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Large-scale preparation of needle-like zinc oxide with high electrical conductivity
Materials Letters 60 (2006) 3133 – 3136 www.elsevier.com/locate/matlet
Large-scale preparation of needle-like zinc oxide with high electrical conductivity Shangfeng Du, Yajun Tian, Jian Liu, Haidi Liu, Yunfa Chen ⁎ Multi-phase Reaction Laboratory, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, PR China Received 16 November 2005; accepted 21 February 2006 Available online 15 March 2006
Abstract Needle-like zinc oxide with high electrical conductivity has been successfully prepared in large-scale from calcining the need-like precursor synthesized by a simple co-precipitation approach with ZnCl2 as Zinc source, GaCl3 as Gallium source and NH4HCO3 as precipitant under an optimized conditions (45 °C and pH = 7.4–7.5). The as-fabricated products were characterized by means of TEM, SEM, XRD, EDS and XPS. Their electrical conductivities were also studied, showing that the volume resistivity of the needle-like zinc oxide with 2.2 mol% Ga3+ dopant was lower than 20 Ω·cm. © 2006 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Needle-like; Gallium-doped; Electrical properties; Powder technology
1. Introduction ZnO is a wide band-gap (3.37 eV) and large exciton binding energy (60 meV) II–VI semiconductor. Besides its outstanding performance in optics [1], optoelectronics [2], piezoelectricity [3], gas sensor [4,5] and so on, more attention has also been paid to ZnO doping for obtaining transparent electro-conductive materials that was used as fillers to polymer or paper for electrostatic prevention or electromagnetic shielding [6,7]. A large-size sphere-like zinc oxide powders with high electrical conductivity (about 3.3 × 10− 2 Ω·cm) has been prepared by Wang [6] and Katsuhiko [8]. But there is a well known problem with this sphere-like ZnO powders as electro-conductive fillers, namely, in many occasions, in order to improve the conductance of bulk materials, an increasing amount of filler has to be added into, which would simultaneously decrease the strength and ductibility of the composite [10]. When fiber-like or needle-like materials are used as filler, plentiful meshwork is likely to be built up by interlacement due to their high slenderness ratio [10]. Because the current-carrying paths in materials are formed by the contacts between the electro-conductive filler particles, this meshwork will increase ⁎ Corresponding author. Tel.: +86 10 82627057. E-mail address: [email protected] (Y. Chen). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.02.068
chance for the creation of such current-carrying paths [11]. In the way, the meshwork would result in a smaller consumption quantity of filler; correspondingly, the related negative impact on the mechanical properties would be reduced; contrarily, these meshwork structures could enforce the mechanical performances of the composite. Because the needle-like particle has peculiar advantages over spherical particle, accordingly it should be interesting and meaningful to synthesize well electro-conductive zinc oxide particles with needle-like morphology. Previously, Zhou [7] and Katsuhiko [9] had synthesized needle-like zinc oxide particles, but their corresponding electrical conductivities were very low. In this letter, we demonstrate that the needle-like zinc oxide material with electrical conductivity lower than 20 Ω·cm is prepared in large-scale by calcining needle-like precursors of Zn and Ga, and the above precursors are obtained by a simple coprecipitation method without any surfactant or template under a mild condition. 2. Experimental 2.1. Sample preparation Firstly, 1.26 mol/L NH4HCO3 solution and 1.5 mol/L mixed solution of ZnCl2 and GaCl3 (about 2.2 mol%) [12] were
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Fig. 1. TEM and SEM images of precursor synthesized by co-precipitation method at 45 °C, pH = 7.4–7.5, respectively, ZnCl2, GaCl3 and NH4HCO3 were used as raw materials.
