Large-Scale Growth of Highly Oriented ZnO Nanorod Arrays in the Zn-NH3 · H2O Hydrothermal System Heqing Yang,*,† Yuzhe Song,† Li Li,† Junhu Ma,† Dichun Chen,‡ Shuling Mai,† and Han Zhao†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 1039–1043
Key Laboratory of Macromolecular Science of Shaanxi ProVince, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an, 710062, China, and AdVanced Material Analysis and Test Center, Xi’an UniVersity of Technology, Xi’an, 710048, China ReceiVed December 6, 2006; ReVised Manuscript ReceiVed NoVember 24, 2007
ABSTRACT: Large-scale highly oriented ZnO nanorod arrays were directly grown on zinc foil through a simple hydrothermal reaction of Zn foil with aqueous ammonia at 100 °C. The products were characterized with X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. It was found that the ZnO nanorods were single crystalline with the wurtzite structure and grown in the [0001] direction, and have a controllable diameter in the range of 250–100 nm with lengths of up to 4.0 µm by varying the growth time. The ammonia plays a key role in the formation of ZnO nanorod arrays, and a possible mechanism is also proposed to account for the growth of the ZnO nanorod arrays. The ZnO nanorod arrays exhibited a UV emission with peak at 396 nm and a blue green emission with a peak at 488 nm. The UV and blue green emissions are considered to originate from the exciton transition and the transition between the oxygen vacancy and interstitial oxygen, respectively. In addition, the UV and blue green emission intensities can be adjusted by changing the growth time.
1. Introduction Zinc oxide is recognized as one of the most important semiconductor materials due to its direct wide band gap (3.37 eV) and large exciton binding energy (59 meV), and its application in optoelectronics, catalyst, sensors, and actuator. In recent years, one-dimensional (1-D) ZnO nanostructures, such as nanobelts,1 nanotubes,2 nanohelices,3 nanorods, nanowires,4 and tower-like structure of ZnO nanocolumns4 have attracted a great research interest for applications in sensing,5 optoelectronics,6 field emission,7 and piezoelectricity.8 Many methods have been employed for fabrication of 1-D ZnO nanostructures, including thermal evaporation,1,3 chemical vapor deposition (CVD),9–12 metal-organic chemical vapor deposition (MOCVD),4,13 pulsed laser deposition (PLD),14 template-based growth15 and various solution phase approaches.16–23 The vertically aligned single-crystal ZnO nanorods on R-plane-oriented Al2O3 (sapphire), GaN, ZnO, Ga-doped ZnO, etc. substrates were prepared by MOCVD,4 PLD,14 and CVD.9,12 The growth of aligned ZnO nanorods is considered to be a good candidate for light emitting and field emission.4 But these gases-phase approaches generally require highly sophisticated equipment, expensive singlecrystalline substrates for oriented growth, and elevated temperatures of 450–900 °C, and often face other limitations in terms of sample uniformity and low product yield.23 Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al.16,17 developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates. The ZnO microrods and nanorods were grown in an aqueous solution of zinc nitrate (Zn(NO3) · 4H2O) and methenamine (C6H12N4) at 95 °C. Subsequently, a seeded growth process18 was used to make helical ZnO rods and columns at a similar temperature. Greene et al.19 expanded on the seeded growth * Corresponding author. E-mail:
[email protected]. † Shaanxi Normal University. ‡ Xi’an University of Technology.
