Zinc Titanate Nanopowders. Synthesis and Characterization

: Zinc titanates nanopowders viz.; Zn 2 TiO 4 , ZnTi 3 O 8 and ZnTiO 3 were synthesized through the thermal decomposition course of ZnC 2 O 4 .2H 2 O-TiO 2 precursor (1:1 mole ratio), prepared via a new co-precipitation method up to 900 o C. Thermogravimetric measurement (TG) was utilized to characterize the precursor decomposition while X-ray diffraction (XRD), Fourier transform infra-red (FT-IR) were used to characterize the decomposition products as well as the phase transitions at different temperatures. XRD revealed the starting of titanates formation at 700 o C via detecting Zn 2 TiO 4 along with ZnO and TiO 2 (anatase) diffraction peaks. By increasing the calcination temperature to 800 o C, the ZnO content vanished with the appearing of Zn 2 Ti 3 O 8 besides ZnTi 2 O 4 and impurities of TiO 2 (anatase). Finally at 900 o C, the Zn 2 Ti 3 O 8 content was decomposed into ZnTiO 3 . Nitrogen adsorption-desorption isotherm of the calcined precursor at 900 o C indicated low specific surface area of 7.1 m 2 g -1 in accordance with the agglomeration nature estimated via transmission electron microscopy (TEM) study. The conductivity measurements showed semiconducting behavior of the prepared titanates with ferroelectric transition in the range 200-308 o C.The obtained low dielectric value suggests the uses of present titanates as a co-fired ceramic or resonator ceramics.

Based on the literature [6,12,13], there are three phases existing in the system of ZnO-TiO2: zinc orthotitanate; Zn2TiO4 having cubic spinel structure, Zn2Ti3O8 having defect cubic structure and metatitanate; ZnTiO3 with ilmenite rhombohedral structure. Zn2TiO4 could be easily prepared using conventional ceramic technique using 2:1 mole ratio of ZnO:TiO2 and is considered to be stable up to 1418 o C. On the other hand, metastable Zn2Ti3O8 is considered as the low temperature form of ZnTiO3 up to 820 o C. Finally, the ilmenite form; ZnTiO3 is very hard to obtain since it decomposes into orthotitanates; Zn2TiO4 and TiO2 (rutile) at 945 o C.
The different preparation methods for all zinc titanate phases are reviewed [6]. Coprecipitation method can be considered as the most successful one for obtaining ultrafine powder with narrow size distribution. Using this method, the more complex steps could be avoided besides less time consumption and high purity compared to other methods.
In the present manuscript, a simple coprecipitation technique will be followed to synthesize nano-crystalline zinc titanate powders. In this technique, a stoichiometric coprecipitated mixture of ZnC2O4.2H2O-TiO2 (1:1 mole ratio) will be thermally decomposed up to the titanate formation. To our best knowledge, this is the first report on the synthesis of zinc titanate nanopowder by this method in the literature. The titanate phase's formation as well as their structure characterization, morphology and phase transitions will be investigated via TG-DTG, XRD, FT-IR, TEM and BET measurements. The different electrical properties viz. acconductivity as well as dielectric property will also be evaluated.

2.b. Synthesis process
The entire titanates were prepared in the present study via the thermal decomposition of oxalate-titania precursor; ZnC2O4.2H2O-TiO2 (1:1 mole ratio). A new innovative route for the preparation of this precursor was utilized in which, a stoichiometric amount of oxalic acid solution equivalent to precipitate zinc oxalate was added drop wisely to the calculated amount of basic zinc carbonate and titania suspension under vigorous stirring at about 60 o C. after complete precipitation, the zinc oxalate is expected to precipitate on TiO2 surface through heterogeneous nucleation [14]. The prepared precursor was filtered, washed with distilled water, dried then given the name; as-prepared precursor. To characterize the precursor decomposition route until titanate formation, different samples were calcined in a muffle furnace at 400 o C for 30 min, 500 and 600 o C for 1h and for 2h at 700, 800 and 900 o C.

