Reproducible Growth of Ultralong ZnO Nanowire Arrays in the

DOI: 10.1021/cg900438c

Reproducible Growth of Ultralong ZnO Nanowire Arrays in the Metastable Supersaturated Solution

2009, Vol. 9 4653–4659

Guomin Hua, Yue Tian, Liangliang Yin, and Lide Zhang* Key Lab of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031 Anhui, People’s Republic of China Received April 20, 2009; Revised Manuscript Received August 30, 2009

ABSTRACT: Vertical aligned ZnO nanowire arrays with a long length and high aspect ratio were prepared successfully in a metastable supersaturated solution which was formed by the introduction of aluminum-iso-propylate into the growth solution. Investigations on the growth revealed that ZnO nanowires were preferentially grown along the c-axis in the metastable supersaturated solution and growth kinetics was subjected to reaction-limited growth. By the cyclic growth in the metastable supersaturated solution, the ZnO nanowire arrays with a length of 10 μm and aspect ratio of 70 could be prepared. The as-prepared ZnO nanowire arrays have potential applications in the next generation nanodevices.

1. Introduction Recently, one-dimensional ZnO nanowires possessing a direct electron pathway and high surface-to-volume ratio have received intensive attention due to their high electron mobility and novel size-dependent optical properties.1-3 The ordered and oriented ZnO nanowire arrays become attractive in the applications of piezoelectric nanogenerators,4 roomtemperature ultraviolet nanowire nanolasers,5 quantum dots sensitized solar cells,6 and field emission devices.7 Moreover, ZnO is a biocompatible material with a high isoelectric point (IEP) of about 9.5, which makes it suitable for adsorption of proteins with low IEPs, as the protein immobilization is primarily driven by electrostatic interaction. They are promising for biosensor applications.8,9 A few novel prototypes of gas, DNA, and glucose sensors have been proposed based on the ZnO nanowire arrays.10-12 For these abovementioned applications, long length and high aspect ratio ZnO nanowire arrays are desired to enhance their performances. So far, solution-route growth appears as the popular method for the preparation of ZnO nanowire arrays on the merit of the low-cost, large-scale fabrication. And remarkable progress has been achieved with respect to the control of the density and orientation of nanowires.13-17 In the solution-route growth, the supersaturation is one of the most important factors for the crystal growth.18 And it can be divided into two major levels: one is termed as the unstable supersaturation where the homogeneous nucleation occurs. At this level, ZnO nuclei form in the bulk solution and consume the ZnO precursor, which is harmful for persistent growth of ZnO nanowire arrays on the substrates. Another is termed the metastable supersaturation where the homogeneous nucleation is suppressed but the crystal growth can proceed. From the thermodynamic viewpoint, such status is conceived as the optimized condition for the anisotropic growth of long ZnO nanowire arrays. To date, there are few effective methods to control the supersaturation of growth solution and it was empirically

adjusted by the concentration of agents and the temperature.19 Although there are some literature that reported on the preparation of ZnO nanowire arrays20-23 where the experimental conditions were widely different and the reproducibility was quite poor, the controlled growth of long length and high aspect ratio of nanowire arrays is difficult. In particular, when the solution route growth is used, the reproducible growth of ultralong ZnO nanowire arrays is still a challenge owing to the lack of effective control and optimization of the growth conditions. Recently, the influences of Al3þ ions on the preparation of ZnO nanowire arrays were intensively investigated.24-26 In our previous work,26 it was observed that the supersaturation of the solution containing Al3þ ions for ZnO nanorod growth could be maintained at the metastable supersaturated level. The typical characterization was the ZnO nucleation and the growth process in the bulk solution was suppressed effectively. In this paper, we investigate the growth of nanowire arrays in the well-defined metastable supersaturated solution and present a facile and reproducible method to prepare ultralong and high aspect ratio ZnO nanowire arrays by cyclic growth in the metastable supersaturated solution. 2. Experimental Details

*To whom correspondence should be addressed. E-mail: ldzhang@issp. ac.cn.

