J. Phys. Chem. B 2005, 109, 12697-12700
12697
Controllable Synthesis of ZnO Nanorod and Prism Arrays in a Large Area Debao Wang* and Caixia Song College of Chemistry and Molecular Engineering, Qingdao UniVersity of Science & Technology, Qingdao 266042, P. R. China ReceiVed: February 3, 2005; In Final Form: May 20, 2005
ZnO nanorod and nanoprism arrays have been directly synthesized on a large-area zinc substrate via a convenient solution method. The products were characterized with XRD, SEM, HRTEM, and photoluminescence (PL) spectroscopy. The influence of the solvent and the concentration of NaOH on the size and shapes of the as-prepared ZnO samples have been studied. It was found that ZnO nanorod or nanowire arrays were fabricated in alcohol, whereas ZnO nanoprisms with pyramid tips were produced in an alcohol-water mixture. The diameters of the nanorods or nanoprisms became thicker when a higher concentration of NaOH was used. Room-temperature PL spectra of the ZnO products showed a UV emission and a broad green band. The mechanism of the nanorods and nanoprisms in two systems is briefly discussed.
Introduction In recent years, one-dimensional (1-D) nanostructure semiconductor materials have attracted much attention due to their great potential for fundamental studies of the roles of dimensionality and size in their physical properties as well as for applications in optoelectronic nanodevices and functional materials.1,2 Zinc oxide (ZnO) is a well-known semiconductor for its wide band gap and high exciton binding energy at room temperature. It possesses unique catalytic, electrical, optoelectronic, and photochemical properties, which stimulate wide research interest in its potential applications. A particularly striking recent observation is that of room-temperature lasing action in oriented ZnO nanowire arrays,3 highlighting the prospects of corresponding research interests in a morphologically controllable synthesis of 1-D ZnO nanocrystals to answer the demand for the development of novel devices. And more recently, the fabrication of a ZnO nanowire transistor has been reported.4 As a consequence, intensive research has been focused on fabricating 1D ZnO nanostructures5 and in correlating their morphology with their size-related optical and electrical properties.3,6 Aligned fabrication of 1-D ZnO nanocrystals has been successfully achieved via a vapor-liquid-solid (VLS) process,7 and gold,6-8 copper,9 and tin10 have been used as catalysts to direct their 1-D structured growth. ZnO nanorod arrays have also been synthesized on the surface of a porous aluminum oxide template.11 Due to the fact that solid-vapor phase synthesis typically occurs at high temperature, the synthesis of ZnO 1-D nanocrystal arrays via chemical solution routes has been explored.12 However, most of these methods generally produce ZnO nanostructures on silicon, glass slide, or sapphire substrates on the microscale level or ZnO nanoparticle seeds are needed, which presents a limit toward the road of large-scale industrial production. Therefore, it is imperative to develop an alternative method to tackle the problem from the science and technology point of view. Tang et al. reported the synthesis of ZnO nanorod arrays on zinc substrate via a H2O2-assisted hydrothermal method.13 * Corresponding author. Phone: +86-532-4022787. E-mail: wangdb@ ustc.edu.
TABLE 1: Summary of Experimental Conditions versus the Products (160 °C, 20 h) sample no.
