Journal of the Korean Physical Society, Vol. 60, No. 2, January 2012, pp. 216∼219
Influences of Annealing Temperature on Characteristics of Composite Materials Consisting of Multi-walled Carbon Nanotubes and Pb(Zr0.52 Ti0.48 )O3 Thin Films Jin Ho Kwak, Jin Kyu Han, Sam Yeon Cho and Sang Don Bu∗ Department of Physics, Chonbuk National University, Jeonju 561-756, Korea (Received 2 May 2011) We report the synthesis and the characterization of composite materials consisting of multi-walled carbon nanotubes and Pb(Zr0.52 Ti0.48 )O3 (CNT-PZT) in a film structure. The CNT-PZT films were prepared by using a sol-gel process, a spin-coating method and a rapid thermal annealing process. CNT bundles in 2-methoxyethanol solution were mixed with a 7-wt% PZT sol-gel solution. The CNT-PZT solution was deposited onto (111) Pt/Ti/SiO2 /Si substrates by spin-coating at 3000 rpm for 60 s and was then pyrolyzed. The samples were annealed at various temperatures in an oxygen atmosphere for crystallization. The morphological, structural and electrical properties of CNT-PZT composite films were studied with a field-emission scanning electron microscope, X-ray diffraction and polarization-voltage hysteresis loops, respectively. We found that, during the annealing process, the thickness of CNT-PZT films dramatically decrease, especially in the range of 500 to 600 ◦ C, which may be related with the loss of CNTs in that temperature region due to burning. The CNTs also affect the orientation of and the number of defects in the PZT films, which change the ferroelectric properties of the films. PACS numbers: 77.84.Lf, 77.84.Dy, 68.55.-a Keywords: Composite materials, Multi-walled carbon nanotubes, Pb(Zr0.52 Ti0.48 )O3 films DOI: 10.3938/jkps.60.216
I. INTRODUCTION
the other hand, Ruangchalermwong et al. [14] reported that their remnant polarization and capacitance were degraded. More studies must be done before the composite will be ready for scientific use. The effects of annealing temperature, especially, may be an important parameter because the characteristics of CNTs are sensitively influenced by heat treatments. The characteristics of CNTs embedded in PZT films are still not understood. In this study, we report the influence of annealing temperature on the morphological, structural and ferroelectric properties of CNT-PZT films. The CNT-PZT films were fabricated spin-coating a PZT solution mixed with CNTs.
Ferroelectric films have attracted considerable interest because they are important materials in memory devices and microelectromechanical systems [1–3]. In particular, the Pb(Zr0.52 Ti0.48 )O3 (PZT) film is a well-known material for its great ferroelectricity and piezoelectricity. These properties should facilitate their applications in systems such as ferroelectric random access memory (FeRAM) devices [4] and ultrasonic image sensors [5]. An important requirement in the applications is to synthesize films having a large remnant polarization and a low coercive voltage [6]. As is well known, the properties of PZT films have been altered or enhanced by adjusting some parameters, such as the film’s thickness [7], orientation [8], and composition. Controlling the composition, especially, has been realized by changing the Zr/Ti ratio or by doping with ions like La and Nb [9,10]. From the point of view of doping, multi-walled carbon nanotubes (CNTs) are excellent candidates to enhance or modify the properties of various oxide materials [11,12]. Nie et al. [13] reported enhanced some ferroelectric properties in PZT films combined with CNT bundles. On ∗ E-mail:
II. EXPERIMENTS The CNT-PZT films were prepared using a sol-gel process with spin-coating of a CNT-PZT solution. The CNT-PZT solution was synthesized by mixing and dispersing CNTs in a PZT solution. The PZT solution was synthesized from lead acetate trihydrate, zirconium n-propoxide and titanium (IV) isopropoxide in a 2methoxyethanol solvent (Sigma Aldrich). In order to disperse the CNTs (Carbon Nano-material Technology, Korea) into the PZT solution, we used the ultrasonic
[email protected]; Fax: +82-63-270-3320
-216-
Influences of Annealing Temperature on Characteristics · · · – Jin Ho Kwak et al.