ZnO nanowires arrays on clean silicon substrates were prepared as follows. A layer of ZnO seeds was covered on the substrate, and the ZnO seeds were prepared according to the method reported by Yang et al.13 Then, the substrate was suspended upside down in the growth solution with a clip to grow ZnO nanowire arrays. The growth solution was composed of zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O) (0.025 ML-1), hexamethylenetetramine (HMT) (0.025 ML-1), and aluminum-iso-propylate (0.005 ML-1). The temperature of growth solution was maintained at 75 °C in a water tank. The pH value of growth solution was monitored at 7.2 during the growth process. After 20 h, the products on the substrate were purged with deionized water several times and annealed at 400 °C for 1 h. As for the cyclic growth, the composition of the growth solution in each cycle was the same as the above-mentioned growth solution, and the duration of the growth in each cycle lasted 4 h and then the growth solution was refreshed. After 12 cycles, the products were purged several times with deionized water. Phase identification of the assynthesized products on the substrate was done with a Philips X’Pert powder X-ray diffractometer (XRD) using Cu KR (0.15419 nm)

r 2009 American Chemical Society

Published on Web 09/30/2009

pubs.acs.org/crystal

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radiation. Field emission scanning electron microscopy (FESEM, Sirion 200) was used to observe the morphologies of the nanoarrays. Transmission electron microscopy (TEM JEM-2010) was used to examine microstructure of the ZnO nanowire. Energy dispersive X-ray (EDX, Inca Oxford) analysis was performed to determine the element composition. The photoluminescence (PL) spectra of the ZnO nanowire arrays were recorded on a LABRAM-HR spectrometer (Jobin-Yvon) excited with a 325 nm He-Cd laser. The average diameter and length of nanowires were acquired by the use of the ruler tool in Adobe Photoshop and a calibrated scale bar. The assembly of the dye-sensitized solar cells (DSC) based on the as-prepared ZnO nanowire arrays were performed according to ref 27. The characteristics of the DSC were derived with a Keithley 2400 source meter under a Xenon lamp (1000 W/m2).

Hua et al.

respectively. The aspect ratio reaches up to 26. It can be found that the growth solution with aluminum-iso-propylate could provide sustainable support for the preparation of long length and high aspect ratio nanowires by preferential growth. Figure 2 is the XRD spectrum of the arrays corresponding to the sample shown in Figure 1c,d. Five peaks exist in the spectrum and can be indexed to the

3. Results and Discussion The FESEM images of the nanowire arrays prepared by different conditions are presented. Figure 1a-d shows the nanowire arrays prepared after 20 h growth in the solution without and with aluminum-iso-propylate, respectively. It can be seen that nanowire are thick and short prepared in the solution without aluminum-iso-propylate, the length and the average diameter are 2 μm and 200 nm, respectively. The aspect ratio is about 10. While the nanowires are thin and long prepared in the solution with aluminum-iso-propylate, the length and the average diameter are 4.2 μm and 160 nm,

Figure 2. The XRD spectrum of nanowire arrays grown in the solution with aluminum-iso-propylate.

Figure 1. The lateral view and top view images of ZnO nanowire arrays of 20 h growing in different growth solutions, (a, b) in the solution without aluminum-iso-propylate; (c, d) in the solution with aluminum-iso-propylate.

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nanowires in the solution with aluminum-iso-propylate could be described as follows:24-26 AlðOC3 H7 Þ3 þ OH - þ 3H2 O f AlðOHÞ4- þ 3C3 H7 OH ð1Þ C6 H12 N4 þ 6H2 O f 4NH3 þ 6HCHO

ð2Þ

H2 O þ NH3 T OH - þ NHþ 4

ð3Þ

Zn2 þ þ 4NH3 T ZnðNH3 Þ2þ 4

ð4Þ

AlðOHÞ4- þ ZnðNH3 Þ2þ 4 þ ðNO3 Þ þ H2 O T ZnAl : LDH þ 4NH3

Figure 3. The TEM image, SAED pattern, and EDX spectrum of nanowire arrays grown in the solution with aluminum-iso-propylate. (a) The TEM image of ZnO nanowires; (b) the SAED pattern of ZnO nanowires; (c) the HRTEM image of ZnO nanowires; (d) the EDX spectrum of nanowires (C and Cu elements come from the TEM grid).