methods to produce homogeneous and dense arrays of ZnO nanowires on ZnO-nanoparticle-coated arbitrary substrates. However, the liquid-phase coating of the substrates with ZnO nanoparticles prepared in solution remains complex and difficult/irreproducible. Recently, a radio frequency magnetron-sputtering technique was utilized to prepare ZnO-filmcoated substrates for subsequent growth of highly oriented ZnO nanorods. The oriented ZnO nanorod arrays were grown from formamide aqueous solution at 65 °C in the presence of zinc foils.20 Tang et al. developed an H2O2-assisted hydrothermal method for ZnO nanorods. The aligned ZnO nanorods were grown directly on zinc foil substrates in an aqueous solution of NaOH and H2O2, but only at high temperature (160–200 °C).21 In addition, little work has been done on the diameter-controlled growth of the highly oriented ZnO nanorods.24 Zhang, et al.25 prepared prismlike and tubular ZnO in the ZnAc2-ammonia-ethanol hydrothermal system for the first time. Very recently, Bardhan, et al.26 synthesized ZnO submicrometer particles with unique morphologies including rings, bowls, hemispheres and disks by decomposing Zn(NH3)42+ precursor in aqueous ethanol solution. The different morphologies can be selectively obtained by varying precursor concentrations within a certain pH range.26 Herein, a novel Zn-NH3 · H2O hydrothermal route was utilized to prepare the ZnO nanorod arrays. The ZnO nanorod arrays were grown directly on zinc foil substrates in aqueous ammonia at 100 °C. The H2O2 and precasting ZnO nanoparticle templetes were unnecessary in the route, and the diameter of ZnO nanorods can be changed by adjusting the reaction time. The growth mechanism and photoluminescence (PL) spectra of the ZnO nanorod arrays were studied in detail.
2. Experimental Section 2.1. Preparation of ZnO Nanorods. Typically, 10 mL of 1% aqueous ammonia (0.6 mol/L) was put into a Teflon-lined autoclave of 25 mL capacity. One 4 cm × 0.5 cm zinc foil was polished with a sand paper, and then treated in ethanol and ion-exchanged water
10.1021/cg060890f CCC: $40.75 2008 American Chemical Society Published on Web 02/20/2008
1040 Crystal Growth & Design, Vol. 8, No. 3, 2008
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Figure 1. (a, b) SEM top-images of ZnO nanorod arrays grown in aqueous ammonia at 100 °C for 12 h at low and high magnifications, respectively; (c-e) tilted view images of the aligned ZnO nanorods grown in aqueous ammonia at 100 °C for 12, 18, and 36 h.
Figure 2. XRD patterns of ZnO nanorod arrays grown on zinc foil for (a) 12 h and (b) 18 h.
for 10 min with ultrasonic irradiation, respectively. The zinc foil was vertically immersed into the autoclave, and the autoclave was sealed, and heated at 100 °C for 12–24 h. After the heating treatment, the autoclave was cooled to room temperature naturally. The zinc foil was removed from solution, rinsed with ion-exchanged water 2–3 times, and then dried at room temperature. 2.2. Characterization of ZnO Nanorods. The products were characterized and analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and PL. The XRD analysis was performed using a Rigaku DMX-2550/PC X-ray diffractometer with Cu KR radiation (λ ) 1.54 Å) at 40 kV and 50 mA. The 2θ range used was from 30° to 80° with a speed of 4°/min. SEM images were obtained on a FEI Quanta 200 scanning electron microscope at an accelerating voltage of 25 kV. HRTEM and electron diffraction images were obtained on a JEOL JEM-3010 transmission electron microscope (TEM) at an accelerating voltage of 300 kV. Samples for TEM were prepared by dispersing powdered ZnO products on carbon-coated copper grids. PL spectra were measured at room temperature on an Edinburgh FLS920 fluorescence spectrophotometer using a Xe lamp with an excitation wavelength of 300 nm.
Figure 3. TEM images of (a) a cluster of ZnO nanorods and (b) single nanorod removed from an array grown for 12 h; (c) a selected area electron diffraction (SAED) of an individual ZnO nanorod; (d) highresolution TEM image of a ZnO nanorod.