Fig. 1. (Color online) TG-DSC curve of the CNT bundles. The heating rate was 5 ◦ C/min. Inset images show the morphologies of CNT bundles annealed at 450 and 650 ◦ C.
method; first, a CNT powder was mixed with a 2methoxyethanol solvent. Then, ultrasonification of the solution was performed for 1 h. Second, a 7-wt% CNT solution was mixed with the PZT solution. The films were fabricated by spin coating the CNT-PZT solution at 3000 rpm for 1 min onto a Pt(111)/Ti/SiO2 /Si substrate. The coated films were dried at 200 ◦ C for 2 min and were pyrolyzed at 400 ◦ C for 5 min on a hot plate in air. The films were finally annealed for 30 min at temperatures ranging from 500 to 700 ◦ C in and oxygen atmosphere. The thermal properties of a CNT bundle were analyzed by using thermo gravimetric (TG) and differential scanning calorimeter (DSC) analyses. The surface and the cross-section morphologies of the films were observed using a field-emission scanning electron microscope (FESEM). The crystal structure and orientation of the films were analyzed using X-ray diffraction (XRD) with Cu Kα radiation. Ferroelectric hysteresis loops of the films were measured by using a TF analyzer 2000 (aixACCT).
III. RESULTS AND DISCUSSION In order to identify the states of the CNTs for various treatments, we analyzed the morphologies, weights and heat flows of the CNT bundles with increasing annealing temperatures. The inset images in Fig. 1 shows the FESEM images of CNT bundles annealed at 450 and 650 ◦ C, respectively. The CNT bundles annealed at 450 ◦ C keep their original morphology while the CNT bundles annealed at 650 ◦ C changed out of shape. The TG and the DSC curves of the CNT bundles measured at a heating rate of 5 ◦ C min−1 are shown in Fig. 1. The TG curve shows a loss of weight in the temperature range of 480 to 650 ◦ C. The corresponding DSC curve shows a strong exothermic peak with a maximum at 638 ◦ C, which is attributed to the evaporation of CNT bundles into air due to their oxidation. According to the experimental results by Eder and Windle [15], the oxidation temperature depends on the degree of crystallinity of the CNT bundles
-217-
Fig. 2. (Color online) FESEM images of CNT-PZT films annealed at various temperatures in an oxygen atmosphere for 30 min: (a) – (c) surface images and (d) – (e) crosssectional images.
and on the concentration of defects. For the case of our CNT bundles, the oxidation temperature was found to be about 640 ◦ C. Figure 2 presents FESEM images of the CNT-PZT films annealed at several temperatures. The FESEM images show the surface (Figs. 2(a) – (c)) and cross-section (Figs. 2(d) – (e)) morphologies. At an annealing temperature of 500 ◦ C, no grains are observed on the surfaces of the CNT-PZT films, as shown in Fig. 2(a). The surfaces of the CNT-PZT films annealed at 600 ◦ C show uniform, dense and fine grains with an average size of ∼10 nm, as shown in Fig. 2(b). In case of the CNT-PZT films annealed at 700 ◦ C, as shown in Fig. 2(c), grains with an average size of ∼200 nm are observed with fine grains with an average size of ∼20 nm, which is similar to the grain size of conventional PZT thin films [16]. Figures 2(d) – (e) show the cross-section morphologies of the CNT-PZT films. Interestingly, their thicknesses decrease with increasing annealing temperatures. The CNT-PZT films annealed at 500 ◦ C had very rough cross sections with many voids as shown in Fig. 2(d). The CNT-PZT films annealed at 600 ◦ C have few voids and less rough surfaces than the CNT-PZT films annealed at 500 ◦ C. The CNT-PZT films annealed at 700 ◦ C show very uniform cross sections without any voids. This result indicates the existence of the CNT bundles in the PZT films because the number of voids and the roughness are reduced with increasing annealing temperature. Beside, the thickness of the films tends to decrease from about 290 nm to 210 nm with increasing annealing temperature. The difference in thickness between the CNT-PZT films annealed at 500 ◦ C and 600 ◦ C may be due to the loss of CNTs with increasing annealing temperatures in the temperature region of 500 – 600 ◦ C, as shown in Fig. 1. XRD measurements were performed to determine the structure of the CNT-PZT and the PZT thin films for various annealing temperatures, as shown in Fig. 3. Figures 3(a) and (b) show the log-scale XRD spectra of CNT-PZT and PZT films for several annealing temperatures. The CNT-PZT and the PZT films exhibit mainly a (111) diffraction peak, but there is no peak of a pyrochlore phase. These results suggest that the
-218-
Journal of the Korean Physical Society, Vol. 60, No. 2, January 2012
Fig. 3. (Color online) XRD patterns of the CNT-PZT films and PZT films annealed at various temperatures: (a) and (b) XRD patterns on a log scale; (c) and (d) magnified XRD patterns.
