ARTICLE pubs.acs.org/IECR
Synthesis and Characterization of Nanocrystalline PbTiO3 Mohamed Abd El-Fattah Gabal Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia ABSTRACT: Nanocrystalline PbTiO3 in tetragonal form has been successfully prepared via a newly developed environmentally friendly coprecipitation process. The formation process of lead titanate from the PbC2O4 3 TiO2 precursor was monitored using DTATG, XRD, FT-IR, and TEM techniques. XRD showed the phase transition of less stable tetragonal PbO decomposition intermediate to a more stable orthorhombic structure by raising the calcination temperature. For the precursor calcined at 600 °C, only XRD lines characteristic of nanocrystalline tetragonal PbTiO3 are detected. TEM of PbTiO3 nanocrystals showed agglomerates with diameters in the range 8590 nm. Consistent with the TEM result, the grain size estimated using surface area measurements indicated a high degree of particle agglomeration. FT-IR measurements gave results which are in close agreement with those revealed by XRD technique. The ac conductivity of PbTiO3 depends on the dielectric property. The kinetics of the nonisothermal TG curves for the precursor decomposition was carried out assuming various solid-state-reaction models and applying three different computational methods. The data analysis showed that the presence of TiO2 during oxalate decomposition reaction does not alter the decomposition mechanism; it drastically affects the activation parameter values.
1. INTRODUCTION Ferroelectric materials based on the perovskite structure ABO3 are important for numerous applications in high-capacity memory cells, catalysts, optical waveguides, integrated optics applications, and high temperature cuprate superconductors.1 Lead titanate (PbTiO3, PT) is one of the simplest and most important members in the perovskite family. It possesses a high Curie temperature and low dielectric constant, which makes it attractive for high-temperature and high-frequency piezoelectric applications.2 It is cubic and paraelectric above the Curie temperature, while its tetragonal structure is stable below the Curie temperature.3 More recently, PT nanostructures have become more and more attractive because they are promising candidates for high-density, nonvolatile information storage and scanning probe based ferroelectric mass storage.4 In addition, PT can be used as an optical sensor since it has a large electrooptic coefficient and high photorefractive sensitivity.1 Therefore, owing to these interesting properties, it is worthy of being investigated and characterized. Many methods have been applied to prepare lead titanate powders, such as the solgel method,5 coprecipitation,6 hydrothermal synthesis,7 single-source precursor route,8 and mechanical alloying9 besides the traditional solid-state reaction of mixed oxides.10 In the present investigation, PbTiO3 nanopowder was prepared using a precursor, synthesized through a coprecipitation method incorporating benefits of both the solid oxide and chemical methods. The titanate formation was characterized by differential thermal analysisthermogravimetry (DTATG), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM) techniques. The surface area of the obtained PT was characterized by the N2 adsorption isotherm. Additionally, the electrical properties of PT were investigated as a function of temperature and frequency. A mechanism for the titanate formation and the kinetic parameters will be achieved using nonisothermal TG curves. r 2011 American Chemical Society
2. EXPERIMENTAL PROCEDURE Pure lead oxalate (PbC2O4) was prepared using the coprecipitation method in which analytically pure reagent of lead nitrate (Pb(NO3)2) was dissolved in bidistilled water, and an equivalent amount of analytical reagent oxalic acid was added until complete precipitation occurred. The fine yellow precipitate was filtered, washed with distilled water, and dried. For the preparation of precursor PbC2O4 3 TiO2, analytical reagent basic lead carbonate (PbCO3 3 Pb(OH)2), titanium dioxide (TiO2 (anatase)), and oxalic acid were chosen as the starting chemicals. TiO2 was added to an aqueous solution of PbCO3 3 Pb(OH)2 containing stoichiometric amount with vigorous stirring. Stoichiometric amount of oxalic acid solution was added dropwise into the suspension during constant stirring. This process precipitated lead oxalate on the surface of TiO2 particles by heterogeneous nucleation.11 The resulting was evaporated to dryness, and the precipitate was dried at 100 °C. Differential thermal analysisthermogravimetry (DTATG) experiments were performed in air atmosphere on a PerkinElmer thermal analyzer, in a Pt crucible at heating rate of 5 °C/min, from room temperature to 1000 °C. For the kinetic measurements, other nonisothermal TG curves were obtained at heating rates of 1, 2, and 3 °C/min. Pure lead oxalate was calcined at 400 °C for 30 min, and samples of the precursor, PbC2O4 3 TiO2, were calcined, at a very slow heating rate in an electrical oven, at 400 °C for 30 min, 500 °C for 2 h, or 600 °C for 2 h. The calcined samples were then characterized using the X-ray diffraction technique. The X-ray diffraction (XRD) pattern of the powder was examined using a D8 advanced diffractometer system (Bruker AXS) with Cu Kα radiation (λ = 1.5418 Å). Received: May 7, 2011 Accepted: November 13, 2011 Revised: November 13, 2011 Published: November 14, 2011 13771
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Figure 1. DTATG curves for the thermal decomposition of PbC2O4 in air at a heating rate of 20 °C min1.