solvent
NaOH (mol/L)
CTAB (mol/L)
ZnO products
1 2 3 4 5 6
alcohol alcohol alcohol:H2O ) 1:3 alcohol:H2O ) 1:3 alcohol:H2O ) 1:3 alcohol:H2O ) 1:3
0.05 0.025 0.025 0.05 0.1 0.05
0.03 0.03 0.03 0.03 0.03 0.0
Figure 2 Figure 3a Figure 3b Figure 3c,d Figure 3e Figure 3f
Here, we demonstrate an effective solvothermal method for large-area synthesis of aligned ZnO nanorod arrays with highdensity and controllable morphologies. Fresh zinc foils were used as both a reagent and a substrate for the direct growth of ZnO nanorod arrays, and no catalysts were used. In addition, by varying the experimental conditions, ZnO nanorods with different morphologies could be easily obtained. These findings provide a convenient, simple technique for production of the novel 1-D ZnO nanocrystals suitable for subsequent processing into nanostructure materials and devices. Experimental Section Fresh zinc foils (15 × 15 × 0.25 mm3) with a purity of 99.9% were used as both a reagent and a substrate for the direct growth of ZnO nanorod arrays. The typical experiment procedure was as follows: An alcohol solution containing 0.05 mol/L of NaOH and 0.03 mol/L of cetyltrimethylammonium bromide (CTAB) was loaded into a Teflon-lined stainless steel autoclave to its 85% capacity. The zinc foils were carefully cleaned with absolute alcohol and deionized water, respectively, in an ultrasound bath. After the zinc substrate was dipped into the solution, the autoclave was sealed and put into an oven. The reaction was conducted at 160 °C for 20 h. After that, the samples were taken out and rinsed with deionized water, which consisted of a pale layer homogeneously coated on the zinc substrate. The different reaction conditions and the products are summarized in Table 1. The as-prepared products on the substrate were directly subjected to characterizations by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The XRD patterns
10.1021/jp0506134 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/15/2005
12698 J. Phys. Chem. B, Vol. 109, No. 26, 2005
Figure 1. XRD patterns of the as-synthesized ZnO samples: (a) sample 1, (b) sample 2, and (c) sample 4. (d) Standard XRD pattern of wurtzite ZnO.
Figure 2. SEM images of ZnO sample 1: (a) low-magnification, (b) high-magnification, and (c) side view. (d) HRTEM image and ED pattern of sample 1.
were recorded by employing a Philips X’pert X-ray diffractometer with Cu KR radiation (λ ) 0.154187 nm). SEM images were obtained on a JSM-6700F field emission scanning electron microscope. The transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and electron diffraction (ED) patterns of the samples were recorded with a JEOL 2010 microscope (200 kV). The photoluminescence (PL) measurements were performed on a Hitachi 850 luminescence spectrometer at room temperature. Results and Discussion Figure 1 shows XRD patterns of the as-prepared ZnO nanorod arrays. All the diffraction peaks can be indexed to wurtzite structured ZnO (JCPDS card no. 36-1451), except those marked with Zn coming from the zinc substrate. No characteristic peaks from other impurities are detected, and the relative peak intensities of the as-prepared samples are somewhat different when compared with the standard powder diffraction pattern of bulk ZnO (Figure 1d). Preferential orientation and alignment of the prepared crystals on the substrate are plainly the explanation. The sharp shape of the diffraction peaks suggests all the ZnO samples should be well crystallized. The morphology of the as-prepared sample was examined by SEM. Figure 2 shows SEM images for the sample synthesized in alcohol. It can be clearly seen that only rodlike features are observed for sample 1, which are aligned in a dense array approximately perpendicular to the substrate surface. The ZnO nanorod arrays uniformly covered the entire surface of the zinc
Wang and Song
Figure 3. SEM images of ZnO samples: (a) sample 2, (b) sample 3, (c, d) sample 4, (e) sample 5, and (f) sample 6. The insets are shown to reveal the hexagonal pyramid tips.