diffraction peak of (100), (002), (101), (102), and (110) planes of the wurtzite ZnO (JCPDS 80-0075), respectively. It confirms that the as-prepared high aspect ratio nanowire arrays are wurtzite ZnO with a vertical orientation along c-axis. The microstructures of ZnO nanowires grown in the solution with aluminum-iso-propylate are examined by the TEM image. As shown in Figure 3a, the diameter of the nanowire is 150 nm, which is consistent with the observation of FESEM images. The selected area electron diffraction (SAED) pattern of ZnO nanowire, as shown in Figure 3b, can be indexed as the zone axis of ZnO crystal with lattice constants a = 0.32 nm, c = 0.52 nm, which means the incident electron beam is vertical to the (1120) planes. From the HRTEM image shown in Figure 3c, it can be observed that the ZnO nanowires were anisotropic growth along the c-axis. Moreover, the composition of the as-prepared ZnO nanowire is examined by the EDX spectrum, as shown in Figure 3d. The EDX spectrum reveals the presence of Zn and O elements without Al element (C and Cu elements come from the TEM grid). These results indicate that the as-prepared ZnO nanowires are the good single-crystal and no Al3þ ions are doped in the ZnO lattice matrix. As it was shown, the preferential and persistent growth of ZnO nanowire arrays was achieved by the introduction of aluminum-iso-propylate in the growth solution. In the following, the influences of aluminum-iso-propylate on the growth of ZnO nanowires in aqueous solution were investigated. The reactions may be involved for the growth of ZnO

ð5Þ

Zn2þ þ 4ðOHÞ - T ZnðOHÞ24 -

ð6Þ

ZnðOHÞ24 - f ZnO þ 2OH - þ H2 O

ð7Þ

Here, the precipitates formed in the bulk solution were examined to check the influences of aluminum-iso-propylate on the growth of ZnO nanowire arrays. Figure 4a,b shows the morphologies of the precipitates obtained from the solution without and with aluminum-iso-propylate, respectively. The phases of the precipitates are shown in Figure 4c. It can be observed that the hexagonal ZnO nanoparticles were obtained in the solution without the aluminum-iso-propylate, while distorted sheets were obtained in the solution with the aluminum-iso-propylate. From the identification of phase and chemical composition, as shown in Figure 4c,d, the obtained distorted sheets could be termed as zinc aluminum layered double hydroxide (ZnAl:LDH) with the same structure of Zn3(OH)4(NO3)2 (JCPDS 70-1361). As described by steps 1, 2, 4 and 5, the introduction of the aluminum-iso-propylate could induce the formation of a mass of ZnAl:LDH instead of ZnO nanoparticles in the bulk solution. It was worth noting that the hexagonal ZnO nanoparticles precipitating in bulk solution without the aluminum-iso-propylate indicated the status of growth solution experienced an unstable supersaturated stage where the homogeneous nucleation occurred and followed by a metastable stage where the growth of ZnO nanoparticles with hexagonal shape occurred.18,28,29 While in the solution with aluminum-iso-propylate, the ZnO nanowires could grow successfully and no ZnO particle precipitates formed in the bulk solution as shown in Figures 1c,d and Figure 4b, which indicated the status of growth solution was kept at a metastable supersaturated level where the homogeneous nucleation of ZnO in bulk solution could not occur and ZnO precipitate in the bulk solution was suppressed. From these evidence, it can be suggested that the introduction of the aluminum-iso-propylate could maintain the status of growth solution at a metastable supersaturated level for the anisotropic growth of ZnO nanowire arrays on the substrates. Most recently, there are renewed interests in understanding the growth kinetics of materials in the nanometeric level. For the ZnO growth in the aqueous solution, two major growth mechanisms of diffusion-limited growth and reaction limited growth have been demonstrated and they were subjected to the relations of d3 µ t and d3 µ t1.5, respectively,30 where the d3 represents the volume of ZnO nanoparticles. In the following, the growth kinetics of ZnO nanowires in the metastable supersaturated condition was investigated. Here the growth time was limited within 5 h to make a convenient comparison with the ZnO nanowire growth without supersaturation

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Figure 4. (a) The morphology of the particle precipitates obtained in the growth solution without aluminum-iso-propylate, (b) the morphology of the precipitates obtained in the growth solution with aluminum-iso-propylate, (c) the XRD spectrum of the precipitates obtained in the growth solution, (d) the EDX spectrum of the precipitates obtained in the growth solution with aluminum-isopropylate.