2.c. Characterization
Thermogravimetric analysis (TG) of the prepared precursor was conducted in air by a Perkin Elmer thermal analyzer (STA 6000) at heating rate of 5  C/min up to 1100  C. The crystal phases were estimated by X-ray diffraction (Bruker AXS) with Cu K radiation (λ = 1.5418 Å). The titanates sample were morphologically characterized using transmission electron micrograph (JEOL-2010) with 100 kV accelerating voltage. FT-IR spectra were measured using a JASCO FT-IR 310 spectrophotometer. Specific surface area was measured using BET adsorption and ASAP 2010 analyzer. In electrical measurements, the calcined powders were pressed into pellets (1 cm diameter and 1mm thickness) using 2 ton.cm -2 pressure. The two probe method was then used for measuring the temperature dependence of conductivity and dielectric constant at different frequencies (1-1000 kHz), using a Hioki LCR bridge model 3531.

3.a. Thermal decomposition of ZnC2O4.2H2O-TiO2 precursor
The full decomposition course of ZnC2O4.2H2O-TiO2 precursor (Fig. 1), up to 1000 o C in air, consists of three well-defined TG steps. According to the calculated weight losses, the first two steps could be attributed to the loss of water with the formation of anhydrous ZnC2O4-TiO2 mixture. The calculated weight loss (14.0 %) for the dehydration process agreed well with that experimentally obtained (13.9 %) up to about 240 o C. The dehydration process was found to occur through two separated TG steps in the temperature ranges; 100-125 and 215-240 o C, respectively which are characterized by two differential thermogravimetric (DTG) peaks at 116 and 226 o C. According to Diefallah [15], the water lost at low temperature range can be considered as crystal water while that lost at higher temperatures as coordinately bounded.
In the third TG step appeared in the range from 306 to 387 o C (with DTG peak at 362 o C), the observed weight loss (25.5 %) agreed well with the calculated one of 26.5 % attributed to the decomposition of the oxalate content with the formation of ZnO-TiO2 mixture and evolution of Co and CO2 as decomposition products [16]. No further weight changes could be observed up to 1000 o C.

3.b. X-ray diffraction and titanates formation
The thermal decomposition course of the studied precursor as well as the titanates formation was successfully followed using XRD measurements of calcined precursor samples at temperatures ranging between 400 and 900 o C.  [17][18][19] were reported on the inability of pure ZnTiO3 formation from 1:1 mole ratios of ZnO and TiO2 mixture. Others [20][21][22] succeeded to prepare pure ZnTiO3 phase using sol-gel methods. On the other hand, many researchers [19,[23][24][25][26] obtained a mixture of titanates such as ZnTiO3, Zn2Ti3O8 and Zn2TiO4 with Zn2TiO4 as major phase. Ivanova et al. [27] and Yan et al. [28] prepared Zn2TiO4, TiO2 and ZnO via sol-gel and hydrothermal routes, respectively using 1:1 mole ratio of Zn:Ti mixture. Siriwong and Phanichphant [29] prepared Zn2TiO4 single-phase using 1: 1 mole ratio of zinc naphthenate and titanium tetra isopropoxide precursors by the flame spray pyrolysis technique.
The precursor calcined at 800 o C showed the complete vanishing of ZnO diffraction peaks with the appearance of diffraction peaks characteristic for the presence of Zn2TiO4 and Zn2Ti3O8 (JCPDS file No. 87-1781) phases indicating the complete titanates formation. The Zn2Ti3O8 with a defect cubic spinel structure, was first discovered by Yamaguchi et al. [17] and is considered as the low temperature phase of ZnTiO3. The presence of very weak peaks attributed to the presence of TiO2 (anatase) could be ascribed to the reduction of very few amount of ZnO to volatile elemental Zn [21].
Liu et al. [26] reported that the formation of Zn2TiO4 and Zn2Ti3O8 phases could only happen in the presence of anatase whereas ZnTiO3 formation is limited only in the presence of rutile. They also reported that, when anatase grains are small enough then, it could be completely utilized in the formation of Zn2TiO4 and Zn2Ti3O8 while if they are large enough, an anatase to rutile transition would be took place with increasing probability for ZnTiO3 formation. In this context, the estimated grain size of the anatase phase (for the sample calcined at 700 o C) using the strongest peak (101) and Scherrer`s equation [30] amounts to 67 nm which is expected to be small enough to form Zn2TiO4 and Zn2Ti3O8 mixture.
The calculated grain size of the very few amount of anatase in the sample calcined at Really, the reason for selecting the (1:1) mole ratio of ZnC2O4:TiO2 in the present preparation was that this ratio would correspond to the stoichiometric ZnTiO3. Instead, the above XRD study indicated the formation of different zinc titanate phases including: ZnTiO3, Zn2TiO4 and Zn2Ti3O8. Accordingly, the phase transitions along the present decomposition could be represented as: ZnC2O4.2H2O-TiO2 → ZnC2O4-TiO2 → ZnO-TiO2 → ZnO + anatase + Zn2TiO4 → Zn2TiO4 + Zn2Ti3O8 + anatase → Zn2TiO4 + Zn2Ti3O8 + anatase + rutile

