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Growth of hexagonal ZnO nanowires and nanowhiskers
Scripta Materialia 48 (2003) 1367–1371 www.actamat-journals.com
Growth of hexagonal ZnO nanowires and nanowhiskers Congkang Xu a
a,*
, Zhihui Liu b, Sheng Liu a, Guanghou Wang
a,*
National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, Nanjing 210093, PR China b Department of Biological Science and Technology, Nanjing University, Nanjing 210093, PR China Received 17 September 2002; received in revised form 18 December 2002; accepted 19 December 2002
Abstract ZnO nanowires and nanowhiskers were prepared via a novel approach in the presence of surfactant. Several techniques confirmed that the nanostructures are single crystalline with the growth direction along the [1 0 1] axis. Fourier transform infrared spectroscopy also showed that typical peak of ZnO nanostructure is shifted to high frequency. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Nanowires (ZnO); X-ray diffraction and selected-area electron; Nanostructure
1. Introduction The preparation of one dimensional (1D) nanocrystal has aroused much interest because of their great potential to test fundamental quantum mechanics concepts [1,2] and to play a vital role in various applications such as photonics [3,4], nanoelectronics [5] and data storage [6]. Over the past few years, many methods e.g. electric arc discharge [7], excimer laser ablation [8] and thermal evaporation [9] have been developed to produce a number of interesting 1D nanomaterials from insulator to metals. Zinc oxide is a directband-gap semiconducting material with an energy gap of 3.3 eV at room temperature. It has higher exciton binding energy and higher optical gain
* Corresponding authors. Tel.: +86-25-3595082; fax: +86-253595535. E-mail addresses: [email protected] (C. Xu), [email protected] (G. Wang).
than GaN at room temperature [10,11]. ZnO is a very attractive and prosperous material for lowvoltage and short wavelength electro-optical devices such as light emitting diodes and laser diodes; its other applications comprise transparent ultraviolet protection films, gas sensors [2,12] and varistors [13]. Recently, considerable attention has been focused on ZnO low dimensional materials such as nanorods, nanowires and nanobelts. In the last two to three years, ZnO nanowires have been prepared by Li et al. using chemical reaction in confined space provided by alumina templates with nanochannels [14], and Wang et al. also have produced ZnO nanowires and nanobelts by thermal evaporation route [9], on the other hand, Li et al. have synthesized ZnO nanorods by a simple gas reaction [15]. However, exploration of novel methods for the large-scale synthesis of 1D ZnO nanostructures is a challenging research field. Herein we report a novel approach to the fabrication of ZnO nanostructures by calcining ZnC2 O4 nanoparticles obtained in novel nonionic (RME)
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(02)00656-5
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C. Xu et al. / Scripta Materialia 48 (2003) 1367–1371
solvent in the presence of a NaCl flux and surfactant as well as argon gas going through the furnace. The as-prepared nanostructures include not only the nanowires but nanowhiskers as well. The sizes and morphology of the nanostructures could be controlled by adjusting the flow rate of argon gas passing through the furnace.
Japanese Rigaku D/max-cA KII X-ray diffractometer. The morphologies and dimensions of the products were observed by JEOL-2010 transmission electron microscope using an accelerating voltage of 200 kV. Meanwhile, infrared spectra (IR) were recorded by using a Nicolet 170 SX Fourier transform infrared spectroscopy (FTIR) spectrometer from 4000 to 400 cm1 at room temperature.
