa [21]. However the decrease of the grain sizes from 69 to 33 nm can be related to the crystalline segregation effect and the feeble coalescence of nano-grains at 900 °C [19]. Similar results have been reported in literature [21,22]. M. S. Kim and al. [21] have observed the decrease of the average grain size of Al-doped ZnO thin films prepared by sol gel spin-

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coating method from 60 to 30 nm when the Al concentration increased from 0 to 3 at.% respectively. You-Hua and Meng Xia [22] have fabricated ZnTiO3 powders at 500 °C by using sol gel process where the average grain sizes are in the range 30-70 nm. Other works [16,23,24] have reported different results that can be essentially attributed to high TiO2

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compositions and the applied synthesis process at low temperature. A. D. BacharovaNedelcheva and al. [23] fabricated nanosized ZnO-TiO2 powders via combustion sol-gel

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method at 450 °C. They have obtained only ZnO phase with crystallite size 16 nm for low TiO2concentration about 5-10 mol%, but for composition 90 mol% TiO2 10 mol% ZnO they observed a mixture of anatase, rutile and ZnTiO3 with average grains size about 20nm. 3.2 Infra Red (IR) analysis: Fig.3 illustrates the IR spectra of ZnO/TiO2 samples sintered at 900 °C in the wave number range 2500-400 cm-1. All the spectra show a series of IR absorption peaks. The band at 2327 cm-1 is associated to CO2 molecule existing in air [25], the absorption peaks at 1980 cm-1 and 1458 cm-1 are ascribed respectively to Zn-H and symmetric stretching CO2 [26] and the band at 1120 cm-1 is attributed to C-O stretching frequency [27]. Concerning the inorganic 4

ACCEPTED MANUSCRIPT vibrations below 1000 cm-1, it is obvious to note the presence of additional peaks in TiO2doped ZnO samples which are located at 497 cm-1, 550 cm-1, 609 cm-1 and shifted to 600 cm-1 by increasing respectively TiO2 concentration from 3 to 7 Wt.%, 645 cm-1 and 702 cm-1. These peaks are assigned to Ti-O-Ti, O-Ti-O and Ti-O stretching vibrations owing to octahedral TiO6 group existing in all forms of spinel Zn2TiO4 and hexagonal ZnTiO3. Our

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results are consistent with the data reported by Shabalin [28]. It is important to note the significant shift and the intensity changes of zinc titanates peaks with TiO2 doping concentrations. These shifts are also observed in XRD spectra.

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3.3 Raman analysis:

Fig.4 presents the Raman spectra of undoped and TiO2 doped ZnO matrix. In addition to the typical ZnO vibrational peaks which are depicted in Table 2 the TiO2 incorporation

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provokes new vibrational peaks assigned to spinel Zn2TiO4 and hexagonal ZnTiO3. As reported by the literature, the obtained modes of undoped ZnO sample are the weak non-polar mode E2 (Low) [29] located at about 101 cm-1, the second order mode E2M [30,31] caused by multi phonic process localized around 345 cm-1, the second order transversal and local modes with A1 symmetry A1 (TO) [29,30] and A1 (LO) [32,33] situated at 381 cm-1 and 540 cm-1 respectively. In addition, it appears at 413 cm-1 and 589 cm-1 the first order transversal mode

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E1(TO) [32,34] and the local order mode E1(LO) [30, 32] respectively. The non-polar optical mode E2 (High) [29,30] is centered nearly at 440 cm-1, while the second order vibrational modes corresponding to the acoustic combination A1 and E1 [30, 32] and A1 (2LO) [29,30] are

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located nearly at 1105 cm-1 and 1144 cm-1 respectively. As shown in the table 1, the Raman peaks have thin FWHM which reflect the high structural quality of ZnO sample.