3.c. FT-IR spectroscopic study
The thermal decomposition course of the entire precursor as well as the titanates formation was also characterized using FT-IR spectroscopy. The characteristic bands obtained for the as-prepared precursor and samples calcined at different temperatures are exhibited in The vibration band at about 1302 cm -1 could be attributed to the symmetric mode of oxalate ion`s carbonyl group [14]. The observed bands at 773 and 506 cm -1 are attributed to the out-ofplane and in-plane bending modes of water and O-C-O of oxalate, respectively while the band at 348 cm -1 could be assigned to the metal ion vibration [31]. Finally, the bands at 680 and 2924 cm -1 are attributed to TiO2 [32].
The calcined precursor at 400°C showed a decrease in the band intensities due to the carbonyl group as a result of oxalate content`s decomposition with the formation of metal oxides as discussed above in TG and XRD results. Thus, the broad band appeared at 525 cm -1 can be assigned to the metal-oxygen (M-O). The further appearance of the bands characteristic of water at 3441 and 1652 cm -1 could be attributed to the presence of adsorbed water formed during sample preparation. The samples calcined at 500 and 600 o C indicated nearly the same bands indicated same composition without any chance for titanate formation.
The calcined samples at 700, 800 and 900 o C indicated alike patterns with an apparent absorption band at 583 cm -1 ascribed to Ti-O stretching vibration due to [TiO6] octahedron group existing in all titanates forms of Zn2TiO4, Zn2Ti3O8 and ZnTiO3 as previously reported [20,21].

3.d. Morphological study
XRD pattern of the sample calcined at 900 o C (Fig. 2), as previously described, indicated two types of particles including Zn2TiO4 and ZnTiO3 besides traces of TiO2. In agreement with this result, TEM image of the calcined precursor at 900 o C (Fig. 4) exhibited two distinct types of particles having different sizes. Generally, the particles are nearly spherical with particle sizes of about 60 and 130 nm and exhibited a dense aggregation. A similar result was obtained by Arin et al. [33] for zinc titanates synthesized via hydrothermal method.

3.e. Surface area characterization
Nitrogen adsorption-desorption isotherm of the calcined precursor at 900 o C (Fig. 5) exhibited, according to the IUPAC classification [34], type II isotherm in which the adsorption is on macroporous adsorbents through strong adsorbate-adsorbent interaction. This type exhibited also a very small hysteresis as appeared in Fig. 5. The specific surface area (SSA) calculated according to BET method is about 7.1 m 2 g -1 which can be considered as the surface area of Zn2TiO4 and ZnTiO3 main contents. This very low surface area could be attributed in accordance with the TEM image (Fig. 4) to the agglomeration nature of the powder which lowering porosity. Pore size distributions (BJH) (inset of Fig. 5) showed narrow distribution characterized by three pore size types located around 19, 28 and 42 nm.

