characterization and optical property of zno nano-,

Characterization and optical property of ZnO nano-, submicro- and microrods synthesized by hydrothermal method on a large-scale Narges Kiomarsipour ⇑ , Reza Shoja Razavi Department of Materials Engineering, Malek Ashtar University of Technology, P.O. Box 83145/115, Shahin Shahr, Isfahan, Iran article info Article history: Received 29 May 2012 Accepted 7 July 2012 Available online 14 July 2012 Keywords: ZnO Hydrothermal synthesis Morphology Photoluminescence Microstructure abstract In the present paper, well-dispersed ZnO nano-, submicro- and microrods with hexagonal structure were synthesized by a simple low temperature hydrothermal process from zinc nitrate hexahy- drate without using any additional surfactant, organic solvent or catalytic agent. The phase and structural analysis were carried out by X-ray diffraction (XRD), the morphological analysis was car- ried out by field emission scanning electron microscopy (FESEM) and the optical property was characterized by room-temperature photoluminescence (PL) spectroscopy. The results revealed the high crystal quality of ZnO powder with hexagonal (wurtzite-type) crystal structure and the formation of well-dispersed ZnO nano-, submicro- and microrods with diameters of about 50, 200 and 500 nm, and lengths of 300 nm, 1 l m and 2 l m, respectively, on a large-scale just using the different temperatures. Room-temper- ature PL spectrum from the ZnO nanorods reveals a strong UV emission peak at about 360 nm and no green emission band at $530 nm. The strong UV photoluminescence indicates the good crystallization quality of the ZnO nanorods. Room-temperature PL spectra from the ZnO submicro- and microrods reveal a weak UV emission peak at $400 nm and a very strong visible green emis- sion at 530 nm, that is ascribed to the transition between V o Zn i and valence band. Ó 2012 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2012.07.003 ⇑ Corresponding author. Tel.: +98 312 5225041; fax: +98 312 5228530. E-mail address: na.kiomarsipour@yahoo.com (N. Kiomarsipour). Superlattices and Microstructures 52 (2012) 704–710 Contents lists available at SciVerse ScienceDirect Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices 1. Introduction Zinc oxide is one of the most promising materials for optoelectronic applications because of its wide direct band gap (3.37 eV) and large excitation binding energy of 60 meV. ZnO micro- and nano- particles have been extensively studied over the past few years because of their size-dependent elec- tronic and optical properties [1]. Particle size and morphology have a strong effect on their properties and application [2–4]. Thus, various ZnO structures including nanostructures [5], nanowires [6], nano- bowls [7], and nanopellets [8] have been produced. They are widely used in many important areas, such as solar cells [9], pigments [10], gas sensors [11], electronics [12] and photocatalysts [13]. Differ- ent methods have been used to prepare ZnO nanostructures, such as hydrothermal [14], sol–gel [15], mechanical milling [16], and chemical vapor deposition [17]. Among all of the methods to prepare ZnO nanostructures, hydrothermal synthesis route, as an important method for wet chemistry, has been attracting material chemists attention. In this work, used a simple process for the preparation of ZnO nano-, submicro- and microrods employing Zn(NO 3 ) 2 Á6H 2 O and KOH as the reactants without using any additional additives. Reactions were car- ried out at about 150 °C for 20 h in an autoclave as reaction vessel. Then the optical property of the products was studied. The photoluminescence spectra indicated a weak emission peak at UV wave- length and a strong green band for submicro- and microrods and a strong UV emission peak for nanorods. 