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A CTAB-assisted hydrothermal orientation growth of ZnO nanorods
Materials Chemistry and Physics 78 (2002) 99–104
A CTAB-assisted hydrothermal orientation growth of ZnO nanorods X.M. Sun, X. Chen, Z.X. Deng, Y.D. Li∗ Key Laboratory of Atomic and Molecular Nanoscience, Department of Chemistry, Tsinghua University, China Ministry of Education, Beijing 100084, PR China Received 29 October 2001; received in revised form 19 April 2002; accepted 30 April 2002
Abstract ZnO nanorods are prepared by cetyltrimethylammonium bromide (CTAB) favored hydrothermal oxidization of zinc metal at 180 ◦ C. The samples are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results show that CTAB played a significant role in the formation of ZnO rods. The presence of CTAB can greatly favor the erosion process and lead to the orientation growth of ZnO nanorods. Possible mechanisms for the CTAB assisted orientation growth of ZnO nanorods are discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Nanorod; Surfactant; Hydrothermal; Erosion
1. Introduction Recently many research efforts have been invested in the area of wide band-gap semiconductor materials due to their potential applications in short wavelength optical devices. So far ZnO-based II–VI compound semiconductor has been studied extensively [1–8] and used widely in the fields of ceramics and varistors as a main body material [9], and in rubbers as intensifier [10,11]. ZnO nanomaterials with one-dimensional (1D) structure, such as nanowires or nanorods, are especially attractive due to their tunable electronic and opto-electronic properties, and the potential applications in the nanoscale electronic and opto-electronic devices [8]. Yang and co-workers have successfully prepared self-organized [0 0 0 1] oriented ZnO nanowire arrays with diameters varying from 20 to 150 nm on sapphire substrates by a vapor transport and condensation process, and room temperature ultraviolet lasing was demonstrated [12]. Thus, the development of mild and lost-cost synthetic route to ZnO nanorods or nanowires was of great significance. However, most of the ZnO whiskers prepared by gas phase reactions were mainly in micrometer scale and with a wide size distribution [13,14]. Hydrothermal method, as an important method for wet chemistry, has been employed for the preparation of ZnO materials in the last decade, and particles with narrow size distribution, little or no microagglomeration, well crystallization and phase homogeneity can be obtained [15–20]. In the exploration to prepare one-dimensional ∗ Corresponding author. Tel.: +86-10-627-72350. E-mail address: [email protected] (Y.D. Li).
(1D) structured ZnO in this method, Nishizawa et al. have decomposed aqueous solution of Na2 Zn–EDTA at 330 ◦ C, and needle-like crystals were grown [16]. Shi et al. have prepared acicular ZnO crystal at 190 ◦ C, but NaNO2 must be used as mineralizer [17]. Under the hydrothermal condition, the growth rate and the orientation control is a contradiction: as the concentration of growth units increases, the growth rate will also increase, while the growth orientation tends to be confused. To achieve high growth rate and good orientation simultaneously is the goal of many materialists. In the present paper, a surfactant assisted hydrothermal method is found to be able to promote the formation of ZnO crystal and control the orientation of products at the same time. Cationic surfactant cetyltrimethylammonium bromide (CTAB), which has been generally used as templating micelle molecule to synthesize mesoporous materials [21–23], is introduced to hydrothermal process to favor the erosion of zinc and control the morphology of ZnO crystals. Comparative experiments show that it is capable of favoring the erosion reaction and inducing the orientation growth of the resultant metal oxide nano-crystals.
2. Experimental Synthesis: Zinc powder (Shuangyan chemical factory, Pinggu, Beijing) and CTAB (Beijing Chemical Corp.; both were of AR grade) were used without further purification. CTAB (1.5 g) was dissolved in 35 ml deionized water to form a transparent solution. Then 1.8 g zinc powder was added to the above solution under continuous stirring. The
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resulting suspension was transferred into a 40 ml Teflon-lined stainless steel autoclave and sealed tightly. Hydrothermal treatments were carried out at 180 ◦ C for 24 h. After that, the autoclave was allowed to cool down naturally. Precipitates were collected, and washed with deionized water several times and dried in air at 80 ◦ C. Characterization: All the samples were characterized by powder X-ray diffraction (XRD) on a D8 Advance Bruker X-ray diffractometer with monochromatized Cu K␣ (λ = 1.5418 Å) incident radiation. XRD patterns were recorded from 10 to 70◦ (2θ ) with a scanning step of 0.01◦ . The size distribution and morphology of the samples were analyzed by transmission electron microscopy (TEM) observation on a H-800 transmission electron microscope operated at 200 kV. Electron diffraction (ED) patterns were used to determine the growth orientation of the as-prepared nanorods.
