Influences of growth temperature and solution concentration on hydrothermal preparation of ZnO nanorod

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0042 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 138-143 This paper is available online at INFLUENCES OF GROWTH TEMPERATURE AND SOLUTION CONCENTRATION ON HYDROTHERMAL PREPARATION OF ZnO NANOROD Nguyen Dinh Lam, Nguyen Thanh Lap, Pham Van Vinh, Vuong Van Cuong, Phuong Thi Thuy Hang, and Nguyen Van Hung Faculty of Physics, Hanoi National University of Education Abstract. ZnO nanorod structures were grown on glass substrates using a hydrothermal method. Influences of growth temperature and hydrothermal solution concentration on ZnO nanorod structures were investigated. The results indicate that diameter and length of the ZnO nanorod increase with an increasing of growth temperature and hydrothermal solution concentration. However, density of the ZnO nanorod can reach a maximum value when growth temperature and hydrothermal solution concentration is 80 ◦C and 20 mM, respectively. The optical transmittance of the ZnO nanorod structure is strongly reduced if growth temperature and hydrothermal solution concentration is increased. This reduction can be explained based on the length of the ZnO nanorod. Keywords: ZnO nanorod, hydrothermal method, photovoltaic. 1. Introduction Zinc oxide (ZnO) is a semiconductor with a wide bandgap of 3.37 eV and large exciton binding energy of 60 meV [1-3]. For optoelectronic device applications such as thin film solar cells, transistors and sensors, ZnO is usually fabricated under film or one dimension (1D) nanostructures [4-6]. Recent reports indicate that the performance of photovoltaic and light emitting diode devices using ZnO nanostructures is much higher than that of those using ZnO film [7, 8]. The improvement in the performances of these devices was explained as being due to a larger effective surface area and higher electrical conductibility. ZnO nanostructures can be grown on substrates via methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), gold-catalyzed vapor transport, and hydrothermal [9-11]. Among them, the hydrothermal method has the advantages of ease of control of chemical components, good uniformity of nanostructure, low temperature synthesis, and economical production. In this work, ZnO nanorod structures on glass substrates are fabricated using the hydrothermal method. Influences of growth temperature and hydrothermal solution concentration on diameter, length and density of ZnO nanorod were investigated in detail. Furthermore, the optical transmittance property of fabricated ZnO nanorod structures was evaluated to determine optimal conditions for photovoltaic fabrication. Received July 22, 2016. Accepted September 14, 2016. Contact Nguyen Dinh Lam, e-mail address: lam.nd@hnue.edu.vn 138 Influences of growth temperature and solution concentration on hydrothermal preparation of ZnO nanorod 2. Content 2.1. Experimental details Glass substrate was cleaned using a NaOH solution, methanol and deionized water in sequence. The ZnO seed layers were coated on the glass substrates via sol-gel method using a 0.3 M solution of zinc acetate dehydrate (Zn(CH3COO)2.2H2O). After the coating process, the ZnO seed layers were dried at 150 ◦C for 20 minutes in an oven to evaporate the solvent and remove organic residuals and then annealed at 500 ◦C for 1 h in air. In the process of hydrothermal growth of ZnO NRs, a glass substrate with a ZnO seed layer and 100 mL solution was transferred together into a Teflon-lined stainless steel autoclave and then annealed for 120 min. The solution concentration varied from 10 mM to 30 mM. The growth temperature was changed from 70 to 90 ◦C. The growth time and volume of solution were kept constant. After the growth process, the autoclaves were allowed to cool naturally. The obtained samples were cleaned ultrasonically in ethanol and distilled water for 30 min, followed by a drying treatment at 100 ◦C. The X-ray diffraction pattern of the ZnO nanorod was measured using a X-Ray Diffractometer (XRD) D5000 with CuKα radiation (λ = 1.5406 A˚) at room temperature. The diameter, length and density of the rod were investigated using a Scanning Electron Microscope (SEM). The optical spectra of the ZnO nanorod structures were studied using an UV-VIS-NIR spectrophotometer in the wavelength range of 300 - 800 nm at room temperature. 2.2. Results and Discussion 2.2.1. ZnO seed layer The SEM image of the ZnO seed layer was shown in Fig.1(a). The image indicates that ZnO seed layer is made up of uniform ten nanometer thick particles. Figure 1. (a) Top-view SEM image and (b) XRD pattern of ZnO seed layer Figure 1(b) shows the X-ray diffraction pattern of the ZnO seed layer. The XRD patterns indicate that the structure of the films is polycrystalline. The presence of the (100), (002), (101), (102), (110), 103 and (112) peaks in the XRD patterns indicate a hexagonal wurtzite structure of the ZnO. Diffraction peaks related to other impurity phases cannot be seen in the XRD patterns. Using the Debye Scherer formula, a crystalline particle size of≈18 nm can be calculated. Furthermore, based on Bragg’s equation, the crystal lattice constants of ZnO were also determined 139 N. D. Lam, N. T. Lap, P. V. Vinh, V. V. Cuong, P. T. T. Hang and N. V. Hung (a = b ≈ 3.2 A˚ and c ≈ 5.2 A˚). The optical transmittance spectrum of the ZnO seed layer is presented in Figure 2. The spectrum shows that the average transmittance of this seed layer is over 95% in the visible range. Based on the transmittance spectrum and J.Tauc equation [12], the bandgap of the ZnO seed layer is determined to be about 3.23 eV (inset Figure 2). Figure 2. Optical transmittance spectrum of ZnO seed layer. (Inset) Plots of (αhν)2 vs. photon energy of the ZnO seed layer. 