Advanced Materials Research Vols. 403-408 (2012) pp 1205-1210 Online available since 2011/Nov/29 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.403-408.1205 ! " " # $ %& ! " " # $ , $ ' ( ! " " # $ " # / $% "# )* + "# ! " " # ' 2 3 $# " 45 6 7 8 '9&( ' ( "# )* + , "# - " . $ . " # %& ' ( "# , ( "# 0 0*& )0+ . ,1 1 , , %1 $1 & " 1 In this work ZnO nanocrystal powders have been synthesized by using Zinc acetate dehydrate as a precursor and sol-gel method. Then the products have been annealed at temperature of 200-1050°C, for 2 hours. The powders were characterized using X-ray diffraction (XRD), UVvis absorption and photoluminescence (PL) spectroscopy. The morphology of refrence ZnO nanoparticles have been studied using Transmission Electron Microscope (TEM). During the annealing process, increase in nanocrystal size, defects and energy gap quantitative, and decrease in specific surface area have been observed. Zinc oxide (ZnO), a versatile white inorganic compound has nearly insoluble in water has attracted intensive interests due to the potential applications optoelectronic devices, including blue-ray light-emitting and room temperature UV lasing diodes, UV emitters and detectors, light emitting diodes, detectors, white light sources, biological markers and UV blocker for skin protection gels and creams[1-4]. This transparent semiconductor with a large binding energy (60 meV) [5] and a wide band gap of about 3.37 eV at room temperature [6,7], good transparency, and high electron mobility [8], crystallizes in three forms: 1) Hexagonal Wurtzite , 2) Cubic Zincblende [9], and 3) Cubic Rocksalt form that rarely observed at high pressure about 10 GPa [10]. ZnO nanocrystals possess noncentrosymmetric structure that used as non-resonant nonlinear investigations for both in-vitro and in-vivo biomedical applications. By possessing large second order and third order nonlinearities, ZnO nanocrystals used in process of second harmonic and sum frequency generation (SHG and SFG) and electric Four-Wave Mixing (FWM) that applied for biological microscopy[3,4]. Annealing process can execute in two ways, in complete annealing both the annealing and cooling procedures perform in the furnace. But in normalizing way ZnO nanocrystals are cooled in air after heating process. ZnO nanocrystals have been prepared by different methods including thermal decomposition, vapor chemical deposition, sol–gel method, wet chemical synthesis, mechanochemical, electrodeposition, gas-phase reaction, hydrothermal synthesis sputtering, spray-pyrolysis and electrodeposition and so on [11,12]. Our route is sol-gel method because in comparison with other methods, the sol-gel method has notable advantages like high purity, low-temperature synthesis, simplicity, and easily controlled reaction condition [12,13]. In this paper we have studied the photoluminescent properties of the ZnO nanocrystals at various annealing temperature. A strong relationship has been observed among the annealing temperature and the green emission band of the ZnO nanocrystals. These results shown that the ZnO nanocrystals are intensively sensitive to temperature changing. By X-ray diffraction (XRD), the effect of temperature on the size and the specific surface area (SSA) of the ZnO particles has been indicated. The diameter of the ZnO nanocrystals has been estimated. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 78.39.205.75-08/01/12,11:05:26) 1206 MEMS, NANO and Smart Systems The ZnO nanocrystals with hexagonal wurtzite form and average particle size of 25 nm have been prepared by sol-gel method. At first zinc acetate dihydrate (99.5 % (CH3COO)2Zn.2H2O, Merck ) was dissolved in methanol at room temperature. The resultant solution of 0.2M was obtained by ultrasonic magnetic stirrer at 25°C for 120 min. The prepared sol was stable and transparent. Then sodium hydroxide (0.1N NaOH, Merck) was added to solution and stirred ultrasonically for 60 min. The final sol was kept for 48h for completing the gelation process. The ZnO nanocrystals were settle down in the bottom of the flask. After washing the products with methanol to removing the starting materials and drying at 120°C for 2 hours, ZnO powders obtained [14].Morphology of ZnO nano particles have been studied using Transmission Electron Microscope (TEM). Then the powders have been annealed at various temperatures from 200°C to 1050°C (200-400-650-850-1050°C) for 2 hours. After annealing, the ZnO nanocrystals have been cooled slowly in the furnace. The structure of the ZnO nanocrystals were scanned using ITALSTRUCTURE ADP200 X-ray diffractometer, equipped with