3. Results and Discussion 3.1. SEM and XRD. Typical SEM top-micrographs of the ZnO nanorod arrays grown on zinc foil in aqueous ammonia at 100 °C for 12 h at low and high magnifications, respectively, are presented in Figure 1, panels a,b. The highly oriented ZnO nanorod arrays with high density were observed on the surface of the zinc foil. The ZnO nanorods have flat hexagonal crystallographic planes, indicating partially that the ZnO nanorods are hexagonal in crystal structure and preferentially oriented in the c-axis direction. The diameters of the nanorods distribute in the range of 150–400 nm, and the average diameter is about
Growth of Highly Oriented ZnO Nanorod Arrays
250 nm. Figure 1c is a tilted view image of the ZnO nanorod arrays, showing that the diameter of each nanorod has little variation from bottom to top. The most ZnO nanorods have a uniform length of about 4 µm. Figure 1c-e is a tilted view image of the ZnO nanorod arrays grown on zinc foil in aqueous ammonia at 100 °C for different reaction times. As the reaction time was increased from 12 to 18 h, the average diameter of the nanorods decreased from 250 to 100 nm, whereas the length has little variation. When the reaction time was increased from 18 to 24 h, the diameters hardly changed. Therefore, the aspect ratio of the nanorods can be adjusted via changing the reaction time in the range of 12–18 h. Figure 2 shows XRD patterns of the ZnO nanorod arrays grown on zinc foil in aqueous ammonia at 100 °C for 12 and 18 h. The eight peaks at 2θ ) 31.7°, 34.4°, 36.3°, 47.5°, 56.6°, 62.8°, 67.9°, and 72.5° are observed from Figure 2a,b. According to JCPDS card no. 36-1451, the products are hexagonal ZnO phases (a ) 0.325 nm, c ) 0.521 nm), and these peaks are assigned to (100), (002), (101), (102), (110), (103), (112), and (004) diffraction lines of hexagonal ZnO phases, respectively. In addition to diffraction lines of ZnO, the diffraction lines of zinc foil were also observed. Furthermore, the relative intensities of these peaks are distinct from those of ZnO powders. The diffraction intensity of the (002) surpasses others, which illustrates the c-oriented nature of the as grown array. The degree of the orientation can be illustrated by the relative texture coefficient,27 which is given by TC002 )
I002 / I 0002 I002 / I 0002 + I100 / I 0100
where TC002 is the relative texture coefficient of diffraction peaks (002) over (100), I002 and I100 are the measured diffraction intensities due to (002) and (100) planes, respectively, and I0020 and I1000 are the corresponding values of standard PDF(36-1451) measured from randomly oriented powder samples. For materials with random crystallographic orientations, e.g., powders, the texture coefficient is 0.5. The TC002 of samples grown at 100 °C for 12 and 18 h is 0.95 and 0.96, respectively. The XRD results suggest that our samples are wurtzite ZnO nanorods with preferable c-orientation. 3.2. HRTEM. To further illuminate the detailed structure of the products, the ZnO nanorods grown in aqueous ammonia at 100 °C for 12 h were characterized with TEM. The lowermagnification TEM images of ZnO nanorods removed from the arrays are shown in Figure 3, panels a and b, respectively. As can be seen, the arrays are composed of two different kinds of ZnO nanorods: (i) single ZnO nanorod with the diameter of 250 nm and with the length of 1 µm and (ii) ZnO nanorods in a cluster. The results suggest that multiple and single nanorods grow from a single aggregate of ZnO nanoparticles and a single nanoparticle, respectively. Figure 3c shows the associated selected area electron diffraction (SAED) pattern, and it can be indexed as the [011j0] zone axis of single-crystalline ZnO with a hexagonal structure. The HRTEM image of the ZnO nanorod is displayed in Figure 3d. The distance between the parallel lattices was measured to be 0.52 nm, corresponding to the (001) crystal planes of the wurtzite-type ZnO. The SAED and HRTEM further confirm that the ZnO nanorods are single crystalline and grow along the [0001] direction. From the crystal structure, ZnO is wurtzite structure consists of polar (0001), (0001j) planes and nonpolar (1000) planes with C6v symmetry. Due to its anisotropic crystal structure, the c-axis is the most preferred growth
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Figure 4. PL spectra of ZnO nanorod arrays grown on zinc foil in aqueous ammonia at 100 °C for 12 (a) and 18 h (b) at room temperature using a Xe lamp with an excitation wavelength of 300 nm.