2.2.2. ZnO nanorod structures * Growth temperature dependence A ZnO nanorod structure was grown on glass substrate coated with a ZnO seed layer. In this work, the hydrothermal solution concentration and growth time were kept constants at 20 mM and 120 min, respectively. Growth temperature was varied from 70 to 90 ◦C. Figure 3. SEM images of ZnO nanorod structures at different growth temperature 140 Influences of growth temperature and solution concentration on hydrothermal preparation of ZnO nanorod Figure 4. Density and length of ZnO nanorod at different growth temperatures Top- and side-view SEM images of ZnO nanorod structures at different growth temperatures are shown in Figure 3. The images show that ZnO nanorods are of uniform size and have a tendency to orient perpendicular to the surface of the glass substrate. Moreover, the diameter, length and density of ZnO nanorod are dependent on growth temperature. These dependent ZnO nanorod were extracted and are depicted in Figure 4. This result indicates that the diameter and lenght of ZnO nanorod increase with an increase in growth temperature. The longest rod length of 275 nm was obtained when the growth temperature was 90 ◦C. This can be attributed to high reaction and growth rates at high temperature. However, the density of the ZnO nanorod reaches a maximum value at the growth temperatue of 80 ◦C. Figure 5. Optical transmittance spectra of ZnO nanorod structures at different growth temperatures The influence of the optical transmittance of the ZnO nanorod structures on growth temperature is shown in Figure 5. The optical transmittance of the ZnO nanorod structue is strongly reduced when the growth temperature increases. This coulkd be due to the increased length of the ZnO nanorod. However, the optical transmittance of the ZnO nanorod structure grown at 80 ◦C is still higher 80% [not understandable] in the visible region. The study results indicate that the density of the ZnO nanorod is greatest when it is grown at 80 ◦C. This means that the surface area of this sample is the largest. In addition, the optical transmittance of this sample is higher than 80% in the 400 - 800 nm range. Therefore, 80 ◦C is a ZnO nanorod growth temperature that can create good structures for photovoltaic application. * Hydrothermal solution concentration dependence In this part of the experiment, growth temperature and growth time were kept constant at 80 ◦C and 120 min, respectively. The hydrothermal solution concentration was varied from 10 to 30 mM. Top- and side-view SEM images of the ZnO nanorod structures grown at different solution concentrations are shown in Figure 6. The ZnO nanorods are of uniform size and are oriented perpendicular to the surface of the glass substrates. The diameter, length and density of the ZnO 141 N. D. Lam, N. T. Lap, P. V. Vinh, V. V. Cuong, P. T. T. Hang and N. V. Hung nanorod are strongly dependent on solution concentration. The diameter and length of the ZnO nanorod increase as the solution concentration increases and reach the highest value when the solution concentration is 30 mM in this investigation. However, the density of the ZnO nanorod is highest when the solution concentration is 20 mM. These results were extracted from the SEM images shown in Figure 6 and depicted in Figure 7. Figure 6. SEM images of ZnO nanorod structures grown at different solution concentrations Figure 7. Density and length of ZnO nanorod grown at different solution concentrations Figure 8. Optical transmittance spectra of ZnO nanorod structures grown at different solution concentrations Figure 8 shows the optical transmittance spectra of the ZnO nanorod structures grown at different solution concentrations. When the solution concentration increases from 10 mM to 30 mM, the optical transmittance in visible region of ZnO nanorod structue is strongly reduced from 142 Influences of growth temperature and solution concentration on hydrothermal preparation of ZnO nanorod 95% to 65%. This reduction in optical transmittance is attributed to the thickness of the ZnO nanorod structure. However, the ZnO nanorod structure grown at a solution concentration of 20 mM shows potential for application in optoelectronic devices fabrication. 3. Conclusion ZnO nanorod structures were successfully grown on glass substrates using the hydrothermal method. The ZnO nanorod are of uniform size and have a tendency to orient perpendicular to the surface of the glass substrate. 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