graphite monochromatized CuKα radiation (λ=1.5405Å). The diffraction pattern was recorded at scanning rate of 2° min in the range of 10° ≤ 2θ ≤ 60° .UV-Vis absorption spectra were recorded on a PerkinElmer 550ES spectrophotometer. The wavelength of incident ray was selected in the range of 190-800 nm with the accuracy of 1 nm. The photoluminescence spectra were measured on a Cary Eclipse with an excitation wavelength of 275 nm. The emission spectrum of ZnO nanocrystals was observed in the wavelength range of 350-650 nm. ! " In our study we used zinc acetate dehydrate as precursor of sol-gel process to investigatethe growth of ZnO nanocrystals. This procedure contains four levels: 1- Solvation, 2- Hydrolysis, 3-Polymerization and 4-Transformation into ZnO. TEM micrograph of reference ZnO nanoparticles is illustrated in Figure 1. Fig. 1. TEM micrograph of reference Zno nanoparticles. # Figure 2 shows the X-ray diffraction (XRD) patterns of ZnO nanocrystals that annealed at various temperatures. In this figure, the diffraction peaks were observed in 100, 002, and 101. The intensity of 002 peaks is obviously lower than 100 and 101 peaks, which protects this idea that the ZnO nanocrystals have a hexagonal wurtzite structure [15]. The broadening of the peaks in the XRD pattern indicates that the size of ZnO nanocrystals is in the nanorange. The diameter of particles estimated from the full width at half maximum (FWHM) of the XRD peaks. By using the Scherrer formula [15], we can estimate the particle size. = λ β θ . (1) In this formula D is the particle size, λ the wavelength of X-ray radiation, β the FWHM of the peak, θ the angle of diffraction and k the correction factor. For precise calculations = 0.9 , λ=1.5405Å. The size of nanocrystals obtained from 24 to 70 nm. Advanced Materials Research Vols. 403-408 1207 The specific surface area estimated according to equation.(2) [16]: = 6 ρ . (2) In this equation ρ is the density ( 3 ), D the size of the ZnO nanoparticles (µm), and S the specific 2 3 surface area (SSA) ( ). For ZnO nanocrystals ρ = 5.6 . The size of particles estimated from Scherrer formula. Figure 3 shows the relation between changing of the particle size and the SSA. It’s clear that with increasing of temperature, particle size was increased and the SSA was decreased. $% & In a typical procedure, 0.005 g zinc oxide nanocrystals was dispersed in 10 ml ethanol, the resultant solution was put in ultrasonic for 10 min, and then the absorption spectra of ZnO nanocrystals have been recorded. These processes have been repeated for every sample. Figure 4 shows the UV-vis spectra of ZnO nanocrystals in various temperatures. All the products exhibit an absorption band at 370 nm, which correspond to the band gap width of ZnO. Comparing the results shows that the absorption intensity decreased when the temperature increased. Fig. 2. XRD Pattern of ZnO Nanocrystals synthesized at various tempreature. 1208 MEMS, NANO and Smart Systems /' /' 5 .' Particle size SSA -' ,' +' +' *' *' )' )' (' (' ) ,' 74 -' ' 1 61 4 .' ' 0 200 400 0 650 850 1050 1234 Fig.3. Changing of SSA and particle size with temperature. 1 0.9 ZnO ZnO ZnO ZnO ZnO ZnO Absorbance% 0.8 0.7 0.6 REF 200 400 650 850 1050 0.5 0.4 0.3 0.2 0.1 0 250 300 350 400 450 500 Wavelength ( nm ) Fig. 4. The absorption spectra of ZnO nanocrystals in various temperatures The energy band gap of ZnO nanocrystals can estimate by use of UV-vis absorbance spectra. The absorption coefficient expressed as [17]: α= ( 0) . (3) In this equation 0 and are the light intensities respectively before and after transmission, and d the light pass length. In our case d=1 cm. In high absorption regions that α (ω ) ≥ 10−4 Davis and Mott equation can be useful [18,19]. α( ν ) = ( ν − ) ν. (4) where B is a constant, r the index determined by the nature of the transitions during the absorption process the energy of optical band, and hν the energy of incident photon. ( = 2,3,1 2,1 3 ), and Figure 5 shows the energy band gap for direct allowed transitions ( = 1 2 ) of ZnO nanocrystals that annealed in 200°C and 400°C. The value can be measured along the tangent to the curve with the horizontal axis. For ZnO nanocrystals energy band gap will be 3.2 eV. It’s obvious that the energy band gap will increase by increasing the temperature. 