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Figure 3. DTATG curves for the thermal decomposition of PbC2O4 3 TiO2 in air at a heating rate of 5 °C min1.
Figure 2. Characteristic part of XRD pattern of pure PbC2O4 calcined at 400 °C.
The transmission electron micrograph (TEM) was taken using a JEOL-2010 instrument running at an accelerating voltage of 100 kV. FT-IR spectra were obtained between 4000 and 200 cm1, using a JASCO Model FT-IR 310 spectrophotometer. The specific surface area of the nanocrystallites was measured with a Micromeritics ASAP 2010 analyzer using the multipoint BrunauerEmmettTeller (BET) adsorption method. The average grain size (d) of the particles was calculated from the formula d = 6/FA, where F is the theoretical density of the material and A is the specific surface area of the powder.12 For the electrical measurement, PT powder calcined at 600 °C was pressed into a pellet with a diameter of 1 cm and a thickness of 1 mm under a pressure of 2 tons cm2. The polished pellet with a silver paint was used for measuring temperature dependent electrical properties, at different frequencies (101000 kHz), using a Hioki LCR bridge Model 3531.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Thermal Decomposition Course of Pure PbC2O4. Figure 1 shows the thermal decom-
position behavior of pure PbC2O4, in air atmosphere, at a heating rate of 20 °C min1 up to 1000 °C. The TG curve shows that most of the decomposition process occurs through a large step started at 310 °C giving a weight loss of 23.1% at 380 °C. This weight loss is comparable with the theoretical weight loss of 24.4% attributed to the formation of PbO. The XRD pattern of pure lead oxalate calcined at 400 °C (Figure 2) shows diffraction
Figure 4. Characteristic parts of XRD patterns of PbC2O4 3 TiO2 precursor calcined at different temperatures. Phases: •, PbC2O4; +, TiO2; o, α-PbO; #, β-PbO; /, PbTiO3.
peaks which can be indexed to PbO (JCPDS File No. 85-1739) and Pb3O4 with a tetragonal structure (JCPDS File No. 411493). The TG curve also shows a very small weight loss step around 480 °C. This weight loss is attributed to the reductive decomposition of Pb3O4 content to PbO which can compensate for the difference between the observed and theoretical weight losses obtained in the first step. The DTA curve shows peaks which closely correspond to the obtained TG steps. The very sharp exothermic DTA peak with peak temperature at 360 °C characterizes the major decomposition step and can be assigned to the catalyzed oxidation of carbon monoxide decomposition product to carbon dioxide.13,14 A very small endothermic DTA peak with peak temperature at 490 °C characterizes the reductive decomposition reaction of Pb3O4.15 With increasing temperature, other very small endothermic DTA peaks can be detected at 760 and 850 °C. The first peak is not accompanied by any weight loss and can be attributed to the melting of PbO. The second peak is accompanied by a successive weight loss and can be assigned to the molten salt evaporation.14,16,17 3.2. Characterization of the Thermal Decomposition Course of PbC2O4 3 TiO2 and Titanate Formation. The thermal decomposition behavior of PbC2O4 3 TiO2 precursor in flowing air measured at a heating rate of 5 °C min1 is shown in Figure 3. From Figure 3 it is clear that the decomposition temperatures in case of the precursor are lower than that of the pure lead oxalate. According to the TG curve, the complete decomposition of the 13772
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Figure 5. TEM image of PbC2O4 3 TiO2 precursor calcined at 600 °C.
