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Synthesis and characterization of ZnO nanoparticles assembled in one-dimensional order
Inorganic Chemistry Communications 6 (2003) 877–881 www.elsevier.com/locate/inoche
Synthesis and characterization of ZnO nanoparticles assembled in one-dimensional order Jinmin Wang, Lian Gao
*
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, PR China Received 4 February 2003; accepted 28 March 2003
Abstract ZnO nanoparticles assembled in one-dimensional order were synthesized by a template-free method. The synthesized ZnO powder have a hexagonal zincite structure. The ZnO aggregates with rod-like shapes are typically 1.2 lm in length and 100 nm in diameter with an aspect ratio as high as 12, which consist of many small nanocrystals with diameters of 15 nm. Longer wires connected by many hexahedral ZnO nanocrystals were obtained after calcination at the temperature over 500 °C. The formation of the rod-shaped ZnO powders is related to the existence of [Zn(NH3 )x ]2þ complex cations. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Nanoparticles; Assembled in one-dimensional order; [Zn(NH3 )x ]2þ complex cations
1. Introduction Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties that depend sensitively on both size and shape [1,2]. Because of the unique quantum confinement effects of nanoparticles, ZnO nanoparticles with different morphologies have potential wide applications in varistors [3,4], gas sensors [5,6], ceramics [7], electrical and optical devices [8,9]. ZnO is one of the few oxides that show quantum confinement effects in an experimentally accessible size range [10]. So the synthesis and morphology control of ZnO nanoparticles has received great attention recently [11,12]. Control of the size and morphology of nanoparticles has aroused the interest of materials scientists [13–15]. Especially one-dimensional nanorods have been the focus due to their important role as interconnect components in future mesoscopic electronic and optical devices [16–18]. Such investigations in nanocrystallites have been limited mostly to sulfides and selenides [19,20]. *
Corresponding author. Tel.: +86-21-52412718; fax: +86-2152413122. E-mail address: [email protected] (L. Gao).
However, the knowledge regarding particle synthesis and surface chemistry is not as mature as with II–VI sulfides and selenides. Peng et al. [21] synthesized CdSe nanorods using Cd(CH3 )2 as the starting material and trioctyl phosphine oxide (TOPO), hexyl-phosphonic acid (HPA) as the surfactants. It is of more significant importance to probe simple methods to synthesize II–VI semiconductors by using inorganic compounds as starting materials. In this work, the synthesis of ZnO nanoparticles with rod-like shapes without using template agents is presented. The phase composition, morphology and crystallite size of the obtained ZnO powder were characterized by XRD and TEM, respectively. The surface state and the thermal decomposition behavior of the precursor were characterized by FT-IR and TGDTA–MS, respectively. 2. Experimental Zn(NO3 )2 6H2 O and (NH4 )2 CO3 were, respectively, dissolved in distilled water to form solutions with a certain concentration, for example, 0.5, 1.0 and 2.0 M Zn(NO3 )2 solution was slowly dropped into the vigorously stirred (NH4 )2 CO3 solution with varying molar
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00134-5
878
J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881
ratio of 1:1.25 or 1:2 to prepare the precursor. A white precipitate occurred immediately when the two solutions mixed each other but it was dissolved with stirring. A stable state slowly occurred due to the concentration of Zn2þ ions was high enough in the mixed solution so that it reached the state of supersaturation. Excess Zn(NO3 )2 solution was dropped into the mixed solution to obtain more solid. The solid was collected by filtration and repeatedly rinsed with ethanol, then dried at 100 °C for 12 h. At last, ZnO nanoparticles were obtained after calcination in the temperature range from 250 to 600 °C for 2 h. The samples were dispersed in ethanol using an ultrasonic bath, and a drop of the suspension was deposited onto carbon film coated copper grids for characterization by transmission electron microscopy (TEM, JEM 200CX). 3. Results and discussion X-ray powder diffraction patterns of ZnO nanoparticles are illustrated in Fig. 1. All peaks can be well indexed to the zincite phase of ZnO (JCPDS No. 36-1451). No peaks from any else phase of ZnO and impurities were observed, which indicates the high purity of the obtained zincite ZnO nanoparticles. The diffraction peaks are sharper with the increase of the calcination temperature, which implies that the crystalline structure tends to more integrity and the average particle size increases with the increase of the calcination temperature. Fig. 2 shows the TEM micrographs of ZnO nanoparticles calcined at 250, 400, 500 and 600 °C for 2 h, respectively. It can be seen that the ZnO aggregates with rod-like shapes are formed by some small nanocrystals
Fig. 1. XRD patterns of ZnO nanoparticles calcined at (a) 250 °C, (b) 400 °C, (c) 500 °C and (d) 600 °C for 2 h.