two films are crystallized well into a perovskite structure with a preferred (111) orientation without any pyrochlore phases. Figures 3(c) and (d) show the XRD spectra near the (111) peak for the CNT-PZT and the PZT films, respectively. The magnified XRD spectra show some interesting differences in their intensities, peak positions and the full widths at half-maximum (FWHM) values, as shown in Fig. 4. Figure 4(a) is a graph showing the variation in the intensity of (111) peak with annealing temperatures for the two films. With increasing annealing temperature, the intensity is increasing. The inset graph of Fig. 4(a) shows that the (111) orientation of CNT-PZT films starts to increase at a temperature of 580 ◦ C while that of the PZT films starts to increase at 560 ◦ C. The temperature of the CNT-PZT films is 20 ◦ C higher than that of the PZT films. Then, after annealing at 650 ◦ C the intensity of the CNT-PZT films does not increase anymore while that of the PZT films keeps increasing at a constant rate. Usually, a decrease or saturation of the intensity in XRD spectra takes place during an annealing process at a high temperature. In the case of our samples, the saturation of the intensity of the XRD spectra may be related to the existence of CNTs in the composite films rather than the annealing treatment at high temperatures. Figure 4(b) presents the positions of the (111) peak for the two films, for various annealing temperatures. The (111) peak shifts to lower angle with increasing annealing temperature from 575 to 700 ◦ C. For the sample annealed at 600 ◦ C, the position of the (111) peaks of the CNT-PZT films is 0.1◦ higher than that of the PZT films. On the other hand, for the sample annealed at 650 ◦ C, the position of the (111) peaks of the CNT-PZT films is 0.1◦ lower than that of the PZT films, which can be related to the evaporation of CNTs
Fig. 4. (Color online) Variations of (a) the intensity, (b) the position and (c) the FWHM of the (111) peaks in the XRD pattern with increasing annealing temperature.
due to an oxidation reaction, as mentioned in the discussion of Fig. 1. According to the Mars-van Krevelen mechanism [17], oxygen vacancies can occur at the CNT surface inside the composite materials. During an annealing treatment, the oxygen vacancies can move into the oxide materials, which can result in a small shift of the (111) peak of the XRD spectrum. Figure 4(c) shows the change in the FWHM of the (111) peak with increasing annealing temperature. The FWHM is considered in evaluating the quality of the CNT-PZT films [18]. The FWHM of the CNT-PZT films decreased from 0.24 to 0.19◦ with increasing annealing temperature while the FWHM of the PZT films decreased from 0.22 to 0.20◦ . For samples annealed at temperatures below 650 ◦ C, the FWHM of the PZT films was smaller than that of the CNT-PZT films. On the other hand, for samples annealed at 700 ◦ C, the FWHM of the CNT-PZT films was smaller than that of the PZT films. Figures 5(a) and 5(b) show the electric polarization (P ) - applied voltage (V ) hysteresis loops of the CNTPZT and the PZT films for several annealing temperatures. According to the P -V hysteresis loops, the remnant polarizations (Pr ) and the coercive fields (Ec ) of
Influences of Annealing Temperature on Characteristics · · · – Jin Ho Kwak et al.
-219-
ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Education, Science and Technology) (Basic Science Research Program No. 20110000258 and Priority Research Centers Program No. 2011-0031400).