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The effect of TiO2 incorporation into ZnO matrix is obviously sensed by the appearance of new peaks attributed to mixed phases. Indeed, the characteristic peaks of hexagonal ZnTiO3 are detected at around 346 and 713 cm-1 which can be assigned to υ2 (LO,TO) and υ1 (TO) vibration modes respectively [35]. The observed Raman peaks at about 242 and 302 cm1

are the characteristic peaks of the spinel-structure Zn2TiO4. They are recognized as the

normal modes of F32g and F22g representations [36,37]. Concerning the distinguished peaks located just about 149 and 469 cm-1, they are respectively assigned to the Raman active modes B1g and Eg of rutile TiO2 phase [38]. The appearance of rutile phase in Raman measurements and not in XRD ones is probably caused by the weakness of its peaks or the masking of TiO2 phase by the ternary titanates in the chosen scan range. To further investigate 5

ACCEPTED MANUSCRIPT the Ti inclusion in ZnO lattice, we try to fit the Raman spectra of figures 4-a, 4-b and 4-c with Gauss-Lorentz peaks using LabSpec software. Compared with undoped ZnO, the fitted spectra of mixed ZnO/TiO2 compounds reveal the broadening of the line widths, the slight shifting towards lower wave numbers and the changing of peaks intensity after the TiO2 incorporation in ZnO matrix. In fact, some peaks of

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zinc titanates composites keep the same position but their intensities still increase up until an amount x=5 wt% and after that they diminish. The other peaks become strong and broad with Ti incorporation until the same rate to decrease later. They also shift faintly to the low energy side. These changes are possibly originated from the residual stress, crystal defect and

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structural disorder induced by Ti substitution. It seems that x= 5 wt% of rutile TiO2 is a critic rate. 4. Conclusion:

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This paper reports the synthesis of ZnO/TiO2 system and their structural characterization. The obtained results reveal the improvement of alloying and crystal quality of ternary phases in favor of undoped ZnO as the TiO2 doping rates increase. It is noticed the decrease of grain size and stress with rising TiO2 rutile content. Up to an amount x = 5 wt.% of TiO2 multiple phonon scattering become weak and broad and the intensities of all observed vibrational

by Ti random substitution. References:

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modes decrease which is probably explicated by the microscopic structural disorder induced

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ACCEPTED MANUSCRIPT [6] M. L. Levy (1888) Chimie—Sur Quatre Nouveaux Titanates de Zinc. Compt. Rend 107:421–423 [7] M. Sugiura and K. Ikeda (1947) Studies On Dielectrics of TiO2–ZnO System. J. Jpn. Ceram. Assoc 55:62–66 [8] F.H. Dulin, D.E. Rase (1960) Phase Equilibria in The System ZnO—TiO2. J. Am. Ceram.

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ACCEPTED MANUSCRIPT [18] Sarra Ayed, Raoudha Ben Belgacem, Jaafar Othman Zayani, Adel Matoussi (2016) Structural and optical properties of ZnO/TiO2 composites. Superlattices and Microstructures 91:118-128 [19] Mariem Chaari, Adel Matoussi (2013) Effect of Sn2O3 Doping on Structural, Optical and Dielectric Properties of ZnO Ceramics. Materials Science and Engineering B 178:1130-1139

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Korean Chem. Soc. 33:1235-1241

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CRYSTALLINITY OF ZnO/TiO2 NANOPOWDERS. Journal of Chemical Technology and [25] T. Rattanaa, S. Suwanboon, P. Amornpitoksuk, A. Haidoux, P. Limsuwan (2009) Improvement of optical properties of nanocrystalline Fe-doped ZnO powders through

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temperature electrosynthesized ZnO thin film with strong (0 0 2) orientation and its optical

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[35] C.H. Perry, D.B. Hall (1965) Temperature Dependence of Raman Spectrum of BaTiO3.

