Appl. Phys. A 91, 157–160 (2008)
Applied Physics A
DOI: 10.1007/s00339-007-4389-7
Materials Science & Processing
dongliang tao1 wallace c.h. choy2,u
The growth mechanism of ZnO single-crystal nanorods synthesized by polymer complexing with zinc salts 1
Laboratory of Nanomaterials Chemistry, Key Laboratory of Science and Technology of Controllable Chemical Reactions, Beijing University of Chemical Technology, Beijing 100029, P.R. China 2 Department of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
Received: 15 January 2007/Accepted: 10 December 2007 Published online: 26 January 2008 • © Springer-Verlag 2008
The growth mechanism of single-crystal ZnO nanorods synthesized by the method of polymer complexing with zinc salts is investigated. The annealing temperature is controlled at about the decomposition temperature of dihydrate zinc acetate (Zn(O2 CCH3 )2 · 2H2 O) of 573 K. By changing the annealing time, the ZnO nanostructures can be modified from nanoparticles to nanorods. As a result, the formation of singlecrystal ZnO nanorods can be observed. Through investigating the Fourier transform infrared spectra of (a) polyvinyl pyrrolidone (PVP), (b) Zn(O2 CCH3 )2 · 2H2 O and (c) the mixture of PVP and Zn(O2 CCH3 )2 (H2 O)2 , the interaction between PVP and Zn(O2 CCH3 )2 · 2H2 O can be observed. PVP plays an important role in the growth of the single-crystal ZnO nanorods. We analyze the growth process of ZnO nanorods by observing their TEM images at different moments. Consequently, our results indicate that the single-crystal ZnO nanorods were formed by self-assembling the ZnO nanoparticles. ABSTRACT
PACS 61.46.Hk;
1
61.46.Df; 78.30.-j; 81.07.-b; 81.16.Be
Introduction
ZnO nanomaterials possess the potential applications of solar cells [1] and nanoscale lasers [2]. Various ZnO nanostructures have been synthesized, such as by thermal evaporation of ZnO powders at high temperature for ZnO nanobelts [1] and carbothermal reduction of ZnO and using Au thin films as the catalysts for nanowire growth [2]. Meanwhile, other methods to synthesize ZnO nanomaterials have been reported, including the hydrothermal method [3, 4], ZnO-film-coated substrates, the liquid phase growth method [5], the nonhydrolytic single-precursor approach [6], the citrate ions controlling orientation approach [7], the aluminum oxide template approach [8], the microemulsion approach [9] and the catalyst approach [10]. Their growth mechanisms have also been actively investigated [4, 8, 11–15]. However, most of the growth mechanisms were proposed based on theoretical presumptions. It is highly u Fax: +852-2559-8738, E-mail:
[email protected]
expected that the growth mechanisms can be visually investigated in the synthesis [13]. Utilizing polymer complexing with metal salts, we have synthesized metal oxide nanomaterials, such as NiO nanoparticles [16], Co3 O4 nanocrystals [17] and ZrO2 nanolaminae [18]. Experimental results indicated that polyvinyl pyrrolidone (PVP) played a key role in synthesizing metal oxide nanomaterials. In this article, through the method of polymer complexing with dihydrate zinc acetate (Zn(O2CCH3 )2 · 2H2O), we can not only synthesize ZnO nanoparticles and nanorods but also visually investigate the formation mechanism of ZnO nanorods. The synthesis details and the Fourier transform infrared (FT-IR) spectra of the precursors will be investigated. 2
Experimental
In the synthesis, 1 g Zn(O2 CCH3 )2 · 2H2O and 2 g PVP (K30, MW = 30 000) were dissolved in 80 ml deionized water. The solution was vaporized in a water bath at 333 K until the solution became sticky. The solution was then transferred into a crucible and dried in an oven at 383 K for 24 h. Finally, the resultant solution containing the mixture of Zn(O2CCH3 )2 · 2H2O and PVP was annealed in a muffle furnace for 0.5, 3 or 12 h to produce the final products. The products were collected from the muffle furnace immediately when the preset annealing time was reached. To determine the phase compositions of the products, powder X-ray diffraction (XRD, Shimadzu XRD-6000, Cu K α radiation) was employed. The interaction between Zn(O2CCH3 )2 · 2H2O and PVP was investigated using a Nicolet FT-IR spectrometer (resolution of 4 cm−1 ). The morphology and structure of the samples were studied by highresolution transmission electron microscopy (HRTEM, JEOL2010, 200 kV). To prepare the samples for HRTEM, the synthesized products were dispersed in ethanol solvent by ultrasonic action, and drops of the solution were dripped onto a copper grid with carbon for TEM observation. To determine the heat-decomposition process of Zn(O2 CCH3 )2 · 2H2 O, thermogravimetric analysis and differential thermal analysis (TGA-DTA) experiments were conducted using a Hi-Res model SDT 2960 thermal analyzer.