3. Results and discussion All the samples are obtained through the reaction between zinc and water under hydrothermal conditions at 180 ◦ C. Fig. 1a shows the XRD patterns of the ZnO products obtained after hydrothermal treatment in the absence of CTAB for 24 h. The strong reflections corresponding to unreacted zinc indicated that the reaction between zinc and water is far from complete. Fig. 1b is the XRD pattern of the sample obtained under the same conditions as in Fig. 1a but with elongated reaction period of 40 h. Peaks corresponding to zinc are substantially weakened but still remain observable. It indicated that an even longer time is needed to complete
the reaction. The XRD patterns of the samples obtained in the presence of CTAB at 180 ◦ C for only 20 h is given in Fig 1c. Typical reflectances of pure hexagonal wurtzite ZnO (JCPDS 75-0576, a = 0.3249 nm, c = 0.5205 nm) could be observed. In addition, all peaks having much higher and narrower shapes than those obtained without the addition of CTAB, indicate higher crystallinity. Further experiments reveal that the minimum time needed for the completion of the reaction can be reduced to 5 h. All these results demonstrate that CTAB is capable of increasing the reaction rate between zinc and water to about eight-fold higher, leading to higher crystallinity of the products. TEM micrographs in Fig. 2 also reveal the difference between the samples obtained in the presence and absence of CTAB. Fig. 2a is the TEM image of the sample obtained in the absence of CTAB for 24 h of processing time. It clearly shows that a layer of ZnO surrounds a core of Zn metal. The corresponding mechanism is shown in part I of Fig. 3. This ZnO layer might hinder the further reaction between zinc and water and correlate well with the XRD results of Fig. 1a. The typical morphology of the samples obtained in the presence of CTAB with 20 h of hydrothermal treatment is shown in Fig. 2b. The mean size of as-grown sample is about 1000 nm in length and 70 nm in diameter. ED pattern in Fig. 2c shows that the obtained nanorods are single crystals with [0 0 0 1] growth direction. The possible formation process for ZnO nanorods under hydrothermal condition can be represented as follows: Zn + H2 O → Zn2+ + H2 ↑ + OH− Zn2+ + OH− → Zn(OH)4 2− → ZnO + H2 O
Fig. 1. X-ray diffraction patterns for ZnO powders prepared from corrosion reaction: (a) hydrothermally treated without CTAB, 180 ◦ C, 24 h; (b) hydrothermally treated without CTAB 180 ◦ C, 40 h and (c) hydrothermally treated with CTAB 180 ◦ C, 20 h (#, reflections of zinc; star, reflections of hexagonal ZnO).
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Fig. 2. (a) TEM image of the sample obtained in the absence of CTAB for 24 h, which correlated well with the XRD results of Fig. 1a; (b) TEM micrograph of the sample obtained in the presence of CTAB with 20 h of hydrothermal treatment at 180 ◦ C and (c) ED pattern of nanorod, which showed a [0 0 0 1] growth direction.
For the reaction system in the absence of CTAB, active Zn atoms located at the surface of zinc particle react with water first. As a result, the concentration of zinc ions and OH− around the zinc particles increases. Zn(OH)2 is then formed in situ via the reaction between Zn2+ and OH− . Under hydrothermal condition, Zn(OH)2 can dehydrate to produce ZnO. The as-formed ZnO layer around the zinc cores would cover the active sites at the surface of the zinc particle (as shown in Fig. 3, part I), and thus prevent the further reaction between Zn and H2 O. As the time prolonged, this layers become thicker, and make deeper erosion harder. The
existence of metal zinc in the final products (as evidenced by XRD), even after a hydrothermal treatment for 40 h is a good evidence for this process. The growth process of ZnO in the presence of CTAB is different. Because of the existence of surfactant, the surface tension of solution is reduced, which lower the energy needed to form a new phase, and ZnO crystal, therefore, could form in a lower supersaturation. On the other hand, CTAB could also be considered to influence the erosion process of zinc and the growth process of ZnO by the electrostatic and stereochemical effects. CTAB is an ionic
Fig. 3. Schematic illustration of the erosion process: (I) in the absence of CTAB; (II) in the presence of CTAB. Black color, Zn; gray color, ZnO. The wave-like patterns indicates the CTA+ ions. A CTA+ ion carrying a zincate ion is shown.