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[37] Y.H. Zhang, C.K. Chan, J.F. Porter, W. Guo (1998) Micro-Raman spectroscopic characterization of nanosized TiO2 powders prepared by vapor hydrolysis. Journal of Materials Research 13:2602–2609

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Figure captions:

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Fundamentals in Anatase TiO2. Phys. Rev. B – Cond. Matter and Mat. Phys. 55:7014-7017

Fig.1: XRD profiles of ZnO-TiO2 composites with rutile amounts x = 3, 5 and 7 wt% Fig.2: (a) Variation of lattice constants, (b) grain size and stress with TiO2 rates x = 3, 5 and 7 wt%

Fig.3: IR spectra of ZnO/TiO2 ceramic composites Fig.4: The spectra of undoped ZnO and mixed ZnO/TiO2 phases Fig.5: a) Gauss-Lorentz fit of TiO2 doped ZnO matrix with rutile rates a) x= 3 wt%, b) x= 5 wt% and c) x= 7 wt% 9

ACCEPTED MANUSCRIPT Table captions: Table 1: Characteristics of zinc titanates obtained by different TiO2 concentrations

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Table 2: Position, Intensity and FWHM of wurtzite ZnO vibration modes

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ACCEPTED MANUSCRIPT Table captions: Table 1: Characteristics of zinc titanates obtained by different TiO2 concentrations

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Table 2: Position, Intensity and FWHM of wurtzite ZnO vibration modes

ACCEPTED MANUSCRIPT Figure captions: Fig.1: XRD profiles of ZnO-TiO2 composites with rutile amounts x = 3, 5 and 7 wt% Fig.2: (a) Variation of lattice constants, (b) grain size and stress with TiO2 rates x = 3, 5

Fig.3: IR spectra of ZnO/TiO2 ceramic composites

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and 7 wt%

Fig.4: The spectra of undoped ZnO and mixed ZnO/TiO2 phases

Fig.5: a) Gauss-Lorentz fit of TiO2 doped ZnO matrix with rutile rates a) x= 3 wt%, b)

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x= 5 wt% and c) x= 7 wt%

ACCEPTED MANUSCRIPT Table 1: Characteristics of zinc titanates obtained by different TiO2 concentrations T (°C)

TiO2 concentration

Grain size (nm)

Solid state sintering method

900

1-7 Wt.%

73-33

Physicochemical route

450 ̶

19

200-600 and 500

50 mol% and 90 mol%

15-20

ZnO:3.10-3.18 ZnTiO3:4.75-5.05

Field of application

̶

̶

TiO2:45% ZnO:260%

Liquefied petroleum gas sensor Inorganic antimicrob ial agents

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Ref

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ZnTiO3: 4.1

̶

% Sensor response

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Nonhydrolytic route

Eg (eV)

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Synthesis method

[15]

[14]

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FWHM (cm-1)

E2 (Low)

101.597

8696.070

3.822

E2M

334.736

1042.850

24.843

A1 (TO)

381.555

981.986

22.932

E1 (TO)

413.087

838.019

11.271

E2 (High)

440.796

5122.980

7.644

A1 (LO)

540.167

485.168

E1 (LO)

588.897

592.814

A1,E1

1105.820

547.557

A1 (2LO)

1144.040

574.174

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Position (cm-1)

45.422

13.798

87.905

66.885

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Mode

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Fig.1: XRD profiles of ZnO-TiO2 composites with rutile amounts x = 3, 5 and 7 wt%

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Fig.2: (a) Variation of lattice constants, (b) grain size and stress with TiO2 rates x = 3, 5

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and 7 wt%

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Fig.3: IR spectra of ZnO/TiO2 ceramic composites

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Fig.4: The spectra of undoped ZnO and mixed ZnO/TiO2 phases

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Fig.5: a) Gauss-Lorentz fit of TiO2 doped ZnO matrix with rutile rates a) x= 3 wt%, b)

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x= 5 wt% and c) x= 7 wt%

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Highlights -

Zinc titanates materials were synthesized via solid state sintering process.

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XRD measurements reveal the formation of Zn2TiO4, hexagonal ZnTiO3 and ZnO phases. IR analysis provokes the presence of Ti-O stretching vibration bands.

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Raman study provokes the appearance of new zinc titanates vibrational peaks.

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The TiO2 effect into ZnO is sensed by the shift and intensity changes of peaks.

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