orientation, and the velocities of growth in different directions under hydrothermal condition are V [0001] > V [0110] > V [1000].22 3.3. PL. The PL spectra of the ZnO nanorod arrays grown on zinc foil in aqueous ammonia at 100 °C for 12 and 18 h was measured at room temperature using a Xe lamp with an excitation wavelength of 300 nm, and the results are shown in Figure 4. An intense UV emission peak at 396 nm and a weak blue green emission peaking at 488 nm were observed from the sample grown for 12 h (curve a). As the growth time increases from 12 to 18 h, the blue green-band emission intensity increases, while the UV emission intensity decreases. The UVband emission corresponds to the recombination of free excitons between conductive band and valence band of ZnO and is called near band edge emission.12,28 The average diameter of the ZnO nanorods measured by SEM (Figure 1, panels b and d) is 250 and 100 nm, respectively. It might be concluded that the strong UV is related to the bigger diameter of nanorods. If this would be the case, no UV emission should have been observed for ZnO nanorods with much lesser diameter ( V (1) because of the complex action of NH3 with Zn2+. In order to understand the formation process of the ZnO nanorod arrays, time-dependent experiments were carried out and the resultant products were analyzed by SEM. The representative SEM images of the products prepared at certain reaction time intervals are shown in Figure 7. The SEM observations of the products obtained for 0.5 h show that there is a large quantity of spherical ZnO nanoparticles with the diameters of 50–100 nm on the Zn substrate (Figure 7a). When the reaction time was prolonged to 1 h, ZnO nanorod arrays were observed on the Zn substrate (Figure 7b). The nanorods have diameters in the range of 100–200 nm and lengths of 150 to 350 nm. With increasing reaction time, the diameter and length of the nanorods increase continuously. When the reaction time is 12 h, the average diameter of the nanorods is about 250 nm. In addition, we found that the density of the nanorod arrays increase with increasing reaction time. In order to further understand the diameter of ZnO nanorods dependence with growth time, the products obtained at 100 °C for 12 h were heated in aqueous ammonia (1%) at 100 °C for 12 h, and the products obtained were characterized with SEM. The result is shown in Figure 7d. It is found that the average diameter of the nanorods is decreased from 250 to 100 nm. On the basis of the investigations described above, it is possible to interpret the formation process of ZnO nanorod arrays. The possible growth process is schematically illustrated in Figure 8. In the initial growth stage zinc foil reacted with aqueous ammonia to form spherical ZnO nanoparticles via the reactions (3) and (4), and coated on the entire surface of the zinc foil. With increasing reaction time, these spherical ZnO nanocrystals grew continuously along the c-axis of wurtzite ZnO into ZnO nanorods. During the growth process, the ZnO nanoparticle served as seeds for subsequent growth of highly oriented ZnO nanorods. Furthermore, as the nanoparticles evolved into nanorods, the fresh nucleation and growth of ZnO nanocrystals were carried out simultaneously. So the density of the nanorod arrays increase with increasing reaction time. As the reaction time increases from 12 to 18 h, the ZnO nanorod arrays with
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(5) (6) (7) (8) (9)
Figure 8. Schematic illustration of growth mechanism of the ZnO nanorod arrays.
(10) (11)
high density suppressed reactions (3) and (4), and limited the growth of the nanorods. In addition, the surface of ZnO nanorods may react with aqueous ammonia to form dissoluble [Zn(NH3)4](OH)2, and then the diameters of ZnO nanorods are reduced. The chemical reaction is contrary reaction of reaction (4). As the reaction time is unceasingly increased, the reversible reaction (4) will reach the equilibrium state, and then the diameters of ZnO nanorods hardly change again. However, when zinc foils reacted with H2O, the formation velocities of the ZnO nanoparticles were too slow to form the ZnO-nanoparticle-film on the surface of zinc foil. Therefore, the ZnO nanorods were randomly distributed on the zinc substrate.
4. Conclusion Highly oriented ZnO nanorod arrays have been successfully prepared on zinc foil via a simple hydrothermal reaction of Zn foil with aqueous ammonia. Precasting ZnO nanoparticle template is unnecessary in the route. The control over the diameter and PL behaviors of ZnO nanorods can be achieved by adjusting reaction time. The ammonia plays a key role in the formation of ZnO nanorod arrays, and the reaction mechanism is discussed. The ZnO nanorod arrays are expected as ideal functional components for photonic and electronic device. Furthermore, this route may be extended to the fabrication of nanostructures of other oxides. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20443006 and 20573072) and Shaanxi Province Natural Science Foundation of China (2003E.06).
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