assembled in one-dimensional order. The average crystallite size of the sample after calcination at 250 °C for 2 h (Fig. 2(a)) is 15–20 nm and it is 30 nm for the sample calcined at 400 °C (Fig. 2(b)). The nanorods are 1.2 lm in length and 100 nm in diameter with an aspect ratio of 12 for the two samples. The particle size of ZnO aggregates increases with the increase of the calcination temperature and it becomes regular hexahedron shape over 500 °C. Some wires were connected by planes of the hexahedral nanocrystals with an average crystallite size 60 nm when the sample was calcined at 500 °C for 2 h (Fig. 2(c)). After being calcined at 600 °C for 2 h, the nanocrystals connect more closely and form longer wires (Fig. 2(d)). There are less nanorods in the sample
Fig. 2. TEM micrographs of ZnO nanoparticles calcined at (a) 250, (b) 400, (c) 500 and (d) 600 °C for 2 h, respectively.
J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881
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Fig. 3. TEM micrograph of the precursor. Fig. 5. IR spectrum of the precursor dried at 100 °C for 12 h.
calcined at 250 °C than that at 400 °C. And the average particle size of the former is smaller than the latter while both are 1.2 lm in length. It is found that the rod-like aggregates consist of small nanocrystals and micropores (Figs. 2(a) and (b)). All of above show that the rodshapes of ZnO aggregates tend to more visible and the nanocrystals tend to more integrate with the increasing of the calcination temperature, which is consistent with the result indicated by XRD patterns. The TEM micrograph and the selected area electron diffraction pattern of the precursor (Fig. 3) reveal it is a polycrystalline aggregate with rod-shaped morphology that can clearly be seen where the arrows point. It is believed that the rod-like morphology of ZnO aggregates is greatly related to that of the precursor. Because the decomposition temperature of the precursor is relatively low (only at 253 °C), the obtained ZnO nanoparticles almost remain the shape of the precursor. In order to identify the precursor and mensurate its decomposition temperature, a TG-DTA–MS coupling technique was used in the work. Fig. 4(a) shows the TGDTA curves of the precursor. Only one endothermic peak appears at 253 °C in the DTA curve and the mass of the precursor rapidly loses at the temperature in the TG curve, which implies the precursor decomposes at 253 °C. The gross mass loss is 56.1% from 100 to 300 °C and it changes very little in the range of 300–600 °C. Fig. 4(b) shows the mass spectra of the escaped gases in the thermal decomposition process of the precursor. There are four
peaks in the mass spectra, they symbolize the mass spectra þ þ for COþ 2 (m=z ¼ 44), NH3 (m=z ¼ 17), C2 H4 (m=z ¼ 28) þ and H2 O (m=z ¼ 18), respectively. It can be found that these peaks almost appear at the same temperature of 253 °C, which shows the precursor decomposes and gives out CO2 , NH3 , C2 H4 and H2 O at the same time. Fig. 5 shows the FT-IR spectrum of the precursor dried at 100 °C for 12 h (to ensure have got rid of the free C2 H5 OH molecule). The peaks at 2980 and 2900 cm1 are due to the stretching vibration of CH3 and CH2 . And the peaks at 1380 and 1500 cm1 are corresponding to the bending vibration of CH3 and CH2 , respectively. The broad peak between 3300 and 3400 cm1 suggests the presence of hydroxyl groups (O–H). These results prove that C2 H5 OH molecule remains in a state of strong chemical adsorption on the surface of the precursor. C2 H5 OH molecule is desorbed and dehydrated into C2 H4 and H2 O during the thermal decomposition process of the precursor. All the results of TG-DTA–MS and FT-IR show that the precursor decomposes and gives out CO2 , NH3 , C2 H4 and H2 O. It can be concluded that the precursor should have the structural formula Zn(NH3 )x CO3 nC2 H5 OH, where x is not a constant value and it is in the range of 1–4. Some experimental parameters, such as the concentrations of the reactants, the molar ratio and the dropping order (Zn(NO3 )2 was dropped into (NH4 )2 CO3 or in reverse order) were considered during the synthesis of
Fig. 4. TG-DTA curves and Mass spectra of the precursor. (a) TG-DTA curves, (b) Mass spectra of the precursor.
