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: inshen1017@hotmail.com 1-4244-0637-4/07/$20.00 ©2007 IEEE 617 [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 X-ray diffraction patterns of ZnO nanorods with different growth temperature. SEM was used to investigate the nanostructure of ZnO nanorods Figures 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 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 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 SEM images of ZnO nanorods with different growth temperatures： （a）65 （d）80 ℃（e）85 ℃ 618 ℃（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 REFERENCES Fig A cross-section view of SEM image of ZnO nanorods with growth temperature equal to 80 ℃. Figure 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 619 [1] M.H Huang, S Mao, H Feick, H Yan, Y Wu, H.Kind,E.Weber, R Russo, P Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science vol.292,p.1897, 2001 [2] J Zhong, A.H Kitai, P Mascher, W Puff, “Effect of substrate temperature on the growth and luminescence properties of ZnO nanostructures,” J Electrochem Soc vol.140,p.3644, 1993 [3] N Beermann, L Vayssieres, S.-E Lindquist, A.Hagfeldt, “Photoelectrochemical studies of oriented nanorod thin films of Hematite,”J Electrochem Soc vol.147,p.2456, 2000. [4] N Yamazoe, “Photoelectrochemical studies of oriented nanorod thin films of Hematite,”Sensors Actuators B vol.5,p.7, 1991 [5] G.S Trivikrama Rao, D Tarakarama Rao, “Study on Sensitivity of Nano-Grain ZnO Gas Sensors,”Sensors Actuators B vol.55, p.166, 1999 [6] M Law, L.E Greene, J.C Johnson, R Saykally, P Yang, “Nanowire dye-sensitized solar cells,” Nat.Mater vol.4 ,p.455, 2005 [7] T Yoshida, K Terada, D Schlettwein, T Oekermann, T Sugiura,H Minoura, “Electrochemical and Photoelectrochemical Properties of Organic Semiconductors - Dye-Sensitization in Nanostructured Hybrid Materials,” Adv Mater vol.12,p.1214, 2000 [8] J.B Baxter, E.S Aydil, “Nanowire-based dye-sensitized solar cells,”Appl Phys Lett vol.86,p.53114, 2005 [9] E Hosono, S Fujihara, I Honma, H Zhou, “The Fabrication of an Upright-Standing Zinc Oxide Nanosheet for Use in Dye-Sensitized Solar Cells,” Adv Mater vol.17,p.2091, 2005 [10] L.E Greene, M Law, D.H.Tan, MMontano,J.Goldberger ,G Somorjai, P Yang, “ZnO Nanowire/p-GaN Heterojunction LEDs,”Nano Lett vol.5, p.1231, 2005 [11] W.U Huynh, J.J Dittmer, A.P Alivisatos, “Investigation of Properties of ZnO Nanorad Structures by Chemical Vapor Deposition,” Science vol.295,p.2425, 2002 [12] T Stu binger, W Bru tting, “Exciton diffusion and optical interference in organic donor-acceptor photovotaic cells,” J Appl Phys vol.90,p.3632, 2001 [13] C.J Brabec, N.S Sariciftci, J.C Hummelen, “Origin of the Open Circuit Voltage of Plastic Solar Cells,” Adv Funct Mater vol.11,p.15, 2001 [14] B Pradhan, A Bandyopadhyay, A J Pal, “Tuning performance of donor-acceptor based self-assembled photovoltaicdevices,”Appl Phys.Lett.vol.85,p.633, 2004 [15] M.H Huang, Y.Wu, H Feick, N Tran, E.Weber, P Yang, “Catalytic growth of zinc oxide nanowires by vapor transport,”Adv.Mater vol.13,p.113, 2001 [16] Y.C Kong, D.P Yu, B Zhang,W Fang, S.Q Feng, “Ultraviolet-emitting ZnO nanowires synthesized by a [17] [18] [19] [20] [21] physical vapor deposition approach,”Appl Phys.Lett vol.78,p.407, 2001 J.-J Wu, S.-C Liu, “Low-Temperature and Catalyst-Free Synthesis of Well-Aligned ZnO Nanorods on Si (100),” Adv Mater vol.14,p.215, 2002 J.-J Wu, S.-C Liu, “Catalyst-Free Growth and Characterization of ZnO Nanorods,”J Phys Chem B vol.106,p.9546, 2002 J Zhang, L Sun, H Pan, C Liao, C Yan, “ZnO nanowires fabricated by a convenient route,” New J Chem vol.26,p.33, 2002 Y Li, G.W Meng, L.D Zhang, F Phillipp, “Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties,” Appl Phys Lett vol.76,p.2011, 2000 L Vayssieres, “Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions,” Adv Mater 620 vol.15,p.464, 2003 [22] L Vayssieres, K Keis, A Hagfeldt, S Lindquist, “Three-dimensional array of highly oriented crystalline ZnO microtubes,”Chem Mater vol.13,p.4395, 2001 [23] L.E Greene,M Law, J Goldberger, F Kim, J.C Johnson, Y Zhang,R.J Saykally, P Yang, Angew “Low-temperature wafer-scale production of ZnO nanowire arrays,” Chem Int Ed vol.42 ,p.3031, 2003 [24] J Choy, E Jang, J Won, J Chung, D Jang, Y Kim, “Soft Solution Route to Directionally Grown ZnO Nanorod Arrays on Si Wafer,”Adv Mater vol.15,p.1911, 2003 [25] K Govender, D Boyle, P Kenway, P O’Brien, “Understanding the factors that govern the deposition and morphology of thin films of ZnO from aqueous solution,”J Mater Chem vol.14,p.2527, 2004 ... nanostructure of ZnO nanorods Figures 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... and the length of nanorods is about 1.3um TABLE I THE AVERAGE DIAMETER OF ZNO NANORODS WITH DIFFERENT GROWTH TEMPERATURES Fig SEM images of ZnO nanorods with different growth temperatures ... 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