2. Experimental Acetate zinc Zn(CH3 COO)2 2H2 O, oxalic acid H2 C2 O4 2H2 O, sodium chloride NaCl, cyclohexane, trixon-100, poly-oxyethylene(9) nonyl phenolether (NP9) and tween-80 are used as starting materials. All chemicals are of analytical grade. The microemulsion reaction medium was prepared using cyclohexane as the continuous phase and a mixture of three chemicals, i.e. Trixon-100, NP9 and tween-80 (TNPT) with volume ratio 1:1.5:1 as nonionic surfactant. Two microemulsion compositions were prepared: 36 ml of cyclohexane þ 36 ml of TNPT þ 12 ml of 0.5 M Zn(CH3 COO)2 2H2 O; 36 ml of cyclohexane þ 36 ml of TNPT þ 12 ml of 0.5 M H2 C2 O4 2H2 O. These solutions were stirred by a magnetic stirrer at 40 °C until they became homogenous and transparent, and then mixed under magnetic stirring at the above temperature to get mixture, the precipitates were separated by centrifugation at high speed. A portion of the mixture was washed by a sufficient amount of acetone and distilled water so as to examine the composition of the precursor, The other (wet precipitates) were mixed with NaCl fine powders according to weight ratio of 1:4 and ground for 15 min. The mixture was placed into a quartz boat in a tubular furnace and heated at 920 °C for 4 h in a flow of Ar gas at 50 and 100 sccm respectively. After heat treatment, air was introduced into the reaction chamber, when the furnace cooled to 700 °C, the quartz tube was removed from the tube furnace and a yellow layer on the inner wall of the tube was found. Finally, the layer was scraped from the inner wall of the tube and washed with distilled water and ethanol. The asprepared product was identified by X-ray powder diffraction (XRD) employing a scanning rate of 0.02 deg/s in a 2h range from 15° to 75°, using a
3. Results and discussion Fig. 1 illustrates the XRD of the as-prepared sample. The diffraction peaks were quite similar to those of bulk ZnO, which can be indexed as the , hexagonal wurtzite structure ZnO (a ¼ 3:249 A ) and diffraction data were in agreec ¼ 5:206 A ment with JCPDS card of ZnO (JCPDS 36-1451). No peaks other than ZnO were detected. The general TEM micrograph of the as-synthesized product at the flow rate of 50 sccm is shown in Fig. 2(a). It demonstrates that the product mainly consists of solid wire-like structures. The diameter of the nanowires ranged from 15 to 80 nm, and the length up to tens of micrometers. The aspect ratio of the sample is up to 500. Fig. 2(b) reveals that the morphology on increasing the flow rate of Ar gas from 50 to 100 sccm and maintaining the remaining parameters,
Fig. 1. The XRD pattern of the as-prepared ZnO nanowires.
C. Xu et al. / Scripta Materialia 48 (2003) 1367–1371
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Fig. 2. TEM image of the as-prepared ZnO samples. (a) Micrograph of the as-prepared ZnO samples at the floe rate of 50 sccm. (b) Morphologies of the as-prepared ZnO samples at the flow rate of 100 sccm. (c) Morphologies of a single ZnO nanowire for SAED.
displayed a distinct change. The nanowires were obtained at a flow rate of 50 sccm whereas the nanowhiskers were obtained at 100 sccm. Fig. 3 illustrates the FTIR spectrum of the asprepared sample. The sharp band at 481.2 cm1 is characteristic of absorption bands for ZnO with a stretching mode, which is shifted to high frequency about 29 cm1 . A sharp absorption band is found at about 3429.5 cm1 arising from the contact of the as-made sample with air resulting in absorption of water vapor. This is in accordance with reported results on this system [16]. Fig. 4(a) is an HRTEM image of a representative nanowire having a diameter of about 50 nm (Fig. 2(c)), which provides further structural information about the sample. The HRTEM image (A local zone in Fig. 1(c)) was recorded along the h1 0 1i zone axis. The clear lattice fringes indicated a crystal structure of the nanowire. In this image, the [1 0 1] direction was parallel to the long axis of the nanowire, indicating that the [1 0 1] direction is a common growth direction in ZnO nanowires. Meanwhile, the observed interplanar spacing (Fig.
Fig. 3. FTIR of the as-prepared ZnO nanowires.
4(a)) is about 0.242 nm, corresponding to the (1 0 1) plane of hexagonal ZnO. This further indicates that the preferred growth plane on the nanowires is along the [1 0 1] plane. The inset (Fig.
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Fig. 4. HRTEM micrograph and SAED of one single crystalline ZnO nanowires. (a) HRTEM micrograph of a 50 nm ZnO nanowire. (b) SAED of a 50 nm ZnO nanowire.
actant provides a sufficient amount of active carbon [20] during the formation of nanowires and nanowhiskers under heating conditions. Hence, the appropriate composition of RME is of importance and significant for the preparation of ZnC2 O4 nanoparticles and subsequent formation of ZnO nanowires and nanowhiskers. Above all, in view of the theory of molten salt synthesis (MSS), NaCl can provide a favorable environment for the nucleation and successive growth. It not only can accelerate the reaction, lower reaction temperature, penetrate among the solid particles and avoid the aggregation of particles but also produce enough thermal evaporation to drive Zn and ZnO into vapor phase. A systemic study for the process parameters is underway.