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J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881
Fig. 6. TEM micrographs of ZnO nanoparticles synthesized under different conditions. (a) CZ ¼ CN ¼ 1.0 M, R ¼ 1:1.25; (b) CZ ¼ CN ¼ 1.0 M, R ¼ 1:2 where CZ , CN and C denote the concentration of Zn(NO3 )2 , the concentration of (NH4 )2 CO3 and the molar ratio of Zn(NO3 )2 to (NH4 )2 CO3 ; Zn(NO3 )2 solution was dropped into (NH4 )2 CO3 solution except (c).
the rod-shaped ZnO nanoparticles. Fig. 6 shows the TEM micrographs of ZnO nanoparticles obtained under different experimental conditions. Fig. 6(a) shows the TEM micrograph of the ZnO nanoparticles obtained in the condition of the concentrations of Zn(NO3 )2 and (NH4 )2 CO3 were both 1.0 M and with a molar ratio 1:1.25. It can be found that the ZnO nanoparticles are also assembled in one-dimensional order to form the rod-like morphology. When the concentrations of Zn(NO3 )2 and (NH4 )2 CO3 were both 1.0 M but the molar ratio was adjusted to 1:2, some regular rodshaped ZnO particles with diameters of 1 lm and lengths up to 10 lm were synthesized (Fig. 6(b)). Both of the experiments were carried out in the condition that Zn(NO3 )2 was dropped into (NH4 )2 CO3 . When these experiments were done in reverse dropping order, that is, (NH4 )2 CO3 was dropped into Zn(NO3 )2 , regular hexahedral nanoparticles with a particle size of 60–70 nm (Fig. 6(c)) were obtained while no rod-shaped particles was found in the TEM micrograph. All results from TEM micrographs show that the existence of excess (NH4 )2 CO3 is important to the formation of rod-shaped ZnO nanoparticles and rod-shaped ZnO particles. It can be speculated that the rod-shaped ZnO nanoparticles were formed in the following steps: firstly, Zn(NO3 )2 solution reacted with (NH4 )2 CO3 solution to form the precipitate; secondly, since (NH4 )2 CO3 was excess during the process of Zn(NO3 )2 was dropped into (NH4 )2 CO3 , the precipitate was dissolved by NH3 H2 O hydrolyzed by NHþ ions to form 4 [Zn(NH3 )x ]2þ complex cations; thirdly, the rod-shaped precursor was steadily obtained when Zn2þ ions were much enough and resulted in the supersaturation of [Zn(NH3 )x ]CO3 in the solution; finally, the rod-shaped precipitate decomposed at 253 °C and obtained ZnO nanoparticles which were connected together one by one to form the ZnO nanorods.
4. Conclusions In conclusion, the unusual rod-shaped aggregates of ZnO nanocrystals in hexagonal phase were synthesized
by a template-free method. TEM micrographs show that the ZnO nanoparticles assembled in one-dimensional order to form the rod-like shapes and there are many micropores among the nanocrystals for the samples calcined at 250–400 °C for 2 h. While the nanocrystals are closely connected together one by one for the sample calcined at over 500 °C. TEM micrograph also shows the precursor exists as rod shapes. So the formation of the unusual morphology of ZnO nanoparticles should be attributed to the decomposition of the rod-shaped precursor. And the formation of the rod-shaped precursor is related to the presence of [Zn(NH3 )x ]2þ complex cations. FT-IR spectroscopy shows that C2 H5 OH molecule remains in a state of strong chemical adsorption on the surface of the precursor. TG-DTA–MS shows that the precursor has the structural formula Zn(NH3 )x CO3 nC2 H5 OH which decomposes and gives out CO2 , NH3 , C2 H4 and H2 O at 253 °C.
