Growth Of ZnO Nanorods by Hydrothermal Method Under Different ...

Growth Of ZnO Nanorods by Hydrothermal Method Under Different Temperatures     T. H. Meen, W. Water, Y. S. Chen*, W. R. Chen, L. W. Ji and C. J. Huang Abstract –In this study, the aqueous solution method was employed to synthesize one-dimensional well-aligned ZnO nano-array on ITO glass substrate. We can find that the dimension of ZnO nanorod will changes with different growth temperature. X-ray diffraction patterns show that the nanorods are high-quality crystals growing along [001] direction with a high consistent orientation perpendicular to the substrate while the growth temperature is equal to 80. SEM images show that the average diameters of ZnO nanorods are about 60-90 nm by changing growth temperature. The smallest diameter of ZnO nanorods is observed while the growth temperature is equal to 75 ℃. The UV/Vis spectra analyses show the absorption peaks appear at 330nm, 370nm and 390nm while growth temperature increases from 65 ℃ to 85 ℃.

I.

INTRODUCTION

ZnO nanostructure has been envisioned to enhance performance of various technologically important devices such as short-wavelength lasers[1], Gratzel-type solar cell [2],[3], and chemical sensors [4],[5]. The interest in synthesis of well-aligned ZnO nanowires or nanorods on substrates keeps growing. ZnO has shown a great deal of research in DSSCs [6]–[9] due to some of its fascinating properties. Comparing with other semiconductors, ZnO has unique excellent properties, such as higher binding energy (60meV), wide band gap (3.37 eV), high breakdown strength, cohesion, and exciton stability. Moreover, ZnO is one of the hardest materials in the family of II–VI semiconductors. Electron mobility in ZnO is more than that in TiO2 making the former suitable for DSSCs. Recently, it has become possible to form vertical nanowires of ZnO [10]. Such nanowires expectedly provide morphology for better electron transport. The vertical geometry also provides a more open structure for filling with hole-transporting materials[11]-[14]. The preparation of 1D ZnO nanostructures has been demonstrated by various methods, including vapor–liquid–solid (VLS) growth T. H. Meen, W. Water, Y. S. Chen W. R. Chen and L. W. Ji are with the Institute of Electro-Optical and Materials Science, and Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan, R.O.C. C. J. Huang is with

the Department of Applied Physics, National University of Kaohsiung, Nan-Tzu 811, Kaohsiung, R. O. C. E-mail: [email protected]

1-4244-0637-4/07/$20.00 ©2007 IEEE

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[15],[16], chemical vapor deposition (CVD) [17],[18], hydrothermal process [19], and template-based methods [20]. However, these growth techniques usually expensive, and the choice of substrate restricted, complex process controlling and high temperature are unfavorable for an industrialized process. Recently, a solution-based approach was developed to achieve highly oriented nanorods film with high surface area on substrate, which has the advantages of mild synthetic conditions, simple manipulation and large scale-up production. It opens a door for future optoelectronic devices based on ZnO nanostructure arrays [21]–[25]. In this work, we report the hydrothermal growth of high quality ZnO nanorods perpendicularly oriented on ITO substrates, and investigate them by X-ray diffraction, scanning electronmicroscopy (SEM) and ultraviolet-visible absorption spectra analyses. These high quality ZnO nanorods can be applied on the electrode of dye-sensitized solar cell to increase the contact area between ZnO and dye, resulting in the enhancement of efficiency for dye-sensitized solar cell. 

II.

EXPERIMENTAL

The ZnO nanorods were prepared from zinc nitrate in a neutral aqueous solution under hydrothermal conditions. The procedure consists of two steps: (1) deposition of ITO substrates with densely and uniformly ZnO films by RF sputter as the buffer layer, and (2) hydrothermal growth of ZnO nanorods in aqueous solution. In detail, the aqueous solutions of zinc nitrate (0.5g) and methenamine (0.5g) were stirred uniformly. An 80 nm thick ZnO layer was first deposited on ITO glass using a RF sputter deposition system under an Ar and O2 pressure of 5x10-2torr. The hydrothermal growth was carried out at 65 ℃ ~ 85 ℃ in a sealed beaker by immersing the modified substrates in the aqueous solution (100ml) containing Zn(NO3)2 (0.5 M) and methenamine (0.35 M) for 10 hours. The morphology, structure, and optical properties of ZnO nanorods were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and Ultraviolet-Visible spectrophotometer (UV/Vis spectrophotometer).  