prepared, then the two solutions were dropwise added together into the mixture of distilled water and ethanol (VWater : VEtOH = 7 : 8) whilst vigorous stirring to generate white precipitation, the pH value of the reaction system was controlled within the range of 7.4–7.5 by regulating the dropping rate of NH4HCO3 solution, and the reaction temperature was around 45 °C. Aging for 1 h, the white precursor powders were obtained by centrifugal separation, then washed with absolute ethanol three times and dried at 45 °C. Finally, the desired products were obtained by calcining the above precursor powders in the atmosphere of H2 to 540 °C with the heating rate of 2 °C/min and held there for 1 h. 2.2. Characterization The morphologies were observed using JSM-6700F field emission scanning electron microscopy (SEM) and H-600A transmission electron microscopy (TEM). The XRD pattern was recorded in 2θ from 20° to 90° with a scanning step of 0.0167° on PANalytical X'Pert PRO X-ray diffraction using CuKα (λ = 0.15418 nm) radiation at 40 kV and 30 mA. The small area element analysis was done on energy dispersive X-ray spectrometer (Oxford INCAEnergyTEM EDS). ESCA Lab220i X-ray photoelectron spectroscopy (XPS) was used to determine the particles surface composition. The electrical conductivities of as-prepared products and the contrast samples were measured by the following method: coinshaped specimen was fabricated with 5 g sample with a
diameter of ϕ 30 mm under uniaxial pressure of 30 MPa. Electrodes with diameter of about 26 mm were formed by Ag paste at 105 °C. The volume resistivity was measured by a low resistance-determining device (My-68, available from MASTECH. Co. Ltd.). 3. Results and discussion Fig. 1 was the TEM and SEM images of the precursors, showing that the precursor particles were of needle-like shape with the diameters from 60 to 280 nm and the lengths of 1–6 μm. Our experiments confirmed that the reactions of Zn2+ and Ga3+ ions with ammonium acid carbonate were two separate processes according with the Reaction (1) and (2), implying that the product from this coprecipitation process should be a homogeneous intermixture of Zn5 (OH)6·(CO3)2 and GaOOH. 5Zn2þ þ 10NH4 HCO3 →Zn5 ðOHÞ6 d ðCO3 Þ2 ↓ þ 10NHþ 4 þ 8CO2 ↑ þ 2H2 O
ð1Þ
Ga3þ þ 3NH4 HCO3 →GaOOH↓ þ 3NHþ 4 þ 3CO2 ↑ þ H2 O
ð2Þ
Many researches have been done to investigate the crystal growth of ZnO particles in hydrothermal conditions [13,14], indicating that the growth procedure was highly influenced by the prepared parameters, including solvent, temperature, supersaturation, and so on. For the growth mechanism of precursor particles of ZnO, especially the Gallium-doped precursor, few reports have been found. In our experiments, it was noticed the applied pH value of the reaction solution was controlled in the range of 7.4–7.5, which was much higher
Fig. 2. TEM images of as-prepared ZnO sample by calcining needle-like precursor under H2 at 540 °C with the heating rate of 2 °C/min and hold there for 1 h.