References [1] C.M. Lieber, Solid State Commun. 107 (1998) 607. [2] R.E. Smalley, B.I. Yakobson, Solid State Commun. 107 (1998) 597. [3] D.R. Clarke, J. Am. Ceram. Soc. 82 (1999) 485. [4] N.T. Hung, N.D. Quang, S. Bernik, J. Mater. Res. 16 (2001) 2817. [5] Y. Shimizu, F.-C. Lin, Y. Takao, M. Egashira, J. Am. Ceram. Soc. 81 (1998) 1633. [6] R. Paneva, D. Gotchev, Sensors Actuator A: Phys. 72 (1999) 79. [7] L. Gao, Q. Li, W.L. Luan, J. am Ceram. Soc. 85 (2002) 1016. [8] B.D. Yao, H.Z. Shi, H.J. Bi, L.D. Zhang, J. Phys.: Condens. Matter 12 (2000) 6265. [9] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [10] Mikrajuddin, F. Iskandar, K. Okuyama, J. Appl. Phys. 89 (2001) 6431. [11] J. Zhang, L.D. Sun, C.S. Liao, C.H. Yan, Chem. Commun. (2002) 262. [12] Y. Dai, Y. Zhang, Q.K. Li, C.W. Nan, Chem. Phys. Lett. 83–86 (2002) 358. [13] M. Chen, Y. Xie, J. Lu, Y.J. Xiong, S.Y. Zhang, Y.T. Qian, X.M. Liu, J. Mater. Chem. 12 (2002) 748. [14] S.M. Liu, L.M. Gan, L.H. Liu, W.D. Zhang, H.C. Zeng, Chem. Mater. 14 (2002) 1391. [15] F. Caruso, M. Spasova, A. Susha, M. Giersig, R.A. Caruso, Chem. Mater. 13 (2001) 109.
J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881 [16] C.-S. Yang, D.D. Awschalom, G.D. Stucky, Chem. Mater. 14 (2002) 1277. [17] Y.W. Wang, G.W. Meng, L.D. Zhang, C.H. Liang, J. Zhang, Chem. Mater. 14 (2002) 1773. [18] Z.Y. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237.
881
[19] M.A. Malik, N. Revaprasadu, P. OÕBrien, Chem. Mater. 13 (2001) 913. [20] T. Trindade, P. OÕBrien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. [21] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59.
Synthesis and characterization of ZnO nanoparticles assembled in one-dimensional order Jinmin Wang, Lian Gao
*
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, PR China Received 4 February 2003; accepted 28 March 2003
Abstract ZnO nanoparticles assembled in one-dimensional order were synthesized by a template-free method. The synthesized ZnO powder have a hexagonal zincite structure. The ZnO aggregates with rod-like shapes are typically 1.2 lm in length and 100 nm in diameter with an aspect ratio as high as 12, which consist of many small nanocrystals with diameters of 15 nm. Longer wires connected by many hexahedral ZnO nanocrystals were obtained after calcination at the temperature over 500 °C. The formation of the rod-shaped ZnO powders is related to the existence of [Zn(NH3 )x ]2þ complex cations. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Nanoparticles; Assembled in one-dimensional order; [Zn(NH3 )x ]2þ complex cations
1. Introduction Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties that depend sensitively on both size and shape [1,2]. Because of the unique quantum confinement effects of nanoparticles, ZnO nanoparticles with different morphologies have potential wide applications in varistors [3,4], gas sensors [5,6], ceramics [7], electrical and optical devices [8,9]. ZnO is one of the few oxides that show quantum confinement effects in an experimentally accessible size range [10]. So the synthesis and morphology control of ZnO nanoparticles has received great attention recently [11,12]. Control of the size and morphology of nanoparticles has aroused the interest of materials scientists [13–15]. Especially one-dimensional nanorods have been the focus due to their important role as interconnect components in future mesoscopic electronic and optical devices [16–18]. Such investigations in nanocrystallites have been limited mostly to sulfides and selenides [19,20]. *
Corresponding author. Tel.: +86-21-52412718; fax: +86-2152413122. E-mail address: [email protected] (L. Gao).