4. Conclusions 4(b)) shows selected-area electron diffraction (SAED) patterns, taken from the same nanowire as shown in Fig. 2(c). This can be indexed to the reflection of wurtzite ZnO structure, which is in accordance with the above XRD results. In our case, the growth mechanism can be more likely explained by a vapor–solid (VS) mechanism [17]. Furthermore, there exists a conical tip at the end of the nanowires (as Fig. 2(a) A and B shown), which is typical of VS mechanism, confirming that the nanowires grow in terms of VS mechanism [18,19]. The explanation is in agreement with that of the literature [20]. The formation of the nanowires and nanowhiskers can be divided into three steps. In the first step ZnO is reduced to Zn in the vapor phase by active carbon, which is attributed to decomposition of the surfactant existing in the precipitates [21]. In the second step vapor of Zn and ZnO is driven by the flowing Ar atmosphere and thermal evaporation of NaCl, and deposited on the inner wall to form oxygen deficient zinc oxide crystalline nuclei. Finally, these nuclei further grow into nanowires with incoming of air. In this route, due to the fact that RME acts as a nanoreactor [22] and the as-grown precursor ZnC2 O4 is nanosized, it is favorable to form a vapor phase of Zn and ZnO. Moreover, the surf-
In summary, ZnO nanowires with diameters of 15–80 nm and lengths of several micrometers have been successfully prepared via annealing ZnC2 O4 precursors obtained in nonionic RME solvent in the presence of a surfactant, a NaCl flux and Ar gas. In this case, the surfactant is important. It acts as a nanoreactor for the preparation of the precursor and provides a source of active carbon. The as-prepared ZnO nanowires and nanowhiskers are single crystalline with a wurtzite structure. The growth mechanism can be more likely explained by VS. The different sizes and shapes of particles or wires can be achieved by controlling the flow rate of Ar gas.
Acknowledgements The work is financially supported by National Nature Science Foundation of China (No. 29890210; 10074024; 10023001).
References [1] Prokes SM, Wang KL. Mater Res Sci Bull 1999;24:13. [2] Hu J, Odom TW, Lieber CM. Acc Chem Res 1999;32:435. [3] Snoke D. Science 1996;273:1531.
C. Xu et al. / Scripta Materialia 48 (2003) 1367–1371 [4] Nakamura S. Science 1998;281:956. [5] Mirkin CA. Science 1999;286:2095. [6] OÕBarr R, Yamamoto SY, Schultz S, Xu WH, Scherer A. J Appl Phys 1997;81:4730. [7] Park GS, Choi WB, Kim JM, Choi YC, Lee YH, Lim CB. J Cryst Growth 2000;220:494. [8] Yu DP, Hang QL, Ding Y, Zhang HZ, Bai ZG, Wang JJ, et al. Appl Phys Lett 1998;73:3076. [9] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947. [10] Wong EW, Searson PC. Appl Phys Lett 1999;74:2939. [11] Choopum S, Vispute RD, Noch W, Balsamo A, Sharma RP, Venkatesan T, et al. Appl Phys Lett 1999;75:3947. [12] Dayan NJ, Sainkar SR, Karekar RN, Aiyer RC. Thin Solid Films 1998;325:254. [13] Chen CS, Kuo CT, Wu TB, Lin IN. Jpn J Appl Phys 1997; 1:1169.
1371
[14] Li Y, Meng GW, Zhang LD, Phillipp F. Appl Phys Lett 2000;76:2011. [15] Li JY, Chen XL, Li H, He M, Qiao ZY. J Cryst Growth 2001;233:5. [16] Davidov AA. IR spectroscopy of absorbed species on the surface of transition metal oxides. New York; 1984, p. 25. [17] Wu XC, Song WH, Zhao B, Sun YB, Du JJ. Chem Phys Lett 2001;349:210. [18] Mizuhara Y, Noguchi M, Ishihara T, Takita Y. J Am Ceram Soc 1995;78:109. [19] Jenning NM. J Mater Sci 1983;18:951. [20] Maeda Y. Phys Rev B 1995;51:1658. [21] Deepak FL, Vinod CP, Mukhopadhyay K, Govindaraj A, Rao CNR. Chem Phys Lett 2002;353:345. [22] Pillai V, Shah DO. Industrial application of microemulsion. New York: Marcel Dekker; 1997. p. 227.