S. Du et al. / Materials Letters 60 (2006) 3133–3136 Table 1 Volume resistivities of the as-prepared product and the contrast samples Sample
As-prepared product
23-K
Pazet GK
Doped element
Gallium-doped ZnO <20
Aluminum-doped ZnO <1000
Gallium-doped ZnO <100
Volume resistivity (Ω-cm)
than the normal 6.8–7.0 to prepare Zn5(OH)6·(CO3)2 according to the stoichiometric ratio [15]; the mixture of distilled water and ethanol (VWater : VEtOH = 7 : 8) was used in co-precipitation process, and the existence of ethanol has important influence on the particle solubility and surface energy [14]. Therefore, it was inferred that the environment of high pH value and the existence of ethanol probably acted specific effects on the crystal growth and resulted in the formation of needlelike morphology. But until now, the detailed growth mechanism is not very clear. Fig. 2 (a) and (b) showed the TEM images of as-fabricated ZnO particles produced by calcining needle-like precursors doped with 2.2 mol% Gallium [12] at 540 °C for 1 h in H2 with heating rate of 2 °C/ min. From the images, it was found that the morphology of product was highly similar to that of precursors, and that their root diameter ranged between 50 and 270 nm and the length varied from hundreds of nanometers to several microns. Fig. 2 (b) presented an individual needle, and the EDS characterization of this single needle indicated that Gallium is about 1.90 mol% of the total amount of Gallium and Zinc, that basically matched with the Gallium amount in above experiment. Table 1 compared the volume resistivities of our as-prepared product and the contrast samples: 23-K and Pazet GK that were the excellent commercial electro-conductive zinc oxide powders prepared by vaporization process available from Hakusui Tech. Co. Ltd. [8]. Excitingly, the volume resistivity of our product was less than 20 Ω·cm and lower about one order than the contrast samples. That is the needlelike zinc oxide product with high concentration of current carries was gained in our lab [6]. Fig. 3 (a) was the XRD pattern of as-prepared product. The Miller indices were specified above the corresponding peaks, and all peaks were indexed to hexagonal crystalline ZnO (JCPDS card No. 79-2205). On the basis of the experimental results of the excellent electrical conductivity of zinc oxide, it was concluded that the doping of Gallium should be achieved during calcining treatment. As seen in Fig. 3 (b) that Ga 2p peak at 1118.00 eV was detected in addition to Zn 2p peak at 1021.90 eV and O 1s peak at 530.65 eV, reasonably supporting that
3135
there was Ga3+ existed in ZnO systems. In conclusion, from above characterizations and analyzing, it is suggested that the doped Gallium exists as the substitution form of Ga•Zn in the single hexagonal crystalline ZnO phase and no new phase was generated during the doping process [6,16]. As the precursor was heated, it decomposed into the intermixture of ZnO and Ga2O3 following the Reactions (3) and (4), and then Ga2O3 doped into ZnO lattices following the Reaction (5) with the increase of heating temperature, where the corresponding interstitial O-atoms (O−i ) were generated [6,17]. Subsequently, with the aid of reducing atmosphere of H2, the interstitial O-atoms were removed as possible as it could (following Reaction (6)) and hence a plentiful corresponding free carries were released. Zn5 ðOHÞ6 d ðCO3 Þ2 →5ZnO þ 2CO2 ↑ þ 3H2 O↑
ð3Þ
2GaOOH→Ga2 O3 þ H2 O↑
ð4Þ
2Ga2 O3 →4GaZn þ 4OO þ 2O−i
ð5Þ
2O−i þ H2 →O2 ↑ þ 4e− þ H2 O↑
ð6Þ
•
According to the above discussion, the Gallium doped into ZnO lattice is the crucial reason for the product electrical conductibility. The higher the doped level is achieved, the better electrical conductivity will be acquired. Although our doped amount is lower than the theoretical doped level of 2.7 mol% estimated by electron microprobe analysis (EMPA) [6], our result is much better than that formerly reported [8,16]. Taking into account of their unique morphology, the needle-like electro-conductive zinc oxide product is prospective to be applied as the high performance transparent electric filler. In conclusion, though the precise controlling of the co-precipitation parameters and the calcination process, Gallium-doped zinc oxide with needle-like morphology and high electrical conductivity has been obtained. This simple synthesis approach does not need any surfactant or template, and has advantages of simplicity, low-temperature, lowcost and large-scale production.
4. Conclusions Calcining the needle-like intermixture precursor of Zinc and Gallium synthesized by a co-precipitation method, the needlelike Gallium-doped zinc oxide with high electrical conductivity below 20 Ω·cm was acquired. In terms of the relevant characterizations, the doping mechanism was briefly discussed.
Fig. 3. (a) XRD pattern and (b) XPS result of as-prepared product.