However, the knowledge regarding particle synthesis and surface chemistry is not as mature as with II–VI sulfides and selenides. Peng et al. [21] synthesized CdSe nanorods using Cd(CH3 )2 as the starting material and trioctyl phosphine oxide (TOPO), hexyl-phosphonic acid (HPA) as the surfactants. It is of more significant importance to probe simple methods to synthesize II–VI semiconductors by using inorganic compounds as starting materials. In this work, the synthesis of ZnO nanoparticles with rod-like shapes without using template agents is presented. The phase composition, morphology and crystallite size of the obtained ZnO powder were characterized by XRD and TEM, respectively. The surface state and the thermal decomposition behavior of the precursor were characterized by FT-IR and TGDTA–MS, respectively. 2. Experimental Zn(NO3 )2 6H2 O and (NH4 )2 CO3 were, respectively, dissolved in distilled water to form solutions with a certain concentration, for example, 0.5, 1.0 and 2.0 M Zn(NO3 )2 solution was slowly dropped into the vigorously stirred (NH4 )2 CO3 solution with varying molar
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00134-5
878
J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881
ratio of 1:1.25 or 1:2 to prepare the precursor. A white precipitate occurred immediately when the two solutions mixed each other but it was dissolved with stirring. A stable state slowly occurred due to the concentration of Zn2þ ions was high enough in the mixed solution so that it reached the state of supersaturation. Excess Zn(NO3 )2 solution was dropped into the mixed solution to obtain more solid. The solid was collected by filtration and repeatedly rinsed with ethanol, then dried at 100 °C for 12 h. At last, ZnO nanoparticles were obtained after calcination in the temperature range from 250 to 600 °C for 2 h. The samples were dispersed in ethanol using an ultrasonic bath, and a drop of the suspension was deposited onto carbon film coated copper grids for characterization by transmission electron microscopy (TEM, JEM 200CX). 3. Results and discussion X-ray powder diffraction patterns of ZnO nanoparticles are illustrated in Fig. 1. All peaks can be well indexed to the zincite phase of ZnO (JCPDS No. 36-1451). No peaks from any else phase of ZnO and impurities were observed, which indicates the high purity of the obtained zincite ZnO nanoparticles. The diffraction peaks are sharper with the increase of the calcination temperature, which implies that the crystalline structure tends to more integrity and the average particle size increases with the increase of the calcination temperature. Fig. 2 shows the TEM micrographs of ZnO nanoparticles calcined at 250, 400, 500 and 600 °C for 2 h, respectively. It can be seen that the ZnO aggregates with rod-like shapes are formed by some small nanocrystals
Fig. 1. XRD patterns of ZnO nanoparticles calcined at (a) 250 °C, (b) 400 °C, (c) 500 °C and (d) 600 °C for 2 h.
assembled in one-dimensional order. The average crystallite size of the sample after calcination at 250 °C for 2 h (Fig. 2(a)) is 15–20 nm and it is 30 nm for the sample calcined at 400 °C (Fig. 2(b)). The nanorods are 1.2 lm in length and 100 nm in diameter with an aspect ratio of 12 for the two samples. The particle size of ZnO aggregates increases with the increase of the calcination temperature and it becomes regular hexahedron shape over 500 °C. Some wires were connected by planes of the hexahedral nanocrystals with an average crystallite size 60 nm when the sample was calcined at 500 °C for 2 h (Fig. 2(c)). After being calcined at 600 °C for 2 h, the nanocrystals connect more closely and form longer wires (Fig. 2(d)). There are less nanorods in the sample
Fig. 2. TEM micrographs of ZnO nanoparticles calcined at (a) 250, (b) 400, (c) 500 and (d) 600 °C for 2 h, respectively.
J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881
879
Fig. 3. TEM micrograph of the precursor. Fig. 5. IR spectrum of the precursor dried at 100 °C for 12 h.