Growth of hexagonal ZnO nanowires and nanowhiskers Congkang Xu a
a,*
, Zhihui Liu b, Sheng Liu a, Guanghou Wang
a,*
National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, Nanjing 210093, PR China b Department of Biological Science and Technology, Nanjing University, Nanjing 210093, PR China Received 17 September 2002; received in revised form 18 December 2002; accepted 19 December 2002
Abstract ZnO nanowires and nanowhiskers were prepared via a novel approach in the presence of surfactant. Several techniques confirmed that the nanostructures are single crystalline with the growth direction along the [1 0 1] axis. Fourier transform infrared spectroscopy also showed that typical peak of ZnO nanostructure is shifted to high frequency. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Nanowires (ZnO); X-ray diffraction and selected-area electron; Nanostructure
1. Introduction The preparation of one dimensional (1D) nanocrystal has aroused much interest because of their great potential to test fundamental quantum mechanics concepts [1,2] and to play a vital role in various applications such as photonics [3,4], nanoelectronics [5] and data storage [6]. Over the past few years, many methods e.g. electric arc discharge [7], excimer laser ablation [8] and thermal evaporation [9] have been developed to produce a number of interesting 1D nanomaterials from insulator to metals. Zinc oxide is a directband-gap semiconducting material with an energy gap of 3.3 eV at room temperature. It has higher exciton binding energy and higher optical gain
* Corresponding authors. Tel.: +86-25-3595082; fax: +86-253595535. E-mail addresses: [email protected] (C. Xu), [email protected] (G. Wang).
than GaN at room temperature [10,11]. ZnO is a very attractive and prosperous material for lowvoltage and short wavelength electro-optical devices such as light emitting diodes and laser diodes; its other applications comprise transparent ultraviolet protection films, gas sensors [2,12] and varistors [13]. Recently, considerable attention has been focused on ZnO low dimensional materials such as nanorods, nanowires and nanobelts. In the last two to three years, ZnO nanowires have been prepared by Li et al. using chemical reaction in confined space provided by alumina templates with nanochannels [14], and Wang et al. also have produced ZnO nanowires and nanobelts by thermal evaporation route [9], on the other hand, Li et al. have synthesized ZnO nanorods by a simple gas reaction [15]. However, exploration of novel methods for the large-scale synthesis of 1D ZnO nanostructures is a challenging research field. Herein we report a novel approach to the fabrication of ZnO nanostructures by calcining ZnC2 O4 nanoparticles obtained in novel nonionic (RME)
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(02)00656-5
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C. Xu et al. / Scripta Materialia 48 (2003) 1367–1371
solvent in the presence of a NaCl flux and surfactant as well as argon gas going through the furnace. The as-prepared nanostructures include not only the nanowires but nanowhiskers as well. The sizes and morphology of the nanostructures could be controlled by adjusting the flow rate of argon gas passing through the furnace.
Japanese Rigaku D/max-cA KII X-ray diffractometer. The morphologies and dimensions of the products were observed by JEOL-2010 transmission electron microscope using an accelerating voltage of 200 kV. Meanwhile, infrared spectra (IR) were recorded by using a Nicolet 170 SX Fourier transform infrared spectroscopy (FTIR) spectrometer from 4000 to 400 cm1 at room temperature.