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Fig. 4. (a) Schematic illustration of ion-pair formed between CTA+ and Zn(OH)4 2− . (b) Schematic illustration of landing process on the surface of ZnO [0 0 0 1] crystal face.
compound, which ionizes completely in water. The resulted cation is a positively charged tetrahedron with a long hydrophobic tail, while the growth unit for ZnO crystal is considered to be Zn(OH)4 2− [20], which also has a tetrahedron geometry, but is negatively charged. Therefore, ion-pairs between CTA+ and Zn(OH)4 2− could form due to electrostatic interaction as described in Fig. 4a. The complimentary between CTA+ and Zn(OH)4 2− endows the surfactant the capability to act as an ionic carrier [24]. The zinc particles are negatively charged, and so CTA+ adsorbs on the surface to form a film. The film is assembled and floatable. When the surfactant molecules leave, zincate will be carried away in the form of ion-pairs, so that the barrier layer becomes thinner (as shown in Fig. 3, part II), which facilitates the erosion process. In the crystallization process, surfactant molecules may serve as a growth controller, as well as an agglomeration inhibitor, by forming a covering film on the newly formed ZnO crystal. It has been known that the adsorption of growth units on crystal surfaces strongly affects the growth speed and orientation of crystals [19]. When a ZnO crystal grew, a surfactant film could form at the interface between solution and ZnO crystal to reduce the interface energy. The film
is floating, and surfactant molecules carrying the growing units will release them at the surface of ZnO single crystal. This landing process on the surface of ZnO [0 0 0 1] crystal face is described graphically in Fig. 4b. Since CTAB favors to form a film in which molecules tend to be perpendicular to the absorbed surface, the growth units would tend to face-land onto the growing interface as shown in Fig. 4b. Since this kind of landing and dehydration will result in three Zn–O–Zn bonds, which make this landing mode predominant in competition with other ones such as vertex- and edge-landing. ZnO crystal should grow preferentially along the c-axis ([0 0 0 1] direction) as this kind of face-landing on [0 0 0 1] crystal face and the following dehydration steps repeated. According to the process described earlier, it is supposed that CTAB not only accelerates the erosion reaction as a transporter of the growth units, but also leads to the orientation growth of ZnO rods. When the same process is carried out in NaOH or ammonia solution with the same amount of CTAB, we find that ammonia leads to bifurcation of the resulted rods and causes the formation of nano particles (shown in Fig. 5a), while NaOH tends to produce ZnO in large blocks (shown in Fig. 5b). As for the reaction in ammonia solution, the
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Fig. 5. TEM micrographs of the samples obtained (a) in the presence of CTAB and ammonia and (b) in the presence of CTAB and NaOH.
formation of Zn(NH3 )4 2+ complex ion might interfere the orientation growth of ZnO rods. In the case of NaOH solution, the sharply increased concentration of Zn(OH)4 2− might make the growth process of ZnO out of control. In addition, the changed behavior of CTAB in NaOH solution might also affect the growth process, but further study is needed to clarify this point of view. We have also extended the surfactant-assisted growth method to the synthesis of aluminum oxide. It is found that
Al(OH)4 − could similarly be transported during the crystallization process. Fig. 6 describes the boehmite (AlOOH) nanowires and nanobelts obtained. Further study is in progress.
4. Conclusion CTAB is used to get ZnO nanorods in a narrow size distribution. We find that in the growth process CTAB could serve as a carrier for negative zincate and organize the landing process. The mechanism of such a process is proposed. Because the preparation condition is simple and easy to control, this method is very promising for industrial production. Moreover, this method should be able to be extended to the synthesis of other similar metalloid oxides nanorods.