calcined at 250 °C than that at 400 °C. And the average particle size of the former is smaller than the latter while both are 1.2 lm in length. It is found that the rod-like aggregates consist of small nanocrystals and micropores (Figs. 2(a) and (b)). All of above show that the rodshapes of ZnO aggregates tend to more visible and the nanocrystals tend to more integrate with the increasing of the calcination temperature, which is consistent with the result indicated by XRD patterns. The TEM micrograph and the selected area electron diffraction pattern of the precursor (Fig. 3) reveal it is a polycrystalline aggregate with rod-shaped morphology that can clearly be seen where the arrows point. It is believed that the rod-like morphology of ZnO aggregates is greatly related to that of the precursor. Because the decomposition temperature of the precursor is relatively low (only at 253 °C), the obtained ZnO nanoparticles almost remain the shape of the precursor. In order to identify the precursor and mensurate its decomposition temperature, a TG-DTA–MS coupling technique was used in the work. Fig. 4(a) shows the TGDTA curves of the precursor. Only one endothermic peak appears at 253 °C in the DTA curve and the mass of the precursor rapidly loses at the temperature in the TG curve, which implies the precursor decomposes at 253 °C. The gross mass loss is 56.1% from 100 to 300 °C and it changes very little in the range of 300–600 °C. Fig. 4(b) shows the mass spectra of the escaped gases in the thermal decomposition process of the precursor. There are four
peaks in the mass spectra, they symbolize the mass spectra þ þ for COþ 2 (m=z ¼ 44), NH3 (m=z ¼ 17), C2 H4 (m=z ¼ 28) þ and H2 O (m=z ¼ 18), respectively. It can be found that these peaks almost appear at the same temperature of 253 °C, which shows the precursor decomposes and gives out CO2 , NH3 , C2 H4 and H2 O at the same time. Fig. 5 shows the FT-IR spectrum of the precursor dried at 100 °C for 12 h (to ensure have got rid of the free C2 H5 OH molecule). The peaks at 2980 and 2900 cm1 are due to the stretching vibration of CH3 and CH2 . And the peaks at 1380 and 1500 cm1 are corresponding to the bending vibration of CH3 and CH2 , respectively. The broad peak between 3300 and 3400 cm1 suggests the presence of hydroxyl groups (O–H). These results prove that C2 H5 OH molecule remains in a state of strong chemical adsorption on the surface of the precursor. C2 H5 OH molecule is desorbed and dehydrated into C2 H4 and H2 O during the thermal decomposition process of the precursor. All the results of TG-DTA–MS and FT-IR show that the precursor decomposes and gives out CO2 , NH3 , C2 H4 and H2 O. It can be concluded that the precursor should have the structural formula Zn(NH3 )x CO3 nC2 H5 OH, where x is not a constant value and it is in the range of 1–4. Some experimental parameters, such as the concentrations of the reactants, the molar ratio and the dropping order (Zn(NO3 )2 was dropped into (NH4 )2 CO3 or in reverse order) were considered during the synthesis of
Fig. 4. TG-DTA curves and Mass spectra of the precursor. (a) TG-DTA curves, (b) Mass spectra of the precursor.
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J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881
Fig. 6. TEM micrographs of ZnO nanoparticles synthesized under different conditions. (a) CZ ¼ CN ¼ 1.0 M, R ¼ 1:1.25; (b) CZ ¼ CN ¼ 1.0 M, R ¼ 1:2 where CZ , CN and C denote the concentration of Zn(NO3 )2 , the concentration of (NH4 )2 CO3 and the molar ratio of Zn(NO3 )2 to (NH4 )2 CO3 ; Zn(NO3 )2 solution was dropped into (NH4 )2 CO3 solution except (c).
the rod-shaped ZnO nanoparticles. Fig. 6 shows the TEM micrographs of ZnO nanoparticles obtained under different experimental conditions. Fig. 6(a) shows the TEM micrograph of the ZnO nanoparticles obtained in the condition of the concentrations of Zn(NO3 )2 and (NH4 )2 CO3 were both 1.0 M and with a molar ratio 1:1.25. It can be found that the ZnO nanoparticles are also assembled in one-dimensional order to form the rod-like morphology. When the concentrations of Zn(NO3 )2 and (NH4 )2 CO3 were both 1.0 M but the molar ratio was adjusted to 1:2, some regular rodshaped ZnO particles with diameters of 1 lm and lengths up to 10 lm were synthesized (Fig. 6(b)). Both of the experiments were carried out in the condition that Zn(NO3 )2 was dropped into (NH4 )2 CO3 . When these experiments were done in reverse dropping order, that is, (NH4 )2 CO3 was dropped into Zn(NO3 )2 , regular hexahedral nanoparticles with a particle size of 60–70 nm (Fig. 6(c)) were obtained while no rod-shaped particles was found in the TEM micrograph. All results from TEM micrographs show that the existence of excess (NH4 )2 CO3 is important to the formation of rod-shaped ZnO nanoparticles and rod-shaped ZnO particles. It can be speculated that the rod-shaped ZnO nanoparticles were formed in the following steps: firstly, Zn(NO3 )2 solution reacted with (NH4 )2 CO3 solution to form the precipitate; secondly, since (NH4 )2 CO3 was excess during the process of Zn(NO3 )2 was dropped into (NH4 )2 CO3 , the precipitate was dissolved by NH3 H2 O hydrolyzed by NHþ ions to form 4 [Zn(NH3 )x ]2þ complex cations; thirdly, the rod-shaped precursor was steadily obtained when Zn2þ ions were much enough and resulted in the supersaturation of [Zn(NH3 )x ]CO3 in the solution; finally, the rod-shaped precipitate decomposed at 253 °C and obtained ZnO nanoparticles which were connected together one by one to form the ZnO nanorods.