3.f.i. ac-conductivity
The ac-conductivity measured at different applied frequencies by changing temperature from 30 to 450 o C have been studied. The ln vs. 1000/T plot at frequencies ranging between 1 kHz and 1 MHz for the sample calcined at 900 o C is shown in Fig. 6(a). The conductivity showed a decreasing behavior by increasing temperature up to about 90 o C. During sample`s preparation for conductivity measurements, some water molecules were adsorbed on the titanates surface. This water can be acting as conductor and its removal by increasing temperature could result in decreasing conductivity. Similar behavior was already reported in literature during conductivity measurements [35].
The conductivity showed frequency dependence at low temperatures, in which conductivity increases with increasing frequency, while it indicated frequency independence at higher temperatures. This behavior could be described based on the pumping force of the applied frequency which facilitate the charge carriers' transfer. By increasing temperature, the generated phonons resist the moving of the charge carriers through phonon-electron collisions and thus canceling the applied frequency effect [36].
The estimated conductivity at 110 o C (i.e. temperature at complete adsorbed water evaporation) and frequency of 1 MHz amounts to 6.4x10 -7 ohm -1 cm -1 . The conductivity is appeared to be temperature dependent after 110 o C and showed a gradual increase with increasing temperature which reflects semiconducting behavior of the present studied titanates.
An anomalous behavior was observed in the rising portion of the curve by increasing temperature in the range 200-308 o C. By a closer look to Fig. 6(a), this anomalous behavior, appeared as a transition peak, is clearer at higher frequencies ( 500 kHz) and also indicated a gradual change in the peak position to lower temperature by increasing frequency.
Chang et al. [37] reported an anomalous strong decrease (exponential decrease) in conductivity vs. reciprocal temperature close to the ferroelectric Curie temperature (TC) in most perovskite type structure such as ZnTiO3. Such anomalous behavior is well-known as positive temperature coefficient of resistivity (PTCR) [38] and was attributed to electrical potential barrier resulted from the existence of a two-dimensional surface layer of acceptor ions or oxygen adsorbed at grain boundaries [39]. The increase in the applied frequency is expected to decrease the electrical potential barrier and thus shifts the Curie temperature towards lower temperature.
The conduction activation energies (Ea), in the high temperature range using Arrhenius equation: , were calculated as a function of frequencies, using ln vs 1000/T plot ( Fig. 6(a)), and summarized in Table 1. The estimated values agree well with the obtained semiconducting behavior and suggested that the conduction is through the electron hopping [36]. The reported values showed a gradual decrease with increasing frequency, agreed well with the obvious increase in conductivity, attributed to the effect of applied frequency in facilitating charge carriers transfer.
It is well known that, ac conductivity depends on the capacitance and thus on the material`s dielectric property. This behavior may be assigned to the space charge polarization present in the material [40]. Consequently, the observed anomalous observed in the dielectric constant (`) vs. temperature relation at different applied frequencies ( Fig. 6 (b)), agrees well with that obtained in the conductivity temperature relation (Fi6. 6(a)). At this observed transition, the electric dipoles are disorderly arranged due to the asymmetric shift in the crystal axis`s symmetry with respect to the effective polarization direction. As a result, an abrupt increase in the value of dielectric constant is observed [41,42].
The room temperature dielectric value obtained at applied frequency of 1MHz ( Fig.   6(b)) is 43 which is higher than that reported for ZnTiO3 prepared by chemical deposition of 25 [43] and solid-state conventional method of 30 [23]. This obtained dielectric value suggests the uses of present titanates as a co-fired ceramic or resonator ceramics [43].