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S. Du et al. / Materials Letters 60 (2006) 3133–3136
Acknowledgments We thank the financial support from the National Natural Science Foundation of China (90406024) and Institute of Process Engineering, Chinese Academy of Sciences. References [1] J.F. Wager, Science 300 (2003) 1245. [2] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [3] Z.L. Wang, Mater. Today (Jun. 2004) 26. [4] B.L. Zhu, C.S. Xie, A.H. Wang, D.W. Zeng, W.L. Song, X.Z. Zhao, Mater. Lett. 59 (2005) 1004. [5] J.D. Choi, G.M. Choi, Sens. Actuators, B 69 (2000) 120. [6] R.P. Wang, A.W. Sleight, Chem. Mater. 8 (1996) 433. [7] Z.W. Zhou, L.S. Chu, W.M. Tang, L.X. Gu, J. Electrost. 57 (2003) 347.
[8] Y. Katsuhiko, K. Hideo, H. Takao, K. Nobuyoshi, Eur. Pat. 0597380A1 (1994). [9] Y. Katsuhiko, T. Hiroyuki, H. Mitsuihiko, Eur. Pat. 0598284A1 (1994). [10] D.D.L. Chung, Carbon 39 (2001) 279. [11] S.G. Chen, C.H. Li, W.H. Xiong, L.M. Liu, H. Wang, Mater. Lett. 59 (2005) 1342. [12] S.F. Du, W.M. Gao, J. Liu, Y.F. Chen, Y. Liang, Rare Metal Mat. Eng. 35 (7) (2006). [13] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem., Int. Ed. 41 (2002) 1188. [14] Z.S. Hu, G. Oskam, P.C. Searson, J. Colloid Interface Sci. 263 (2003) 454. [15] F.L. Yuan, P. Hu, C.L. Yin, S.L. Huang, J.L. Li, J. Mater. Chem. 13 (2003) 634. [16] M.H. Yoon, S.H. Lee, H.L. Pard, H.K. Kim, M.S. Jang, J. Mater. Sci. Lett. 21 (2002) 1703. [17] N. Roberts, R.P. Wang, A.W. Sleight, W.W. Warren, Phys. Rev., B 57 (1998) 5734.
Large-scale preparation of needle-like zinc oxide with high electrical conductivity Shangfeng Du, Yajun Tian, Jian Liu, Haidi Liu, Yunfa Chen ⁎ Multi-phase Reaction Laboratory, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, PR China Received 16 November 2005; accepted 21 February 2006 Available online 15 March 2006
Abstract Needle-like zinc oxide with high electrical conductivity has been successfully prepared in large-scale from calcining the need-like precursor synthesized by a simple co-precipitation approach with ZnCl2 as Zinc source, GaCl3 as Gallium source and NH4HCO3 as precipitant under an optimized conditions (45 °C and pH = 7.4–7.5). The as-fabricated products were characterized by means of TEM, SEM, XRD, EDS and XPS. Their electrical conductivities were also studied, showing that the volume resistivity of the needle-like zinc oxide with 2.2 mol% Ga3+ dopant was lower than 20 Ω·cm. © 2006 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Needle-like; Gallium-doped; Electrical properties; Powder technology
1. Introduction ZnO is a wide band-gap (3.37 eV) and large exciton binding energy (60 meV) II–VI semiconductor. Besides its outstanding performance in optics [1], optoelectronics [2], piezoelectricity [3], gas sensor [4,5] and so on, more attention has also been paid to ZnO doping for obtaining transparent electro-conductive materials that was used as fillers to polymer or paper for electrostatic prevention or electromagnetic shielding [6,7]. A large-size sphere-like zinc oxide powders with high electrical conductivity (about 3.3 × 10− 2 Ω·cm) has been prepared by Wang [6] and Katsuhiko [8]. But there is a well known problem with this sphere-like ZnO powders as electro-conductive fillers, namely, in many occasions, in order to improve the conductance of bulk materials, an increasing amount of filler has to be added into, which would simultaneously decrease the strength and ductibility of the composite [10]. When fiber-like or needle-like materials are used as filler, plentiful meshwork is likely to be built up by interlacement due to their high slenderness ratio [10]. Because the current-carrying paths in materials are formed by the contacts between the electro-conductive filler particles, this meshwork will increase ⁎ Corresponding author. Tel.: +86 10 82627057. E-mail address: [email protected] (Y. Chen). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.02.068