References
Fig. 6. TEM micrograph of the AlOOH nanowires and nanobelts.
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A CTAB-assisted hydrothermal orientation growth of ZnO nanorods X.M. Sun, X. Chen, Z.X. Deng, Y.D. Li∗ Key Laboratory of Atomic and Molecular Nanoscience, Department of Chemistry, Tsinghua University, China Ministry of Education, Beijing 100084, PR China Received 29 October 2001; received in revised form 19 April 2002; accepted 30 April 2002
Abstract ZnO nanorods are prepared by cetyltrimethylammonium bromide (CTAB) favored hydrothermal oxidization of zinc metal at 180 ◦ C. The samples are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results show that CTAB played a significant role in the formation of ZnO rods. The presence of CTAB can greatly favor the erosion process and lead to the orientation growth of ZnO nanorods. Possible mechanisms for the CTAB assisted orientation growth of ZnO nanorods are discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Nanorod; Surfactant; Hydrothermal; Erosion
1. Introduction Recently many research efforts have been invested in the area of wide band-gap semiconductor materials due to their potential applications in short wavelength optical devices. So far ZnO-based II–VI compound semiconductor has been studied extensively [1–8] and used widely in the fields of ceramics and varistors as a main body material [9], and in rubbers as intensifier [10,11]. ZnO nanomaterials with one-dimensional (1D) structure, such as nanowires or nanorods, are especially attractive due to their tunable electronic and opto-electronic properties, and the potential applications in the nanoscale electronic and opto-electronic devices [8]. Yang and co-workers have successfully prepared self-organized [0 0 0 1] oriented ZnO nanowire arrays with diameters varying from 20 to 150 nm on sapphire substrates by a vapor transport and condensation process, and room temperature ultraviolet lasing was demonstrated [12]. Thus, the development of mild and lost-cost synthetic route to ZnO nanorods or nanowires was of great significance. However, most of the ZnO whiskers prepared by gas phase reactions were mainly in micrometer scale and with a wide size distribution [13,14]. Hydrothermal method, as an important method for wet chemistry, has been employed for the preparation of ZnO materials in the last decade, and particles with narrow size distribution, little or no microagglomeration, well crystallization and phase homogeneity can be obtained [15–20]. In the exploration to prepare one-dimensional ∗ Corresponding author. Tel.: +86-10-627-72350. E-mail address: [email protected] (Y.D. Li).
(1D) structured ZnO in this method, Nishizawa et al. have decomposed aqueous solution of Na2 Zn–EDTA at 330 ◦ C, and needle-like crystals were grown [16]. Shi et al. have prepared acicular ZnO crystal at 190 ◦ C, but NaNO2 must be used as mineralizer [17]. Under the hydrothermal condition, the growth rate and the orientation control is a contradiction: as the concentration of growth units increases, the growth rate will also increase, while the growth orientation tends to be confused. To achieve high growth rate and good orientation simultaneously is the goal of many materialists. In the present paper, a surfactant assisted hydrothermal method is found to be able to promote the formation of ZnO crystal and control the orientation of products at the same time. Cationic surfactant cetyltrimethylammonium bromide (CTAB), which has been generally used as templating micelle molecule to synthesize mesoporous materials [21–23], is introduced to hydrothermal process to favor the erosion of zinc and control the morphology of ZnO crystals. Comparative experiments show that it is capable of favoring the erosion reaction and inducing the orientation growth of the resultant metal oxide nano-crystals.
2. Experimental Synthesis: Zinc powder (Shuangyan chemical factory, Pinggu, Beijing) and CTAB (Beijing Chemical Corp.; both were of AR grade) were used without further purification. CTAB (1.5 g) was dissolved in 35 ml deionized water to form a transparent solution. Then 1.8 g zinc powder was added to the above solution under continuous stirring. The
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resulting suspension was transferred into a 40 ml Teflon-lined stainless steel autoclave and sealed tightly. Hydrothermal treatments were carried out at 180 ◦ C for 24 h. After that, the autoclave was allowed to cool down naturally. Precipitates were collected, and washed with deionized water several times and dried in air at 80 ◦ C. Characterization: All the samples were characterized by powder X-ray diffraction (XRD) on a D8 Advance Bruker X-ray diffractometer with monochromatized Cu K␣ (λ = 1.5418 Å) incident radiation. XRD patterns were recorded from 10 to 70◦ (2θ ) with a scanning step of 0.01◦ . The size distribution and morphology of the samples were analyzed by transmission electron microscopy (TEM) observation on a H-800 transmission electron microscope operated at 200 kV. Electron diffraction (ED) patterns were used to determine the growth orientation of the as-prepared nanorods.