2. Experimental Acetate zinc Zn(CH3 COO)2 2H2 O, oxalic acid H2 C2 O4 2H2 O, sodium chloride NaCl, cyclohexane, trixon-100, poly-oxyethylene(9) nonyl phenolether (NP9) and tween-80 are used as starting materials. All chemicals are of analytical grade. The microemulsion reaction medium was prepared using cyclohexane as the continuous phase and a mixture of three chemicals, i.e. Trixon-100, NP9 and tween-80 (TNPT) with volume ratio 1:1.5:1 as nonionic surfactant. Two microemulsion compositions were prepared: 36 ml of cyclohexane þ 36 ml of TNPT þ 12 ml of 0.5 M Zn(CH3 COO)2 2H2 O; 36 ml of cyclohexane þ 36 ml of TNPT þ 12 ml of 0.5 M H2 C2 O4 2H2 O. These solutions were stirred by a magnetic stirrer at 40 °C until they became homogenous and transparent, and then mixed under magnetic stirring at the above temperature to get mixture, the precipitates were separated by centrifugation at high speed. A portion of the mixture was washed by a sufficient amount of acetone and distilled water so as to examine the composition of the precursor, The other (wet precipitates) were mixed with NaCl fine powders according to weight ratio of 1:4 and ground for 15 min. The mixture was placed into a quartz boat in a tubular furnace and heated at 920 °C for 4 h in a flow of Ar gas at 50 and 100 sccm respectively. After heat treatment, air was introduced into the reaction chamber, when the furnace cooled to 700 °C, the quartz tube was removed from the tube furnace and a yellow layer on the inner wall of the tube was found. Finally, the layer was scraped from the inner wall of the tube and washed with distilled water and ethanol. The asprepared product was identified by X-ray powder diffraction (XRD) employing a scanning rate of 0.02 deg/s in a 2h range from 15° to 75°, using a
3. Results and discussion Fig. 1 illustrates the XRD of the as-prepared sample. The diffraction peaks were quite similar to those of bulk ZnO, which can be indexed as the , hexagonal wurtzite structure ZnO (a ¼ 3:249 A ) and diffraction data were in agreec ¼ 5:206 A ment with JCPDS card of ZnO (JCPDS 36-1451). No peaks other than ZnO were detected. The general TEM micrograph of the as-synthesized product at the flow rate of 50 sccm is shown in Fig. 2(a). It demonstrates that the product mainly consists of solid wire-like structures. The diameter of the nanowires ranged from 15 to 80 nm, and the length up to tens of micrometers. The aspect ratio of the sample is up to 500. Fig. 2(b) reveals that the morphology on increasing the flow rate of Ar gas from 50 to 100 sccm and maintaining the remaining parameters,
Fig. 1. The XRD pattern of the as-prepared ZnO nanowires.
C. Xu et al. / Scripta Materialia 48 (2003) 1367–1371
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Fig. 2. TEM image of the as-prepared ZnO samples. (a) Micrograph of the as-prepared ZnO samples at the floe rate of 50 sccm. (b) Morphologies of the as-prepared ZnO samples at the flow rate of 100 sccm. (c) Morphologies of a single ZnO nanowire for SAED.
displayed a distinct change. The nanowires were obtained at a flow rate of 50 sccm whereas the nanowhiskers were obtained at 100 sccm. Fig. 3 illustrates the FTIR spectrum of the asprepared sample. The sharp band at 481.2 cm1 is characteristic of absorption bands for ZnO with a stretching mode, which is shifted to high frequency about 29 cm1 . A sharp absorption band is found at about 3429.5 cm1 arising from the contact of the as-made sample with air resulting in absorption of water vapor. This is in accordance with reported results on this system [16]. Fig. 4(a) is an HRTEM image of a representative nanowire having a diameter of about 50 nm (Fig. 2(c)), which provides further structural information about the sample. The HRTEM image (A local zone in Fig. 1(c)) was recorded along the h1 0 1i zone axis. The clear lattice fringes indicated a crystal structure of the nanowire. In this image, the [1 0 1] direction was parallel to the long axis of the nanowire, indicating that the [1 0 1] direction is a common growth direction in ZnO nanowires. Meanwhile, the observed interplanar spacing (Fig.
Fig. 3. FTIR of the as-prepared ZnO nanowires.
4(a)) is about 0.242 nm, corresponding to the (1 0 1) plane of hexagonal ZnO. This further indicates that the preferred growth plane on the nanowires is along the [1 0 1] plane. The inset (Fig.
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Fig. 4. HRTEM micrograph and SAED of one single crystalline ZnO nanowires. (a) HRTEM micrograph of a 50 nm ZnO nanowire. (b) SAED of a 50 nm ZnO nanowire.
actant provides a sufficient amount of active carbon [20] during the formation of nanowires and nanowhiskers under heating conditions. Hence, the appropriate composition of RME is of importance and significant for the preparation of ZnC2 O4 nanoparticles and subsequent formation of ZnO nanowires and nanowhiskers. Above all, in view of the theory of molten salt synthesis (MSS), NaCl can provide a favorable environment for the nucleation and successive growth. It not only can accelerate the reaction, lower reaction temperature, penetrate among the solid particles and avoid the aggregation of particles but also produce enough thermal evaporation to drive Zn and ZnO into vapor phase. A systemic study for the process parameters is underway.