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Results and discussion
In the process of vaporizing the aqueous solution, we find that heating the aqueous solution of Zn(O2 CCH3 )2 · 2H2 O at 333 K will produce a white precipitate of Zn(OH)2 from the hydrolysis of Zn(O2CCH3 )2 · 2H2O. However, no precipitate is produced in the process if PVP is dissolved in the solution with Zn(O2 CCH3 )2 · 2H2 O. This indicates that in the process of vaporization, PVP might interact with Zn(O2CCH3 )2 · 2H2 O to prevent the hydrolysis. In order to understand the interaction mode between PVP and Zn(O2CCH3 )2 · 2H2O, FT-IR spectra of PVP, Zn(O2 CCH3 )2 · 2H2 O and the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O are investigated. Figure 1 shows the FT-IR spectra from 1800 cm−1 to 1500 cm−1 . The band at 1661.50 cm−1 is attributed to the carbonyl of PVP, and the band at 1558.44 cm−1 is attributed to the carboxyl of Zn(O2CCH3 )2 · 2H2O. Due to the interaction with Zn(O2 CCH3 )2 · 2H2O, the carbonyl of PVP is blue shifted to 1668.01 cm−1 . This indicates that the carbonyl of PVP does not coordinate with Zn2+ of Zn(O2 CCH3 )2 · 2H2O; otherwise, the carbonyl of PVP would be red shifted. The interaction mode between PVP and Zn(O2 CCH3 )2 · 2H2 O is obviously different from that between PVP and AgNO3 , which belongs to the coordination mode [19]. The annealing temperature is chosen at the heat-decomposition temperature of Zn(O2 CCH3 )2 · 2H2 O of 573 K. In order to investigate the morphology and the lattice structure of the nanostructures synthesized from PVP and Zn(O2 CCH3 )2 · 2H2 O dried at 383 K for 12 h, the TEM images of the nanostructures are studied. From Fig. 2a, several well-dispersed spherical nanoparticles with diameter of about 50 nm can be observed. Figure 2b shows the image of a nanoparticle with high magnification. This result indicates that nanoparticles are obtained from the mixture of PVP and Zn(O2CCH3 )2 · 2H2O after being dried in an oven for 24 h. Figure 2c shows the HRTEM image of a nanoparticle of the mixture containing PVP and Zn(O2CCH3 )2 · 2H2O, which confirms that ZnO nanocrystals form. Two nanocrystals with diameters of ∼ 3 nm and ∼ 5 nm can be observed. Figure 2d shows the selected-area electron diffraction (SAED) pattern of a nanoparticle, which reconfirms the formation of ZnO nanocrystals. However, no characteristic peaks of ZnO are ob-
TEM image of the mixture containing PVP and Zn(O2 CCH3 )2 · 2H2 O that was dried at 383 K for 12 h. a Nanoparticles for the mixture containing PVP and Zn(O2 CCH3 )2 · 2H2 O at low magnification; b a nanoparticle of the mixture containing PVP and Zn(O2 CCH3 )2 · 2H2 O at high magnification; c high-resolution TEM image for a nanoparticle of the mixture containing PVP and Zn(O2 CCH3 )2 · 2H2 O; d selected-area electron diffraction (SAED) pattern of a nanoparticle FIGURE 2
XRD pattern of the mixture containing PVP and Zn(O2 CCH3 )2 · 2H2 O annealed for different times FIGURE 3
FT-IR spectra of PVP, Zn(O2 CCH3 )2 · 2H2 O and the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O from 1800 cm−1 to 1500 cm−1 FIGURE 1
served in the XRD pattern of the mixture containing PVP and Zn(O2CCH3 )2 · 2H2O, as shown in Fig. 3a. This might be because the quantity of the ZnO nanocrystals is too small to be determined by XRD. Consequently, our results indicate that the ZnO nanocrystals have formed in the surroundings of PVP. Figure 4a shows the TEM image of ZnO nanorods produced by calcining the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O for 0.5 h. From the image, the ZnO nanorods can be divided into two parts; one is the central ZnO nanorods,
TAO et al.