precursor’s oxalate content into PbO and Pb3O4 mixture occurred at 354 °C. The stability of the produced Pb3O4 was lowered to 400 °C. The DTA curve (Figure 3) shows a sharp endothermic peak at 343 °C, with lower exothermicity than that obtained in the case of pure PbC2O4, attributed to the decomposition of the oxalate content. The very broad endothermic peak in the range 400470 °C is due to the reductive decomposition of Pb3O4. The very large broad endothermic peak in the range 540750 °C can be assigned to the formation of PbTiO3. To characterize the precursor’s decomposition course and following the titanate formation, XRD measurement was carried out for the as-prepared precursor and samples calcined at different temperatures. Figure 4 demonstrates the characteristic parts of the XRD patterns. The as-prepared precursor gave individual XRD lines resembling those reported for triclinic PbC2O4 and tetragonal TiO2 (anatase) (JCPDS File No. 110723 and JCPDS File No. 86-1156, respectively). For the precursor calcined at 400 °C, only characteristic lines for the presence of tetragonal PbO (litharge) and tetragonal TiO2 (anatase) were detected (JCPDS File No. 85-1739 and JCPDS File No. 83-2243, respectively). No diffraction peaks characteristic for the presence of Pb3O4 were observed. By raising the calcination temperature to 500 °C, XRD lines show a phase transition from less stable tetragonal PbO (α-PbO) to the more stable orthorhombic structure (β-PbO) (JCPDS File No. 78-1666). The absence of any indication about this transition in the DTA curve (Figure 3) can be attributed to the inability of this transformation without calcining the precursor at 500 °C for 2 h as revealed by Salavati-Niasari et al.18 The pattern also shows some weak diffraction lines attributed to the tetragonal TiO2 (anatase) (JCPDS File No. 84-1286) besides low intensity diffraction lines characteristic for tetragonal lead titanate (macedonite) (JCPDS File No. 74-2495). This result indicates the starting of the titanate phase formation at this calcination temperature. A similar behavior was obtained during the preparation of PbTiO3 through a coprecipitation process by Fang et al.19 At 600 °C, all the diffraction peaks in the pattern can be indexed to the tetragonal single phase PbTiO3 with lattice parameters a = 0.3899 nm, c = 0.4128 nm, and c/a ratio = 1.059, which are very close to the reported data.20,21 No characteristic peaks of impurities, such as PbO or TiO2, are detected in the
Figure 6. N2 adsorption/desorption isotherm obtained for PbTiO3. Inset: pore size distribution.
Figure 7. FT-IR spectra of PbC2O4 3 TiO2 precursor calcined at different temperatures. Inset: spectra of pure PbC2O4 calcined at 400 °C.
pattern. The obtained diffraction peaks are obviously broad, which suggests the nanosized character of the obtained titanate. The average crystallite size estimated using the Scherrer formula22 is 44 nm. A TEM image of PbTiO3 nanocrystals is shown in Figure 5. The image indicates that the PbTiO3 powder consists of nanometric agglomerated particles5 with diameters in the range 85 90 nm. An independent analysis of the textural features of the particles was performed by the classical BET procedure. Figure 6 reports the N2 adsorption/desorption isotherms obtained for PbTiO3. According to the IUPAC classification of adsorption isotherms,23 the obtained isotherm shows a hysteresis loop of the H3 type which does not show any limiting adsorption at high relative pressure. This type describes adsorption on mesoporous adsorbents 13773
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Figure 8. Relation between dielectric constant and absolute temperature as a function of applied frequency for PbTiO3.