4. Conclusions 4(b)) shows selected-area electron diffraction (SAED) patterns, taken from the same nanowire as shown in Fig. 2(c). This can be indexed to the reflection of wurtzite ZnO structure, which is in accordance with the above XRD results. In our case, the growth mechanism can be more likely explained by a vapor–solid (VS) mechanism [17]. Furthermore, there exists a conical tip at the end of the nanowires (as Fig. 2(a) A and B shown), which is typical of VS mechanism, confirming that the nanowires grow in terms of VS mechanism [18,19]. The explanation is in agreement with that of the literature [20]. The formation of the nanowires and nanowhiskers can be divided into three steps. In the first step ZnO is reduced to Zn in the vapor phase by active carbon, which is attributed to decomposition of the surfactant existing in the precipitates [21]. In the second step vapor of Zn and ZnO is driven by the flowing Ar atmosphere and thermal evaporation of NaCl, and deposited on the inner wall to form oxygen deficient zinc oxide crystalline nuclei. Finally, these nuclei further grow into nanowires with incoming of air. In this route, due to the fact that RME acts as a nanoreactor [22] and the as-grown precursor ZnC2 O4 is nanosized, it is favorable to form a vapor phase of Zn and ZnO. Moreover, the surf-
In summary, ZnO nanowires with diameters of 15–80 nm and lengths of several micrometers have been successfully prepared via annealing ZnC2 O4 precursors obtained in nonionic RME solvent in the presence of a surfactant, a NaCl flux and Ar gas. In this case, the surfactant is important. It acts as a nanoreactor for the preparation of the precursor and provides a source of active carbon. The as-prepared ZnO nanowires and nanowhiskers are single crystalline with a wurtzite structure. The growth mechanism can be more likely explained by VS. The different sizes and shapes of particles or wires can be achieved by controlling the flow rate of Ar gas.
Acknowledgements The work is financially supported by National Nature Science Foundation of China (No. 29890210; 10074024; 10023001).
References [1] Prokes SM, Wang KL. Mater Res Sci Bull 1999;24:13. [2] Hu J, Odom TW, Lieber CM. Acc Chem Res 1999;32:435. [3] Snoke D. Science 1996;273:1531.
C. Xu et al. / Scripta Materialia 48 (2003) 1367–1371 [4] Nakamura S. Science 1998;281:956. [5] Mirkin CA. Science 1999;286:2095. [6] OÕBarr R, Yamamoto SY, Schultz S, Xu WH, Scherer A. J Appl Phys 1997;81:4730. [7] Park GS, Choi WB, Kim JM, Choi YC, Lee YH, Lim CB. J Cryst Growth 2000;220:494. [8] Yu DP, Hang QL, Ding Y, Zhang HZ, Bai ZG, Wang JJ, et al. Appl Phys Lett 1998;73:3076. [9] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947. [10] Wong EW, Searson PC. Appl Phys Lett 1999;74:2939. [11] Choopum S, Vispute RD, Noch W, Balsamo A, Sharma RP, Venkatesan T, et al. Appl Phys Lett 1999;75:3947. [12] Dayan NJ, Sainkar SR, Karekar RN, Aiyer RC. Thin Solid Films 1998;325:254. [13] Chen CS, Kuo CT, Wu TB, Lin IN. Jpn J Appl Phys 1997; 1:1169.
1371
[14] Li Y, Meng GW, Zhang LD, Phillipp F. Appl Phys Lett 2000;76:2011. [15] Li JY, Chen XL, Li H, He M, Qiao ZY. J Cryst Growth 2001;233:5. [16] Davidov AA. IR spectroscopy of absorbed species on the surface of transition metal oxides. New York; 1984, p. 25. [17] Wu XC, Song WH, Zhao B, Sun YB, Du JJ. Chem Phys Lett 2001;349:210. [18] Mizuhara Y, Noguchi M, Ishihara T, Takita Y. J Am Ceram Soc 1995;78:109. [19] Jenning NM. J Mater Sci 1983;18:951. [20] Maeda Y. Phys Rev B 1995;51:1658. [21] Deepak FL, Vinod CP, Mukhopadhyay K, Govindaraj A, Rao CNR. Chem Phys Lett 2002;353:345. [22] Pillai V, Shah DO. Industrial application of microemulsion. New York: Marcel Dekker; 1997. p. 227.