5" Photoluminescence (PL) is an important property that gives information about the optically active defects and relaxation pathways of excited states [20]. PL measurements are investigated at room temperature with an excitation wavelength of 275 nm for various samples. Figure 6 shows the PL of ZnO nanocrystals. PL of ZnO nanocrystals shows three emission bands: a band at approximately 433 nm, a weak blue band at approximately 483 nm, and a green band a approximately 529 nm. The visible emissions are usually related to the defects of the ZnO nanocrystals. The first emission indicates the exciton recombination-related near-band edge emission of ZnO. The intensity of this emission Advanced Materials Research Vols. 403-408 1209 9 8 ZnO REF ZnO 200 2 (αһν) (eV/cm) 2 7 ZnO 400 6 5 4 3 2 1 0 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4 һν (eV) Fig.5. The energy band gap for ZnO nanocrystals in 200°C and 400°C is decreased when the annealing temperature is increased. The blue and green emissions are the results of the surface defect in the ZnO nanocrystals. The green band is attributed to the singly ionized oxygen vacancy in ZnO and this emission is from the recombination of photo-generated hole with an electron occupying the oxygen vacancy [16, 21]. Because the insterstitial oxygen indicates the yellow emission, this green emission peak is the result of oxygen vacancies. By increasing the annealing temperature the intensity of green emission increased. From these results it is obvious that the annealing temperature has a great influence on the type of ZnO nanocrystal defects. Fig. 6. Room Temperature PL of ZnO nanocrystals of reference and annealed at 200,400,650,850 and 1050°C 3 In summary, the ZnO nanocrystals have been prepared by sol-gel method. The XRD spectroscopy of ZnO nanocrystals shows that the ZnO nanocrystal has a hexagonal wurtzite structure. The size and SSA of the ZnO nanocrystals are determined using XRD. By increasing the annealing temperature the size of the nanoparticles are increased and the SSA was decreased. The UV-vis spectroscopy of ZnO nanocrystals shows that the absorbance of the ZnO nanocrystals decreased and the energy bang gap increased, by increasing the annealing temperature. The PL spectra have demonstrated the influence of annealing temperature on the ZnO defects for three region spectra, and show that by increasing the annealing temperature the defects in ZnO increased. 1210 MEMS, NANO and Smart Systems [1] Khalifa Al-Azri, Roslan Md Nor, Y.m. Amin, Majid. S. Al-Ruqeishi. Applied Surface Science, 2010 [2] J. V. Foreman, H. O. Everitt. Physical Review B, 81 (2010) 115318. [3] Ben E. Urban, Jie Lin, Os Kumar, Kasilingam Senthilkumar, Yasuhisa Fujita, Arup Neogi. Optical Materials Express, 1,4 (2011) 658-669. [4] Aliaksandr V. Kachynski, Andrey N. Kuzmin, Marcin Nyk, Indrajit Roy, Paras N. Prasad . J Phys Chem C Nanomater Interfaces. 112(29) (2008) 10721-10724. [5] WANG Li, ZHAO Bin, CHANG LiXian & ZHENG WenJun. Science in China Series B: Chemistry, 50 (2007) 224-229 [6] N.Faal Hamedani and F. Farzaneh. Journal of Science, I. R. Iran, 17 (2006) 231-234 [7] Ming Wang, Lide Zhang. Materials Letters, 63 (2009) 301-303 [8] George Cramer, NNIN REU, Research Accomplishments, (2006), 34-35 [9] Dojalisa Sahu, B. S. Acharya, B. P. Bag, Th. Basanta Singh and R. K. Gartia, ,Journal of Luminescence 130 (2010) 1371-1378 [10] Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.J.Cho, and H. Morko. Journal of Applied Physics 9 (2005) 041301 [11] Santi Maensiri, Paveena Laokul, Vinich Promarak, Journal of Crystal Growth 289 (2006) 102106 [12] T.Ivanova, A.Harizanova, T. Koutzarova, B. Vertruyen, Materials Letters 64 (2010) 1147-1149 [13] Jinato Tian, LIjuan Chen, Jinhui Dai, Xin Wang, Yansheng Yin, Pingwei Wu, Ceramics International 35 (2009) 2261-2270 [14] Seema Rani, Poonam Suri, P.K. Shishodia, R.M. Mehra, Solar Energy Materials & Solar Cells 92 (2008) 1639-1645 [15] Yongfa Zhu, Li Zhang, Chong Gao, Lili Cao, The synthesis of nanosized TiO2 powder using a sol– gel method with TiCl4 as a precursor, J.Mater.Sci.35 (2000)4049–4054. [16] Rizwan Wahab, S.G. Ansari, Young-Soon Kim, Hyung-Kee Seo, Hyung-Shik Shin, Room temperature synthesis of needle-shaped ZnO nanorods via sonochemical Method, Appl. Surf. Sci. 253 (2007) 7622–7626. [17] http://en.wikipedia.org/wiki/Sphere [18] Junyong kang, Shin Tsunekawa, Atsuo Kasuya, Applied Surface Science, 174 (2001) 306-309 [19] A. A. Higazy, A. Hussein, M. A. Ewaida and M. El-Hofy, Journal of material science Letters, (1988) 453-456 [20] S. Mandal, M.L.N. Goswami, K. Das, A. Dhar, S.K. Ray, Thin Solid Films 516 (2008) 8702– 8706 [21] Aurangzeb Khan, Martin E. Kordesch. Physica E, 30 (2005) 51–54 MEMS, NANO and Smart Systems 10.4028/www.scientific.net/AMR.403-408 Study of Heating Effect on Specific Surface Area, and Changing Optical Properties of ZnO Nanocrystals 10.4028/www.scientific.net/AMR.403-408.1205