substrate based on the SEM observation (Figure 2a). Close examination of the top tips of the ZnO nanorods reveals that an individual ZnO nanorod shows a flat hexagonal section (Figure 2b). The ZnO nanorods are about 50 nm in diameters and 2 µm in length (Figure 2c). Most of the thinner ZnO nanorods align into a thicker bundle. Structural characterization of the products was carried out with a high-resolution transmission electronic microscope. Figure 2d shows the HRTEM image of a nanorod with a diameter of 45 nm. The inset of Figure 2d shows the corresponding ED pattern, which can be identified as the [01-10] zone axis projection of the ZnO reciprocal lattice. The slighter spot of (0001) in the inset of Figure 2d might result from the secondary reflection of (0002) in ZnO crystals. This ED pattern can be easily obtained from various ZnO nanorods, suggesting that ZnO nanorods are single-crystalline in structure. A lattice-resolved HRTEM image (inset of Figure 2d) shows that a clear interplanar spacing of 0.52 and 0.28 nm corresponds to the lattice spacing of (0001) and (10-10) planes, respectively, indicating that the ZnO nanorods grow preferentially along the [0001] direction. The dependence of the 1-D ZnO nanostructures on the solvents and the alkaline conditions is shown in Figure 3. As exhibited in the SEM images, with lower concentration of NaOH in alcohol (sample 2), quasialigned ZnO nanowire arrays were observed also in high density (Figure 3a), and show diameters of 30 nm on average. While the samples obtained in mixed solvent consist of nanometer to submicrometer rods in high quantity, some of them do not show well-aligned pattern when compared with sample 1. The results for the lower concentration of NaOH are shown in Figure 3b: flowerlike ZnO nanorod arrays together with quasialigned ones were observed via SEM (sample 3). The individual ZnO nanorod has a diameter of 100 nm on average. When the synthesis was carried out in a high concentration of NaOH, ZnO nanorods increase in diameters, and more flowerlike patterns appear in the SEM image (sample 4, Figure 3c). A higher magnification image in Figure 3d reveals the general morphology of the flowerlike ZnO rod arrays. It can be clearly seen that the individual ZnO rod has a diameter in the range of 500 nm to 1 µm and a length of more than 10 µmand exhibits hexagonal prism-like morphology. The magnified SEM image in the inset of Figure 3d reveals that all the ZnO hexagonal prisms have a hexagonal pyramid-like sharp tip. When a higher concentration of NaOH was used, prismshaped ZnO crystals were formed with random orientation and showed pyramid-like tips (Figure 3e and the inset). In addition, some thinner rods about 100 nm in diameter and thicker ones (∼1 µm) coexist in the sample. It has been reported that the presence of CTAB is important for the formation of ZnO
Controllable Synthesis of ZnO Nanorod and Prism Arrays
Figure 4. Room-temperature photoluminescence spectra of ZnO products: (a) sample 1 and (b) sample 4.
nanorods.14 We observed that only random ZnO prisms and ZnO nanowires were formed (Figure 3f) in the absence of CTAB. A typical room-temperature photoluminescence (PL) spectrum of sample 1 is shown in Figure 4a. A sharp and strong UV emission band was recorded at around 380 nm, which can be assigned to the emission of the free exciton.15 A broad and weak green emission was observed centering at 527 nm, which is related to the singly ionized oxygen vacancy.16 For sample 3, however, a weak UV emission and a broad and relative strong green emission were observed (Figure 4b), suggesting that the optical properties of ZnO crystals are sensitive to the size and the morphologies of the crystals. The overall reaction for the growth of ZnO crystals may be simplified as following:
Zn f Zn2+ h [Zn(OH)4]2- f ZnO Although the chemical reaction is rather simple, the growth process of ZnO nano-/microrods would be very complicated. The growth scheme of ZnO nanorod arrays and ZnO prisms is summarized in Figure 5. The usage of a basic condition and an alcoholic environment are two crucial keys in ensuring the formation of the growth unit of Zn(OH)42- and a controlled release of the units from solution. Under solvothermal (hydrothermal) condition, the Zn-foil substrate could be easily oxidized, and consequently the ZnO thin layer may be generated on the Zn-foil substrate. The resulting ZnO layer may be dissolved in alkaline condition as zinc hydroxide species (Zn(OH)42-) into the solution. Consequently, the dissolved Zn(OH)42- species could be easily stabilized by CTAB as a positive surfactant and served as growth units for the formation of ZnO nuclei and epitaxal growth of ZnO nuclei into 1-D structures. A ZnO crystal is a polar crystal, O2- is in hexagonal closest packing (HCP), and each Zn2+ lies within a tetrahedral group of four oxygen ions (Figure 5a). Zn and oxygen atoms are arranged alternatively along the c-axis so that the top surface is Zn-terminated (0001) and the bottom surface is O2--terminated