with weak adsorbateadsorbent interactions. The measured surface area (SBET) is 8.6 m2 g1. The average grain size (d), calculated using the estimated surface area, is 88 nm, which is nearly double that estimated using XRD measurements. This result suggests the presence of a high degree of agglomeration as appeared from the TEM image. The inset of Figure 6 illustrates the pore size distribution (the distribution of pore volume with respect to pore size) in microporous PbTiO3 calculated from the adsorption isotherm. FT-IR spectroscopy is a useful tool to help in understanding the functional group behavior of any molecule. The FT-IR spectrum of PbC2O4 3 TiO2 precursor and its calcined samples are shown in Figure 7. The spectrum of the as-prepared precursor shows a broad absorption band around 3470 cm1 corresponding to the υOH stretching vibrations of water molecules.24 This observation provides evidence for the presence of a small amount of water absorption on the surface of the precursor during measurements. The double absorption peaks appearing at 1663 and 1632 cm1 are due to the asymmetric vibrations of the carbonyl group. The closely spaced absorption peaks at 1388 and 1300 cm1 are assigned to υs(CO)+δ(OCdO) modes.18 Two distinct peaks at 773 and 512 cm1 are attributed to the out-ofplane bending mode of water and the OCO in-plane bending mode of oxalate, respectively,25 which indicate the formation of Pb(O4C2).18 The band at 352 cm1 can be assigned to υ(MO). The absorption bands appeared at 675 and 2929 cm1 are attributed to the presence of TiO2.26 The FT-IR spectrum of the precursor calcined at 400 °C shows the disappearance of the absorption bands characteristic for the oxalate organic moiety. For comparison, the FT-IR spectrum of pure PbC2O4, calcined at 400 °C, is recorded as the standard reference and is represented in the inset of Figure 7. The inset exhibits a strong characteristic absorption band at 1411 cm1, together with other absorption bands at 686 and 401 cm1, which can be assigned to the presence of PbO as a major phase.18 Thus, based on the above results, the absorption bands obtained in the case of the precursor calcined at 400 °C can be assigned to the presence of PbOTiO2 mixture. The new band appearing at 525 cm1 is assigned to TiO2.26 Increasing the precursor’s calcination temperature to 500 °C results in the appearance of characteristic absorption bands of PbTiO3 and a decrease in the intensity of PbO and TiO2 intermediates, which became minor phases. Eventually, the FTIR spectrum corresponding to the vanishing of PbO and TiO2 bands with the formation of single-phase PbTiO3 was obtained
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Figure 9. Relation between ln σ and reciprocal of absolute temperature as a function of applied frequency for PbTiO3.
when the precursor was calcined at 600 °C. The new bands appearing at 717, 582, and 396 cm1 are attributed to the presence of crystalline PbTiO3.19,27,28 Generally, it is apparent that FT-IR results are in close agreement with what has been revealed by phase analysis using the XRD technique. 3.3. Electrical Properties of PbTiO3. The variation of the dielectric constant (ε0 ) with temperature as a function of different frequencies for PbTiO3 is shown in Figure 8. The dielectric constant was found to be frequency independent at lower temperatures, while at temperatures higher than 513 K it becomes frequency dependent. The decrease in the dielectric values with increasing frequency can be considered a normal dielectric behavior.29 This decrease was rapid at lower frequencies and slower at higher frequencies. The dielectric structure was supposed to be composed of two layers. The highly conducting grains are separated by relatively poor conducting substances called the grain boundaries. This causes the localized accumulation of charges under the influence of the electric field, which results in interfacial polarization. At higher frequencies, the electron exchange cannot follow the alternating field, which causes a decrease in the contribution of interfacial polarization in the dielectric constant, and thus a decrease in the dielectric constant obtained. The dielectric constant is observed to increase with a rise in temperature for a given frequency. The increasing tendency of ε0 is due to the increased conductivity in the samples. It is observable that no Curie temperature can be detected even by raising the measuring temperature to 903 K. This suggests a promising property for the prepared lead titanate under the present preparation technique. Figure 9 shows the ac conductivity of the sample as a function of the reciprocal temperature at different frequencies. From Figure 9, it can be concluded that the conductivity for all the samples is frequency independent at high temperature regions, while at low temperature region its values changed with changing frequency. The ac conductivity of the system depends on the dielectric property and sample capacitance. This behavior may be attributed to the presence of space charge polarization in the material.30,31 The observed phase transition obtained in the temperature range 383483 K agrees well with that obtained in the dielectric constant curves (as appeared from the inset of Figure 8). This transition is called the diffuse phase transition.32 A great number of models have been developed to interpret this transition which invoked chemical heterogeneities,33,34 a 13774
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Figure 10. Dynamic measurements for the thermal decomposition in air of (a) pure PbC2O4 and (b) PbC2O4 3 TiO2 precursor. Heating rates: curve A, 1 °C min1; curve B, 2 °C min1; curve C, 3 °C min1; curve D, 5 °C min1.