chance for the creation of such current-carrying paths [11]. In the way, the meshwork would result in a smaller consumption quantity of filler; correspondingly, the related negative impact on the mechanical properties would be reduced; contrarily, these meshwork structures could enforce the mechanical performances of the composite. Because the needle-like particle has peculiar advantages over spherical particle, accordingly it should be interesting and meaningful to synthesize well electro-conductive zinc oxide particles with needle-like morphology. Previously, Zhou [7] and Katsuhiko [9] had synthesized needle-like zinc oxide particles, but their corresponding electrical conductivities were very low. In this letter, we demonstrate that the needle-like zinc oxide material with electrical conductivity lower than 20 Ω·cm is prepared in large-scale by calcining needle-like precursors of Zn and Ga, and the above precursors are obtained by a simple coprecipitation method without any surfactant or template under a mild condition. 2. Experimental 2.1. Sample preparation Firstly, 1.26 mol/L NH4HCO3 solution and 1.5 mol/L mixed solution of ZnCl2 and GaCl3 (about 2.2 mol%) [12] were
3134
S. Du et al. / Materials Letters 60 (2006) 3133–3136
Fig. 1. TEM and SEM images of precursor synthesized by co-precipitation method at 45 °C, pH = 7.4–7.5, respectively, ZnCl2, GaCl3 and NH4HCO3 were used as raw materials.
prepared, then the two solutions were dropwise added together into the mixture of distilled water and ethanol (VWater : VEtOH = 7 : 8) whilst vigorous stirring to generate white precipitation, the pH value of the reaction system was controlled within the range of 7.4–7.5 by regulating the dropping rate of NH4HCO3 solution, and the reaction temperature was around 45 °C. Aging for 1 h, the white precursor powders were obtained by centrifugal separation, then washed with absolute ethanol three times and dried at 45 °C. Finally, the desired products were obtained by calcining the above precursor powders in the atmosphere of H2 to 540 °C with the heating rate of 2 °C/min and held there for 1 h. 2.2. Characterization The morphologies were observed using JSM-6700F field emission scanning electron microscopy (SEM) and H-600A transmission electron microscopy (TEM). The XRD pattern was recorded in 2θ from 20° to 90° with a scanning step of 0.0167° on PANalytical X'Pert PRO X-ray diffraction using CuKα (λ = 0.15418 nm) radiation at 40 kV and 30 mA. The small area element analysis was done on energy dispersive X-ray spectrometer (Oxford INCAEnergyTEM EDS). ESCA Lab220i X-ray photoelectron spectroscopy (XPS) was used to determine the particles surface composition. The electrical conductivities of as-prepared products and the contrast samples were measured by the following method: coinshaped specimen was fabricated with 5 g sample with a
diameter of ϕ 30 mm under uniaxial pressure of 30 MPa. Electrodes with diameter of about 26 mm were formed by Ag paste at 105 °C. The volume resistivity was measured by a low resistance-determining device (My-68, available from MASTECH. Co. Ltd.). 3. Results and discussion Fig. 1 was the TEM and SEM images of the precursors, showing that the precursor particles were of needle-like shape with the diameters from 60 to 280 nm and the lengths of 1–6 μm. Our experiments confirmed that the reactions of Zn2+ and Ga3+ ions with ammonium acid carbonate were two separate processes according with the Reaction (1) and (2), implying that the product from this coprecipitation process should be a homogeneous intermixture of Zn5 (OH)6·(CO3)2 and GaOOH. 5Zn2þ þ 10NH4 HCO3 →Zn5 ðOHÞ6 d ðCO3 Þ2 ↓ þ 10NHþ 4 þ 8CO2 ↑ þ 2H2 O
ð1Þ
Ga3þ þ 3NH4 HCO3 →GaOOH↓ þ 3NHþ 4 þ 3CO2 ↑ þ H2 O
ð2Þ
Many researches have been done to investigate the crystal growth of ZnO particles in hydrothermal conditions [13,14], indicating that the growth procedure was highly influenced by the prepared parameters, including solvent, temperature, supersaturation, and so on. For the growth mechanism of precursor particles of ZnO, especially the Gallium-doped precursor, few reports have been found. In our experiments, it was noticed the applied pH value of the reaction solution was controlled in the range of 7.4–7.5, which was much higher
Fig. 2. TEM images of as-prepared ZnO sample by calcining needle-like precursor under H2 at 540 °C with the heating rate of 2 °C/min and hold there for 1 h.