2. Experimental Three different solutions, A, B and C, were used to synthesize ZnO structures with different mor- phologies. In the first step, for preparation of ZnO nanorods from solution A, 0.5 M zinc nitrate aqueous solution was prepared by adding 14.868 g Zn(NO 3 ) 2 Á6H 2 O (Reagent Grade, 98% Sigma–Aldrich) to 50 ml distilled water at room temperature. The pH of solution increased to 12 by adding dropwise a 1.5 M solution of KOH and stirring vigorously for 10 min. Then the resulting slurry mixture was transferred into a 100 ml Teflon-lined stainless steel autoclave. Hydrothermal reaction was conducted at 120 °C for 20 h in an oven. The B solution, for preparation of ZnO submicrorods, was prepared using the same reactants, but with a different reaction temperature. In B case, hydrothermal reaction was conducted at 150 °C for 20 h. The C case, for preparation of ZnO microrods, hydrothermal reaction was conducted at 180 °C for 20 h. After the reaction was completed, the final product was collected by pressure filtration. Powdered sample was thoroughly washed with distilled water and then dried in air at 120 °C for 12 h. Crystal structure of as-prepared products was characterized by powder X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer using Cu-K a radiation (40 kV, 40 mA and k = 0.1541 nm). XRD patterns were recorded from 0° to 90° with a scanning step of 0.02°/s. Morphologies and sizes of the samples were analyzed by Hitachi S-4160 field emission scanning electron microscopy (FE- SEM) at an accelerating voltage of 15 kV. Room-temperature photoluminescence spectra (PL) were achieved on an Edinbergh instrument FLS 920 spectroscope using a 250 nm excitation line. 3. Results and discussion The typical XRD patterns of the products are shown in Fig. 1a–c. All of the diffraction peaks can be indexed as hexagonal wurtzite ZnO with cell constants a = 3.2490 and c = 5.2050 Å for products, which is in good agreement with the reported data for ZnO of JCPDS Card Files No. 00-005-0664 for nano- and microrods and No. 01-079-020 for submicrorods. The very sharp diffraction peaks were indicated the good crystallinity of the prepared crystals and no characteristic peaks were detected for the other impurities such as Zn(NO 3 ) 2 Á6H 2 O and Zn(OH) 2 . The morphologies and structural characterizations of the ZnO structures are shown in Fig. 2. Fig. 2a shows the top view of the ZnO nanorods. It is found that the product is composed of well-dispersed crystals with column-like structures on a large scale, and all of the columns have fairly uniform diam- eters of about 50 nm and lenghts of 300 nm. The magnified SEM image, shown in Fig. 2b, indicates the N. Kiomarsipour, R. Shoja Razavi /Superlattices and Microstructures 52 (2012) 704–710 705 Fig. 1. XRD patterns of: (a) ZnO nanorods; (b) ZnO submicrorods and (c) ZnO microrods. 706 N. Kiomarsipour, R. Shoja Razavi /Superlattices and Microstructures 52 (2012) 704–710 detailed morphology of the ZnO nanorods. Fig. 2c and d shows the SEM images of the ZnO submicro- rods. The lower magnification image (2c) indicates that ZnO submicrorods highly disperse without any aggregation and have approximately uniform morphologies. The well-dispersed submicrorods are shown in the higher magnification image (2d). The SEM observations demonstrate that the submi- crorods have fairly uniform diameters of about 200 nm and lenghts of 1 l m. Fig. 2e and f shows the SEM images of the ZnO microrods. The lower magnification image (2e) indicates that ZnO microrods highly disperse have diameters of about 500 nm and lenghts of 2 l m. The ZnO is in the regular hex- agonal columnar form and oriented along the [00 01] crystal direction [18]. The increasing length and diameter of the products with the increase of temperature is attributed to the fact that high temper- ature can promote the grain growth. With increase in the reaction temperature from 120 to 180 °C, the morphology of the synthesized products was changed from nanorods to microrods. When the reaction Fig. 2. (a) Low- and (b) high-magnification FESEM images of the ZnO nanorods; (c) low- and (d) high-magnification FESEM images of the ZnO submicrorods and (e) low- and (f) high-magnification FESEM images of the ZnO microrods. N. Kiomarsipour, R. Shoja Razavi /Superlattices and Microstructures 52 (2012) 704–710 707 Fig. 3. Room-temprature photoluminescence spectra of: (a) ZnO nanorods; (b) ZnO submicrorods and (c) ZnO microrods measured at room temperature. 