III. RESUATS AND DISCUSSION The crystal structure of as-prepared ZnO nanorods was analyzed by XRD. X-ray diffraction patterns of ZnO nanorods with different growth temperature are shown in Fig.1. All diffraction peaks well indexed to the standard diffraction pattern of hexagonal ZnO phase except for 2θ=36o and 37o. In comparison with the standard XRD

pattern of ZnO, the much higher relative intensity of the (002) diffraction peak provides further evidence that the nanorods are preferentially oriented in the c-axis direction. The strongest (002) peak of diffraction pattern appears while the growth temperature is equal to 80 ℃.

 

 

 

Fig. 1. X-ray diffraction patterns of ZnO nanorods with different growth temperature. 

  SEM was used to investigate the nanostructure of ZnO nanorods. Figures 2 show the SEM images of ZnO nanorods obtained under different growth temperatures. They show that a dense array of hexagonal ZnO nanorods having a diameter of from 30nm to 150nm are formed under different growth temperatures, and the average diameters of ZnO nanorods are listed in Table I. It is noted that Fig. 2(d) shows the best nanostructure of ZnO nanorods. From the results of Fig. 1 and Figs. 2, the best growth temperature of ZnO nanorods is 80 ℃.  The cross-section image of ZnO nanorods arrays grown at 80 ℃ is shown in Fig. 3. It is found that all ZnO nanorods grow almost vertically from the substrate, and the length of nanorods is about 1.3um.

 

TABLE I THE AVERAGE DIAMETER OF ZNO NANORODS WITH DIFFERENT GROWTH TEMPERATURES

  Fig. 2. SEM images of ZnO nanorods with different growth temperatures： （a）65 （d）80 ℃（e）85 ℃

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℃（b）70 ℃（c）75 ℃

electrode of dye-sensitized solar cell to increase the contact area between ZnO and dye, resulting in the enhancement of efficiency for dye-sensitized solar cell. ACKNOWLEDGEMENT The research is supported by National Science Council, R.O.C. under contract Nos. NSC 96-2622E-150-027-CC3 and NSC 96-2221-E-150-028.

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Fig. 3. A cross-section view of SEM image of ZnO nanorods with growth temperature equal to 80 ℃.    Figure 4 shows the UV-Vis absorption spectra of ZnO nanorods under different growth temperatures. The absorption peaks appear at 330nm, 370nm and 390nm while the growth temperature increases from 65 ℃ to 85 ℃, and the strongest absorption peak at 390nm is observed while the growth temperature is equal to 75 ℃. It is indicated that the smallest average diameter of ZnO nanorods has the best absorption for UV light. From the results of XRD, SEM and UV-Vis analyses for ZnO nanorods, we can apply these high quality ZnO nanorods on the electrode of dye-sensitized solar cell to increase the contact area between ZnO and dye, resulting in the enhancement of efficiency for dye-sensitized solar cell. 

Fig.4 The UV-Vis absorption spectra of ZnO under different growth temperature from 65 ℃ to 85 ℃.  

 

IV. CONCLUSION In this study, we have successfully synthesized ZnO nanorods on ITO glass substrate. From the results of XRD and SEM, the best growth temperature of ZnO nanorods is 80 ℃, at which the average diameter and length of ZnO nanorods are about 70.4 nm and 1.3um. The absorption peaks appear at 330nm, 370nm and 390nm while the growth temperature increases from 65 ℃ to 85 ℃, and the strongest absorption peak at 390nm is observed while the growth temperature is equal to 75 ℃. These high quality ZnO nanorods can be applied on the

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