S. Du et al. / Materials Letters 60 (2006) 3133–3136 Table 1 Volume resistivities of the as-prepared product and the contrast samples Sample
As-prepared product
23-K
Pazet GK
Doped element
Gallium-doped ZnO <20
Aluminum-doped ZnO <1000
Gallium-doped ZnO <100
Volume resistivity (Ω-cm)
than the normal 6.8–7.0 to prepare Zn5(OH)6·(CO3)2 according to the stoichiometric ratio [15]; the mixture of distilled water and ethanol (VWater : VEtOH = 7 : 8) was used in co-precipitation process, and the existence of ethanol has important influence on the particle solubility and surface energy [14]. Therefore, it was inferred that the environment of high pH value and the existence of ethanol probably acted specific effects on the crystal growth and resulted in the formation of needlelike morphology. But until now, the detailed growth mechanism is not very clear. Fig. 2 (a) and (b) showed the TEM images of as-fabricated ZnO particles produced by calcining needle-like precursors doped with 2.2 mol% Gallium [12] at 540 °C for 1 h in H2 with heating rate of 2 °C/ min. From the images, it was found that the morphology of product was highly similar to that of precursors, and that their root diameter ranged between 50 and 270 nm and the length varied from hundreds of nanometers to several microns. Fig. 2 (b) presented an individual needle, and the EDS characterization of this single needle indicated that Gallium is about 1.90 mol% of the total amount of Gallium and Zinc, that basically matched with the Gallium amount in above experiment. Table 1 compared the volume resistivities of our as-prepared product and the contrast samples: 23-K and Pazet GK that were the excellent commercial electro-conductive zinc oxide powders prepared by vaporization process available from Hakusui Tech. Co. Ltd. [8]. Excitingly, the volume resistivity of our product was less than 20 Ω·cm and lower about one order than the contrast samples. That is the needlelike zinc oxide product with high concentration of current carries was gained in our lab [6]. Fig. 3 (a) was the XRD pattern of as-prepared product. The Miller indices were specified above the corresponding peaks, and all peaks were indexed to hexagonal crystalline ZnO (JCPDS card No. 79-2205). On the basis of the experimental results of the excellent electrical conductivity of zinc oxide, it was concluded that the doping of Gallium should be achieved during calcining treatment. As seen in Fig. 3 (b) that Ga 2p peak at 1118.00 eV was detected in addition to Zn 2p peak at 1021.90 eV and O 1s peak at 530.65 eV, reasonably supporting that
3135
there was Ga3+ existed in ZnO systems. In conclusion, from above characterizations and analyzing, it is suggested that the doped Gallium exists as the substitution form of Ga•Zn in the single hexagonal crystalline ZnO phase and no new phase was generated during the doping process [6,16]. As the precursor was heated, it decomposed into the intermixture of ZnO and Ga2O3 following the Reactions (3) and (4), and then Ga2O3 doped into ZnO lattices following the Reaction (5) with the increase of heating temperature, where the corresponding interstitial O-atoms (O−i ) were generated [6,17]. Subsequently, with the aid of reducing atmosphere of H2, the interstitial O-atoms were removed as possible as it could (following Reaction (6)) and hence a plentiful corresponding free carries were released. Zn5 ðOHÞ6 d ðCO3 Þ2 →5ZnO þ 2CO2 ↑ þ 3H2 O↑