control, which had been performed by Aydil et al.31 The lateral-view images of ZnO nanowires with different growth times in the metastable supersaturation are presented in Figure 5. It can be seen the length and the diameter increase with the increase of growth time and have monodisperse size distributions. The relation between the average size and growth time is plotted in Figure 6, and the averages are taken from 30 nanowires at random for each sample. It can be observed that the lengths of ZnO nanowires have a linear relation of L µ t with the growth time, while the diameters of ZnO nanowires have a relation of D µ t0.24 with the growth time. Considering the volume of ZnO nanowires can be described as V = (πD2/4)L, there is a relation of V µ t1.48 between the volume of ZnO nanowires and the growth time. This evidence suggests that the growth of ZnO nanowire arrays is subjected to the reaction-limited growth in the metastable supersaturated solution by the introduction of aluminum-iso-propylate. In addition, considering the cyclic growth could be adopted to acquire longer ZnO nanowire arrays, the cyclic growth of ZnO nanowire arrays in the metastable supersaturated solution was done. The ZnO nanowire arrays prepared by 12 growth cycles are shown in Figure 7. It can be observed that the ZnO nanowire arrays with a length of up to 10 μm and a diameter of 140 nm were prepared successfully. The

higher aspect ratio of ZnO nanowires was achieved, which is up to 70. From the HRTEM images of ZnO nanowires, as shown in Figure 8, the ZnO nanowires prepared by cyclic growth were good crystalline. Moreover, the photoluminescence (PL) of ZnO nanowires prepared by different growth cycles is presented in Figure 9. It can be seen that two emission bands appeared in the PL spectra. The emission band centered at 380 nm could be attributed to the transition between the band edges, and the band centered at 490 nm could be attributed to the oxygen vacancies related emission.32 The intensity of defect-related emission bands increased with the increase of growth cycles, and it indicated that the defect concentration increased with the increase of growth cycles. Considering the ZnO nanowire arrays hold promising potential for the application of the utilization of solar energy, the performance of high aspect ratio ZnO nanowire arrays prepared by cyclic growth was investigated on the application of dye-sensitized solar cells (DSC). The performance of the DSC is presented in Figure 10. The conversion efficiency (η) of the ZnO DSC is 0.4%. The short-circuit current density (Jsc), the open-circuit voltage (Voc), and the fill factor (FF) are 1 mA/cm2, 0.6 V, and 0.66, respectively. From the diagram of the dark I-V curve, it can be seen that an ideal rectifying junction was formed at the interface of the ZnO nanowire/dye.

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Figure 5. The lateral view of ZnO nanowire arrays with different growth times in the low supersaturated solution. (a) 30 min; (b) 1 h; (c) 2 h; (d) 3 h; (e) 4 h; (f) 5 h.

curve based on an equivalent circuit of solar cells.33 For high performance solar cells, higher shunt resistances and lower series resistances are required. The shunt resistance (Rsh) of ZnO nanowire arrays DSC under the irradiation is 500 KΩ cm2. The series resistance (Rs) under the irradiation is 80 Ω cm2. Here, the shunt resistance is an order of magnitude larger than that of the ZnO nanowire arrays prepared by the cyclic growth in the solution without the supersaturation control,31 which in turn caused a higher FF value. The higher shunt resistance may be originated from the suppression of the charge recombination at the interface of ZnO/dye.33 4. Conclusions

Figure 6. The diagram of the size of ZnO nanowire versus growth time.

As to the FF, one of important parameters for the output of the electric power, it is influenced by the shunt resistances and series resistances which can be derived from the I-V

Vertical aligned ZnO nanowire arrays with a long length and high aspect ratio were prepared successfully in metastable supersaturated solution, which was formed by the introduction of aluminum-iso-propylate into the growth solution. The kinetics analysis revealed the growth of ZnO nanowires was subjected to reaction-limited growth. By cyclic growth in the metastable solution, ZnO nanowire arrays with a length of

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Figure 7. FESEM images of ZnO nanowire arrays prepared by cyclic growth in the metastable supersaturated solution with duration of 4 h: (a) the lateral view; (b) the top view.

Figure 8. the TEM images of ZnO nanowires prepared by 12 growth cycles.

Figure 9. The photoluminescence of ZnO nanowire arrays prepared by different growth cycles in the metastable supersaturated solution: (a) 3 cycles; (b) 6 cycles; (c) 12 cycles.

Figure 10. The performance of the ZnO nanowire arrays on DSC, (a) the curve of the photocurrent density vs voltage, (b) the curve of dark current density vs voltage.

10 μm and aspect ratio of 70 were achieved. The ultralong and high aspect ratio ZnO nanowire arrays have potential applications in next generation nanodevices such as novel sensors, photocatalysts, dye-sensitized solar cells, etc.

Acknowledgment. This work is financially supported by the National Major Project of Fundamental Research: Nanomaterials and Nanostructures (Grant No. 2005CB623603).

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Reproducible Growth of Ultralong ZnO Nanowire Arrays in the

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