708 N. Kiomarsipour, R. Shoja Razavi /Superlattices and Microstructures 52 (2012) 704–710 temperature increases to 180 °C, the reverse micelles in the microemulsion will not be maintained. This will result in fast cluster nucleation oriented in random directions, thereby forming large and irregular particles [19]. The room-temperature PL spectrum of as-prepared ZnO nanorods, shown in Fig. 3a, was obtained with an excitation wavelength of 250 nm. The ZnO nanorods exhibit a strong near band edge ultravi- olet (UV) emission peak centered at $360 nm, which is attributed to the radiative recombination of a hole in the valence band and an electron in the conduction band (excitonic emission), whereas the de- fect-related emission (green or yellow emission) centered at about 520 nm is too broad weak to be observed, which may be due to singly ionized the oxygen deficiency or zinc interstitials in ZnO [20]. This finding may indicate that the ZnO nanorods synthesized by the simple low-temperature hydrothermal process way possess high crystalline perfection. The PL spectra of ZnO submicro- and microrods are shown in Fig. 3b and c. The spectra of the ZnO microrods mainly consists of four emission bands: (i) a weak UV emission band centered at 403.50 nm, (ii) two weak blue-green bands at 423.17 and 464.36 nm and (iii) a strong broad green emission band with peak located at 530 nm. The weak UV emission corresponds to the exciton recom- bination related near-band edge emission of ZnO, that is to say coming from the direct recombination of the conduction band electrons and the valence band holes [21–26]. The broad green emission band at about 530 nm is generally attributed to the radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy. However, surface states have also been identified as a pos- sible cause of the visible emission in the ZnO nanomaterials. It is reasonable that there are some de- fects in the column-like ZnO microrods at the surface and subsurface due to their fast reaction formation process and large surface-to-volume ratio [27]. Usually, the UV emission is attributed to the near band edge emission of the wide band gap of ZnO due to the annihilation of excitons. And the green luminescence is considered to be the result of radiative recombination of photo-generated holes with singularly ionized oxygen vacancies. In the present work, the stronger green emission should be attributed to much more defective of the microstructures prepared at lower temperature than those deposited at much higher temperatures, at which the UV emission is stronger [28,29]. Un- like those reported in many ZnO nanostructures synthesis, the green emission band (around 510– 550 nm) due to the presence of the singly ionized oxygen vacancies (or other point defects) [30],is clearly observable in our samples. The peak on 530 nm relates to the transition between complex oxygen vacancy and interstitial zinc (V o Zn i ) and valence band, and the peak on 574 nm relates to the transition between complex oxygen vacancy and interstitial zinc (V o Zn i ) and valence band or between exciton level and antisite oxygen. It can be deduced that a very strong green emission band near 576 nm observed in the PL spectra of as- produced ZnO micro- and submicrorods should originate from the transition between V o Zn i and va- lence band in ZnO structures [31]. 4. Conclusions Large-scale, well-dispersed column-like ZnO nano-, submicro- and microrods were successfully synthesized in a simple system at about 150 °C for 20 h via the hydrothermal method. Zn(NO 3 ) 2 Á6H 2 O and KOH were used as the reactants without using any additives. The structural analysis confirms that the as-syntesized ZnO structures are of hexagonal wurtzite phase. These obtained ZnO microstruc- tures exhibit the very different photoluminescence spectra dependence of particle size and morphol- ogies. The photoluminescence spectra indicated a weak emission peak at UV wavelength and a strong green band for submicro- and microrods and a strong UV emission peak for nanorods. References [1] S.H. Ko, I. Park, H. Pan, N. Misra, M.S. 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Characterization and optical property of ZnO nano-, submicro- and microrods synthesized by hydrothermal method on a large-scale Narges Kiomarsipour ⇑ , Reza Shoja Razavi Department of Materials. Low- and (b) high-magnification FESEM images of the ZnO nanorods; (c) low- and (d) high-magnification FESEM images of the ZnO submicrorods and (e) low- and (f) high-magnification FESEM images of the