J. Phys. Chem. B, Vol. 109, No. 26, 2005 12699 (000-1).7 The inherent asymmetry along the c-axis leads to the anisotropic growth of 1-D ZnO crystallites. The formation of hexagonal prism shaped ZnO crystals in the present study is suggested to be attributed to the difference in the growth rate of the various crystal facets. It is acceptable that the hydrothermal growth rate of different planes is as follows: (0001) > (10-1-1) > (10-10).17 As we know, the more rapid the growth rate, the quicker the disappearance of the plane. Therefore, the relative growth rate of these crystal faces will determine the aspect ratio and the final shape of the ZnO nanostructures. The (0001) plane disappears due to the most rapid growth rate leading to the pointed shape at the end of the c axis. The growth rate of the {10-10} planes is small and they remain to form the hexagonal prisms, while the {10-11} planes remain to form hexagonal pyramid-like tips. In alcohol condition, in contrast, just the (0001) plane remains to form flat tips. The different growth behavior of ZnO 1-D structures might also relate to the different surface properties of ZnO nuclei formed in different solvents. It has been reported that (0001) is not a stable face under hydrothermal conditions,17 and it would disappear and result in hexagonal prisms with an hexagonal pyramid-like sharp tip in alcohol and water mixture (Figure 5c) whereas the (0001) plane may become stable in alcohol media and remain to form flat tips (Figure 5b). In addition, more ZnO nuclei would form in the initial stage in water than in alcohol due to the higher reactivity of metal zinc in water. And these ZnO nuclei might agglomerate, leading to the formation of flowerlike patterned ZnO nanoprisms (Figure 5c). With the increase of concentration of the growth units Zn(OH)42- in higher concentrations of NaOH, the preferential growth difference between 〈0001〉 and other directions would diminish, and more thicker ZnO prisms appear in the products. Of course, further studies are needed to understand the exact mechanism for the different growth behaviors of ZnO 1-D structures observed in our experiments. Conclusion In conclusion, a simple and convenient method for the preparation of ZnO nanorod and nanoprism arrays has been introduced. The morphologies and the shapes of the 1-D ZnO nanostructure could be controlled conveniently by varying the solvent and the alkaline condition. The controllable growth of the ZnO nanorod and nanoprism arrays would open up the possibility of finding new applications or improving existing performances, for example, in the areas of nanoelectronics, catalysts, sensors, and optoelectronics. It is anticipated that this solution method can be extended to the large-area preparation of aligned 1-D nanostructure arrays of other oxides and chalcogenides under mild condition.
Figure 5. Schematic sketch of ZnO crystal structure (a) and possible growth routes for aligned 1-D ZnO nanostructures in alcohol (b) and wateralcohol mixture (c).
12700 J. Phys. Chem. B, Vol. 109, No. 26, 2005 Acknowledgment. The authors express their gratitude to Dr. G. F. Zou for his kind help. Supporting Information Available: TEM image of ZnO nanorod array in Sample 1, HRTEM image of an individual ZnO nanorod in Sample 1, and TEM image and corresponding ED pattern of the thinner ZnO nanorods in sample 4. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Lieber, C. M. Solid State Commun. 1998, 107, 607. (b) Hu, J.; Odom, T. W.; Liber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (3) (a) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (b) Johnson, J. G.; Yan, H.; Yang, P.; Soykally, R. J. J. Phys. Chem. B 2003, 107, 8816. (c) Liu, C.; Zapien, J. A.; Yao, Y.; Meng, X.; Lee, C. S.; Fan, S.; Lifshitz, Y.; Lee, S. T. AdV. Mater. 2003, 15, 838. (4) Goldberger, J.; Sirbuly, D. J.; Law, M.; Yang, P. J. Phys. Chem. B 2005, 109, 9. (5) (a) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (b) Liu, F.; Cao, P. J.; Zhang, H. R.; Li, J. Q.; Gao, H. J. Nanotechnology 2004,15, 949. (c) Bae, S. Y.; Seo, H. W.; Choi, H. C.; Park, J.; Park, J. J. Phys. Chem. B 2004, 108, 12318. (d) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (e) Huang, L. S.; Wright, S.; Yang, S. G.; Shen, D.
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