REFERENCES
Fig. 5. (Color online) P -V loops of (a) CNT-PZT films and (b) PZT films; and (c) Pr and (d) Ec values obtained from graphs (a) and (b).
the two films are described in Figs. 5(c) and 5(d), respectively. With increasing annealing temperature from 650 ◦ C to 700 ◦ C, the P r of the PZT films increases while that of the CNT-PZT films decreases. The decrease in the Pr of the CNT-PZT films may originate from a decrease in the intensity of the (111) peak in the temperature range of 650 to 700 ◦ C. The Pr of the CNTPZT films annealed at 700 ◦ C is almost the same as that of typical (111)-oriented PZT films deposited by using a sol-gel method, as marked by the orange-colored star in Fig. 5(c) [19]. Figure 5(d) shows that the Ec of the CNTPZT films annealed at 700 ◦ C is smaller than that of the PZT films. The Ec values of the two films are larger than that of the (111)-oriented PZT films, as marked by the orange-colored star in Fig. 5(d) [19].
IV. CONCLUSION We have studied the influence of annealing temperature on the characteristics of composite films consisting of CNTs and PZT. During the heating process to the annealing temperature, oxygen vacancies can occur at the CNT surface inside the composite materials, and the oxygen vacancies can move into the PZT, which can result in a the shift in the position of the (111) peak in the XRD spectrum. In addition, the evaporation of CNTs at about 640 ◦ C results in a decrease in the Pr .
[1] B. H. Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee and W. Jo, Nature 401, 682 (1999). [2] J. F. Scott, Science 315, 954 (2007). [3] S. D. Bu, M. K. Lee, C. B. Eom, W. Tian, X. Q. Pan, S. K. Streiffer and J. J. Krajewski, Appl. Phys. Lett. 79, 3482 (2001). [4] J. F. Scott, F. D. Morrison, M. Miyhake, P. Zubko, X. Lou, V. M. Kugler, S. Rios, M. Zhang, T. Tatsuta, O. Tsusi and T. J. Leedham, J. Am. Ceram. Soc. 88, 1691 (2005). [5] K. Yamashita, H. Katata, M. Okuyama, H. Miyoshi, G. Kato, S. Aoyagi and Y. Suzuki, Sens. Actuators, A 97, 320 (2002). [6] S. N. Ryoo, S. G. Yoon and S. H. Kim, Appl. Phys. Lett. 83, 2880 (2003). [7] Y. Sakashita, H. Segawa, K. Tominaga and M. Okada, J. Appl. Phys. 73, 7857 (1993). [8] D. V. Taylor and D. Damjanovic, Appl. Phys. Lett. 76, 1615 (2002). [9] I. Kanno, H. Kotera, K. Wasa, T. Matsunage, T. Kamnada and R. Takayama, J. Appl. Phys. 93, 4091 (2003). [10] Q. Zou, H. Ruda, B. G. Yacobi and M. Farrell, Thin Solid Films 402, 65 (2002). [11] D. Eder and A. H. Windle, Adv. Mater. 20, 1787 (2008). [12] L. Huang, S. P. Lau, H. Y. Yang, E. S. P. Leong, S. F. Yu and S. Prawer, J. Phys. Chem. B 109, 7746 (2005). [13] H. C. Nie, N. B. Feng, X. F. Chen, G. S. Wang, X. L. Dong, Y. Gu, H. L. He and Y. S. Liu, J. Am. Ceram. Soc. 93, 642 (2010). [14] C. Ruangchalermwong and S. Muensit, Cmu. J. Nat. Sci. 7, 1 (2008). [15] D. Eder and A. H. Windle, J. Mater. Chem. 18, 2063 (2008). [16] A. Bhaskar, T. H. Chang, H. Y. Chang and S. Y. Cheng, Thin Solid Films 515, 2891 (2007). [17] P. Mars and D. W. V. Krevelen, Chem. Eng. Sci 3, 41 (1954). [18] C. C. Chang and Y. C. Lai, J. Appl. Phys. 101, 104106 (2007). [19] M. H. M. Zai, A. Akiba, H. Goto, M. Matsumoto and E. M. Yeatman, Thin Solid Films 394, 97 (2001).