ð3Þ
2GaOOH→Ga2 O3 þ H2 O↑
ð4Þ
2Ga2 O3 →4GaZn þ 4OO þ 2O−i
ð5Þ
2O−i þ H2 →O2 ↑ þ 4e− þ H2 O↑
ð6Þ
•
According to the above discussion, the Gallium doped into ZnO lattice is the crucial reason for the product electrical conductibility. The higher the doped level is achieved, the better electrical conductivity will be acquired. Although our doped amount is lower than the theoretical doped level of 2.7 mol% estimated by electron microprobe analysis (EMPA) [6], our result is much better than that formerly reported [8,16]. Taking into account of their unique morphology, the needle-like electro-conductive zinc oxide product is prospective to be applied as the high performance transparent electric filler. In conclusion, though the precise controlling of the co-precipitation parameters and the calcination process, Gallium-doped zinc oxide with needle-like morphology and high electrical conductivity has been obtained. This simple synthesis approach does not need any surfactant or template, and has advantages of simplicity, low-temperature, lowcost and large-scale production.
4. Conclusions Calcining the needle-like intermixture precursor of Zinc and Gallium synthesized by a co-precipitation method, the needlelike Gallium-doped zinc oxide with high electrical conductivity below 20 Ω·cm was acquired. In terms of the relevant characterizations, the doping mechanism was briefly discussed.
Fig. 3. (a) XRD pattern and (b) XPS result of as-prepared product.
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S. Du et al. / Materials Letters 60 (2006) 3133–3136
Acknowledgments We thank the financial support from the National Natural Science Foundation of China (90406024) and Institute of Process Engineering, Chinese Academy of Sciences. References [1] J.F. Wager, Science 300 (2003) 1245. [2] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [3] Z.L. Wang, Mater. Today (Jun. 2004) 26. [4] B.L. Zhu, C.S. Xie, A.H. Wang, D.W. Zeng, W.L. Song, X.Z. Zhao, Mater. Lett. 59 (2005) 1004. [5] J.D. Choi, G.M. Choi, Sens. Actuators, B 69 (2000) 120. [6] R.P. Wang, A.W. Sleight, Chem. Mater. 8 (1996) 433. [7] Z.W. Zhou, L.S. Chu, W.M. Tang, L.X. Gu, J. Electrost. 57 (2003) 347.
[8] Y. Katsuhiko, K. Hideo, H. Takao, K. Nobuyoshi, Eur. Pat. 0597380A1 (1994). [9] Y. Katsuhiko, T. Hiroyuki, H. Mitsuihiko, Eur. Pat. 0598284A1 (1994). [10] D.D.L. Chung, Carbon 39 (2001) 279. [11] S.G. Chen, C.H. Li, W.H. Xiong, L.M. Liu, H. Wang, Mater. Lett. 59 (2005) 1342. [12] S.F. Du, W.M. Gao, J. Liu, Y.F. Chen, Y. Liang, Rare Metal Mat. Eng. 35 (7) (2006). [13] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem., Int. Ed. 41 (2002) 1188. [14] Z.S. Hu, G. Oskam, P.C. Searson, J. Colloid Interface Sci. 263 (2003) 454. [15] F.L. Yuan, P. Hu, C.L. Yin, S.L. Huang, J.L. Li, J. Mater. Chem. 13 (2003) 634. [16] M.H. Yoon, S.H. Lee, H.L. Pard, H.K. Kim, M.S. Jang, J. Mater. Sci. Lett. 21 (2002) 1703. [17] N. Roberts, R.P. Wang, A.W. Sleight, W.W. Warren, Phys. Rev., B 57 (1998) 5734.