4. Conclusions In conclusion, the unusual rod-shaped aggregates of ZnO nanocrystals in hexagonal phase were synthesized
by a template-free method. TEM micrographs show that the ZnO nanoparticles assembled in one-dimensional order to form the rod-like shapes and there are many micropores among the nanocrystals for the samples calcined at 250–400 °C for 2 h. While the nanocrystals are closely connected together one by one for the sample calcined at over 500 °C. TEM micrograph also shows the precursor exists as rod shapes. So the formation of the unusual morphology of ZnO nanoparticles should be attributed to the decomposition of the rod-shaped precursor. And the formation of the rod-shaped precursor is related to the presence of [Zn(NH3 )x ]2þ complex cations. FT-IR spectroscopy shows that C2 H5 OH molecule remains in a state of strong chemical adsorption on the surface of the precursor. TG-DTA–MS shows that the precursor has the structural formula Zn(NH3 )x CO3 nC2 H5 OH which decomposes and gives out CO2 , NH3 , C2 H4 and H2 O at 253 °C.
References [1] C.M. Lieber, Solid State Commun. 107 (1998) 607. [2] R.E. Smalley, B.I. Yakobson, Solid State Commun. 107 (1998) 597. [3] D.R. Clarke, J. Am. Ceram. Soc. 82 (1999) 485. [4] N.T. Hung, N.D. Quang, S. Bernik, J. Mater. Res. 16 (2001) 2817. [5] Y. Shimizu, F.-C. Lin, Y. Takao, M. Egashira, J. Am. Ceram. Soc. 81 (1998) 1633. [6] R. Paneva, D. Gotchev, Sensors Actuator A: Phys. 72 (1999) 79. [7] L. Gao, Q. Li, W.L. Luan, J. am Ceram. Soc. 85 (2002) 1016. [8] B.D. Yao, H.Z. Shi, H.J. Bi, L.D. Zhang, J. Phys.: Condens. Matter 12 (2000) 6265. [9] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [10] Mikrajuddin, F. Iskandar, K. Okuyama, J. Appl. Phys. 89 (2001) 6431. [11] J. Zhang, L.D. Sun, C.S. Liao, C.H. Yan, Chem. Commun. (2002) 262. [12] Y. Dai, Y. Zhang, Q.K. Li, C.W. Nan, Chem. Phys. Lett. 83–86 (2002) 358. [13] M. Chen, Y. Xie, J. Lu, Y.J. Xiong, S.Y. Zhang, Y.T. Qian, X.M. Liu, J. Mater. Chem. 12 (2002) 748. [14] S.M. Liu, L.M. Gan, L.H. Liu, W.D. Zhang, H.C. Zeng, Chem. Mater. 14 (2002) 1391. [15] F. Caruso, M. Spasova, A. Susha, M. Giersig, R.A. Caruso, Chem. Mater. 13 (2001) 109.
J. Wang, L. Gao / Inorganic Chemistry Communications 6 (2003) 877–881 [16] C.-S. Yang, D.D. Awschalom, G.D. Stucky, Chem. Mater. 14 (2002) 1277. [17] Y.W. Wang, G.W. Meng, L.D. Zhang, C.H. Liang, J. Zhang, Chem. Mater. 14 (2002) 1773. [18] Z.Y. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237.
881
[19] M.A. Malik, N. Revaprasadu, P. OÕBrien, Chem. Mater. 13 (2001) 913. [20] T. Trindade, P. OÕBrien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. [21] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59.