3. Results and discussion All the samples are obtained through the reaction between zinc and water under hydrothermal conditions at 180 ◦ C. Fig. 1a shows the XRD patterns of the ZnO products obtained after hydrothermal treatment in the absence of CTAB for 24 h. The strong reflections corresponding to unreacted zinc indicated that the reaction between zinc and water is far from complete. Fig. 1b is the XRD pattern of the sample obtained under the same conditions as in Fig. 1a but with elongated reaction period of 40 h. Peaks corresponding to zinc are substantially weakened but still remain observable. It indicated that an even longer time is needed to complete
the reaction. The XRD patterns of the samples obtained in the presence of CTAB at 180 ◦ C for only 20 h is given in Fig 1c. Typical reflectances of pure hexagonal wurtzite ZnO (JCPDS 75-0576, a = 0.3249 nm, c = 0.5205 nm) could be observed. In addition, all peaks having much higher and narrower shapes than those obtained without the addition of CTAB, indicate higher crystallinity. Further experiments reveal that the minimum time needed for the completion of the reaction can be reduced to 5 h. All these results demonstrate that CTAB is capable of increasing the reaction rate between zinc and water to about eight-fold higher, leading to higher crystallinity of the products. TEM micrographs in Fig. 2 also reveal the difference between the samples obtained in the presence and absence of CTAB. Fig. 2a is the TEM image of the sample obtained in the absence of CTAB for 24 h of processing time. It clearly shows that a layer of ZnO surrounds a core of Zn metal. The corresponding mechanism is shown in part I of Fig. 3. This ZnO layer might hinder the further reaction between zinc and water and correlate well with the XRD results of Fig. 1a. The typical morphology of the samples obtained in the presence of CTAB with 20 h of hydrothermal treatment is shown in Fig. 2b. The mean size of as-grown sample is about 1000 nm in length and 70 nm in diameter. ED pattern in Fig. 2c shows that the obtained nanorods are single crystals with [0 0 0 1] growth direction. The possible formation process for ZnO nanorods under hydrothermal condition can be represented as follows: Zn + H2 O → Zn2+ + H2 ↑ + OH− Zn2+ + OH− → Zn(OH)4 2− → ZnO + H2 O
Fig. 1. X-ray diffraction patterns for ZnO powders prepared from corrosion reaction: (a) hydrothermally treated without CTAB, 180 ◦ C, 24 h; (b) hydrothermally treated without CTAB 180 ◦ C, 40 h and (c) hydrothermally treated with CTAB 180 ◦ C, 20 h (#, reflections of zinc; star, reflections of hexagonal ZnO).
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Fig. 2. (a) TEM image of the sample obtained in the absence of CTAB for 24 h, which correlated well with the XRD results of Fig. 1a; (b) TEM micrograph of the sample obtained in the presence of CTAB with 20 h of hydrothermal treatment at 180 ◦ C and (c) ED pattern of nanorod, which showed a [0 0 0 1] growth direction.
For the reaction system in the absence of CTAB, active Zn atoms located at the surface of zinc particle react with water first. As a result, the concentration of zinc ions and OH− around the zinc particles increases. Zn(OH)2 is then formed in situ via the reaction between Zn2+ and OH− . Under hydrothermal condition, Zn(OH)2 can dehydrate to produce ZnO. The as-formed ZnO layer around the zinc cores would cover the active sites at the surface of the zinc particle (as shown in Fig. 3, part I), and thus prevent the further reaction between Zn and H2 O. As the time prolonged, this layers become thicker, and make deeper erosion harder. The
existence of metal zinc in the final products (as evidenced by XRD), even after a hydrothermal treatment for 40 h is a good evidence for this process. The growth process of ZnO in the presence of CTAB is different. Because of the existence of surfactant, the surface tension of solution is reduced, which lower the energy needed to form a new phase, and ZnO crystal, therefore, could form in a lower supersaturation. On the other hand, CTAB could also be considered to influence the erosion process of zinc and the growth process of ZnO by the electrostatic and stereochemical effects. CTAB is an ionic
Fig. 3. Schematic illustration of the erosion process: (I) in the absence of CTAB; (II) in the presence of CTAB. Black color, Zn; gray color, ZnO. The wave-like patterns indicates the CTA+ ions. A CTA+ ion carrying a zincate ion is shown.