superparelectric behavior,35 or the existence of nanometer “polar clusters” with random interactions.36 After the phase transition, the conductivity was found to be temperature dependent and increases with increasing temperature, which is the normal character of semiconductors. This behavior may be related to the increase in drift mobility of the thermally activated charge carriers (electron and hole) according to the hopping conduction mechanism.37 3.4. Kinetic Studies for the Oxalate Decomposition Reactions and Titanate Formation. Kinetic investigations concerned with the characterization of solid-state decomposition mechanisms, thermal behavior studying, and understanding the processes involving adsorption and surface reactions is very important. In this section, the kinetic analysis, under nonisothermal conditions, of oxalate decomposition reactions in PbC2O4 3 TiO2 precursor was carried out using different heating rates and assuming different solid-state-reaction equations.38 The kinetic parameters E, the activation energy (kJ mol1) and A, the frequency factor (min1), were calculated in view of three integral methods: Diefallah’s composite method based on either the modified CoatsRedfern method (composite method I) or Doyle’s equation (composite method II),3840 the Coats Redfern method,41 and the Ozawa method.42 The details of these methods were reported elsewhere.3840 The results of activation parameters obtained using the different methods are then compared with each other and discussed. The kinetic investigation of pure lead oxalate will also be studied for comparison and to estimate the effect of TiO2 on the obtained activation parameters and the decomposition mechanism. Analysis of dynamic data were carried out at heating rates of 1, 2, 3, and 5 °C min1. The fractional reactions remaining as a function of temperature for the oxalate decomposition reactions are shown in Figure 10. The composite method involves a model-fitting kinetic approach since it does not assume a particular reaction model, but it allows the choice of the kinetic mechanism which best fits the data and gives the highest correlation coefficient.3840 Generally, both the composite methods gave equivalent curves and nearly identical values for the activation parameters.38 The values of the activation parameters, calculated according to composite method II, assuming the different kinetic models, are reported in Table 1. From Table 1, it is clear that the oxalate decomposition reactions in both cases are best described by AvramiErofeev random nucleation mechanisms (A2 and A3) characteristic for random nucleation growth mechanisms in which the reaction is controlled by initial random nucleation followed by overlapping growth in two or three dimensions.43 This result agrees well with
Table 1. Activation Parameters for Nonisothermal Decomposition in Air of PbC2O4 and PbC2O4 3 TiO2 Precursor, Calculated According to Composite Method II, Assuming Different Kinetic Models decomposition of pure PbC2O4 decomposition of PbC2O4 3 TiO2 E
log A
model (kJ mol1)
(min1)
r
E
log A
(kJ mol1)
(min1)
r
D1
354 ( 24
29.3 ( 2.2 0.861
253 ( 23
20.5 ( 2.1
0.778
D2
374 ( 26
30.9 ( 2.4 0.855
268 ( 25
21.7 ( 2.3
0.770
D3 D4
401 ( 28 383 ( 27
30.7 ( 2.7 0.847 31.1 ( 2.5 0.852
287 ( 27 274 ( 25
22.9 ( 2.5 21.6 ( 2.4
0.760 0.767
R2
228 ( 9
18.3 ( 0.8 0.947
169 ( 10
13.3 ( 0.9
0.883
R3
234 ( 10
18.7 ( 0.9 0.942
173 ( 11
13.5 ( 1.0
0.876
F1
248 ( 11
20.5 ( 1.0 0.930
184 ( 12
15.0 ( 1.1
0.861
F2
157 ( 9
13.1 ( 0.8 0.893
127 ( 8
10.7 ( 0.8
0.860
F3
247 ( 21
21.5 ( 1.9 0.807
195 ( 19
17.1 ( 1.8
0.745
A2
158 ( 2
12.7 ( 0.2 0.995
122 ( 4
9.6 ( 0.4
0.963
A3 A4
128 ( 3 113 ( 5
10.1 ( 0.3 0.973 8.8 ( 0.5 0.931
101 ( 3 91 ( 4
7.9 ( 0.3 7.0 ( 0.3
0.968 0.944
E1
76 ( 3
8.3 ( 2.6 0.298
45 ( 23
6.8 ( 2.1
0.211