The growth mechanism of ZnO single-crystal nanorods synthesized by polymer complexing with zinc salts
a TEM image of ZnO nanorods produced by calcining the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O for 0.5 h. b Selected-area electron diffraction (SAED) pattern. c HRTEM image of ZnO crystallite at the outer space of the ZnO nanorod as pointed to by the white arrow in (a) FIGURE 4
the other is ZnO nanograins at the surface of the ZnO nanorods. Figure 4b shows the selected-area electron diffraction (SAED) pattern of the ZnO nanorods. Besides the circularity of ZnO nanograins, we can also observe the spot pattern of the central single-crystal ZnO nanorods. Figure 4c shows the HRTEM image of the surface of the ZnO nanorods. It is clearly seen that the ZnO nanograins assemble randomly around the central ZnO nanorods. The diameter of the ZnO nanograins is around 4 nm. Figure 3b shows the XRD pattern of the ZnO nanorods. Besides the peaks of PVP and heat-decomposed PVP, the peaks of ZnO can be obviously observed. This result indicates that the ZnO nanograins begin to assemble into single-crystal ZnO nanorods under the calcination of 573 K. This implies that the single-crystal ZnO nanorods might be self-assembled from inside to outside. Figure 5a shows the TEM image of the nanorods produced by calcining the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O for 3 h. The nanorods have a smoother surface and no obvious nanograins can be observed. The diameter of the nanorods is about 50 nm. The SAED pattern of the nanorods as shown in Fig. 5b shows the spot patterns of a regular lattice of single-crystal ZnO nanorods. Figure 3c shows the XRD pattern of the mixture containing PVP and Zn(O2 CCH3 )2 · 2H2 O annealed for 3 h. The intensity of the ZnO peaks becomes much stronger than the peak of PVP. This indicates that the
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FIGURE 5 a TEM image of ZnO nanorods produced by calcining the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O for 3 h. b Selected-area electron diffraction (SAED) pattern. c HRTEM image of ZnO nanorods
ZnO nanorods are well crystallized by using calcination of the mixture of PVP and Zn(O2 CCH3 )2 · 2H2O for 3 h. The HRTEM image of Fig. 5c confirms that the nanorods are ZnO nanorods. In addition, the regular candy strip of the crystal lattice as shown in Fig. 5a also reveals that the ZnO products exhibit a single-crystal structure. PVP plays a key role in the growth process of ZnO nanorods. In the process of calcining the mixture of PVP and Zn(O2 CCH3 )2 · 2H2 O, Zn(O2 CCH3 )2 · 2H2O will decompose gradually and produce ZnO molecules. Through interacting with PVP, the produced ZnO molecules self-assemble into ZnO nanocrystals after drying the mixture of PVP and Zn(O2CCH3 )2 · 2H2O. With the annealing temperature controlled at 573 K for 0.5 h, the produced ZnO nanocrystals selfassemble into ZnO nanorods from inside to outside. However, the outer-space nanocrystals are disordered and the center parts are single-crystal ZnO nanorods. When the time of the synthesis is increased to 3 h and the annealing temperature is maintained at 573 K, the outer-space ZnO nanocrystals selfassemble into the ZnO nanorods. Consequently, the whole growth process of ZnO nanorods can be summarized as shown in Fig. 6. In summary, we have investigated the growth process of ZnO nanorods by drying and calcining the mixture of PVP and Zn(O2 CCH3 )2 · 2H2O at various reaction times. Based on the experimental results, a scheme for the whole growth process of ZnO nanorods is proposed. Firstly, ZnO nanocrystals
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Applied Physics A – Materials Science & Processing REFERENCES
The growth process of ZnO nanorods. Zn(O2 CCH3 )2 · 2H2 O is also named as Zn(OAc)2 · 2H2 O FIGURE 6
produced within PVP. Then, the produced ZnO nanocrystals self-assemble into ZnO nanorods. Finally, the ZnO nanorods self-assemble into single-crystal ZnO nanorods. ACKNOWLEDGEMENTS This work was supported by a grant from the NSFC key program (No. 20236020), the NSFC (Nos. 20076004 and 20571007), the Beijing NSFC (No. 2073029) and the University Development Fund (UDF) and the Seed Funding (No. 10207444) of the University of Hong Kong.
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