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Fig. 4. (a) Schematic illustration of ion-pair formed between CTA+ and Zn(OH)4 2− . (b) Schematic illustration of landing process on the surface of ZnO [0 0 0 1] crystal face.
compound, which ionizes completely in water. The resulted cation is a positively charged tetrahedron with a long hydrophobic tail, while the growth unit for ZnO crystal is considered to be Zn(OH)4 2− [20], which also has a tetrahedron geometry, but is negatively charged. Therefore, ion-pairs between CTA+ and Zn(OH)4 2− could form due to electrostatic interaction as described in Fig. 4a. The complimentary between CTA+ and Zn(OH)4 2− endows the surfactant the capability to act as an ionic carrier [24]. The zinc particles are negatively charged, and so CTA+ adsorbs on the surface to form a film. The film is assembled and floatable. When the surfactant molecules leave, zincate will be carried away in the form of ion-pairs, so that the barrier layer becomes thinner (as shown in Fig. 3, part II), which facilitates the erosion process. In the crystallization process, surfactant molecules may serve as a growth controller, as well as an agglomeration inhibitor, by forming a covering film on the newly formed ZnO crystal. It has been known that the adsorption of growth units on crystal surfaces strongly affects the growth speed and orientation of crystals [19]. When a ZnO crystal grew, a surfactant film could form at the interface between solution and ZnO crystal to reduce the interface energy. The film
is floating, and surfactant molecules carrying the growing units will release them at the surface of ZnO single crystal. This landing process on the surface of ZnO [0 0 0 1] crystal face is described graphically in Fig. 4b. Since CTAB favors to form a film in which molecules tend to be perpendicular to the absorbed surface, the growth units would tend to face-land onto the growing interface as shown in Fig. 4b. Since this kind of landing and dehydration will result in three Zn–O–Zn bonds, which make this landing mode predominant in competition with other ones such as vertex- and edge-landing. ZnO crystal should grow preferentially along the c-axis ([0 0 0 1] direction) as this kind of face-landing on [0 0 0 1] crystal face and the following dehydration steps repeated. According to the process described earlier, it is supposed that CTAB not only accelerates the erosion reaction as a transporter of the growth units, but also leads to the orientation growth of ZnO rods. When the same process is carried out in NaOH or ammonia solution with the same amount of CTAB, we find that ammonia leads to bifurcation of the resulted rods and causes the formation of nano particles (shown in Fig. 5a), while NaOH tends to produce ZnO in large blocks (shown in Fig. 5b). As for the reaction in ammonia solution, the
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Fig. 5. TEM micrographs of the samples obtained (a) in the presence of CTAB and ammonia and (b) in the presence of CTAB and NaOH.
formation of Zn(NH3 )4 2+ complex ion might interfere the orientation growth of ZnO rods. In the case of NaOH solution, the sharply increased concentration of Zn(OH)4 2− might make the growth process of ZnO out of control. In addition, the changed behavior of CTAB in NaOH solution might also affect the growth process, but further study is needed to clarify this point of view. We have also extended the surfactant-assisted growth method to the synthesis of aluminum oxide. It is found that
Al(OH)4 − could similarly be transported during the crystallization process. Fig. 6 describes the boehmite (AlOOH) nanowires and nanobelts obtained. Further study is in progress.
4. Conclusion CTAB is used to get ZnO nanorods in a narrow size distribution. We find that in the growth process CTAB could serve as a carrier for negative zincate and organize the landing process. The mechanism of such a process is proposed. Because the preparation condition is simple and easy to control, this method is very promising for industrial production. Moreover, this method should be able to be extended to the synthesis of other similar metalloid oxides nanorods.
References
Fig. 6. TEM micrograph of the AlOOH nanowires and nanobelts.
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