that obtained using TEM and surface area measurements, which revealed the presence of agglomerated nanoparticles. It also indicates that the presence of TiO2 does not alter the decomposition mechanism. The other reaction models gave less satisfactory results, and the exponential law gave the least satisfactory one. The activation parameters, assuming the A2 model, were also calculated according to the CoatsRedfern method (using different heating rates, β) and the Ozawa method (using different fractional reactions, α-values). The calculated results according to the three computational methods are summarized in Table 2. The values of activation parameters calculated according to the different computational methods showed a good agreement, within the experimental error, among each other except in the case of precursor assuming Ozawa method. The obtained activation energy of pure lead oxalate agrees well with that obtained by L’vov.44 The results also showed that both the composite and Ozawa methods gave a very low standard deviation in the calculated activation parameters compared with the Coats Redfern method. The very low standard deviation obtained using the Ozawa method assumed approximately constant E with changing α-values along decomposition, which enhances that the rate-limiting step is a single reaction step.39,43 13775
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Table 2. Activation Parameters Calculated Using Different Methods for the Nonisothermal Decomposition in Air of PbC2O4 and PbC2O4 3 TiO2 Precursor, Assuming A2 Model decomposition of pure
decomposition of
PbC2O4
PbC2O4 3 TiO2
method
E
log A
E
log A
of analysis
(kJ mol1)
(min1)
(kJ mol1)
(min1)
composite II
158 ( 2
12.7 ( 0.2
122 ( 4
9.6 ( 0.4
CoatsRedfern
161 ( 12
12.9 ( 1.1
138 ( 11
10.1 ( 0.6
Ozawa
156 ( 7
12.6 ( 0.1
109 ( 1
9.1 ( 0.1
A closer look at Table 2 reveals that even the presence of TiO2 in the precursor does not alter the oxalate decomposition mechanism; it affects the calculated activation parameters. The decrease in the activation parameters by the addition of TiO2 can be attributed to the spacing effect of TiO2 resulting from the coprecipitation of lead oxalate on the suspended TiO2. In this case, TiO2 acts as a spacer causing a fewer points of contact between lead oxalate particles and increases the interfaces, which facilitate the decomposition and decrease the activation energy. This obtained result agrees well with the decrease of the oxalate decomposition temperatures in the case of the precursor compared with pure lead oxalate. Moreover, the effect of the enthalpy of the reaction on the decomposition temperature and activation energy cannot be ignored.23,24 The exothermicity due to pure oxalate decomposition is more than that present in the precursor as evidenced from the DTA curve (Figure 1). This is due to the fact that PbC2O4 decomposition in air is accompanied by an exothermic oxidation reaction.14 The presence of TiO2 would allow ease rupture of the bonds in the precursor compared with the pure lead oxalate.
4. CONCLUSIONS A chemical coprecipitation route is developed to synthesize PbC2O4 3 TiO2 precursor at room temperature. The synthetic process is environmentally friendly and economical, employs inexpensive and nontoxic reagents, and can be applied to synthesize different kinds of perovskite oxides or mixed oxides. The decomposition of precursor and titanate formation were followed by DTATG, XRD, and FT-IR techniques. The pyrolysis of the precursor at 600 °C in air produced nanocrystalline tetragonal PbTiO3 forming agglomerates with diameters in the range 8590 nm. Electrical property measurements showed that ac conductivity depends on the dielectric property. Kinetics of nonisothermal decomposition of the precursor revealed that the presence of TiO2 does not alter the decomposition mechanism but drastically changes the activation parameter values. ’ AUTHOR INFORMATION Permanent Address
Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. E-mail:
[email protected].
’ ACKNOWLEDGMENT The author is grateful to the Deanship of Scientific Research (DSR), King Abdulaziz University (KAU), Jeddah, KSA, for providing financial support for this work. Thanks are extended
also to Mr. Mohamed El-Garni, Thermal Analysis Lab (KAU), for thermal analysis measurements and to Dr. A. Awad, Chemistry Department, Benha University, Egypt, for his help and cooperation.
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dx.doi.org/10.1021/ie200988g |Ind. Eng. Chem. Res. 2011, 50, 13771–13777
