Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architectures S. Ashoka a , G. Nagaraju a , C.N. Tharamani b , G.T. Chandrappa a, ⁎ a Department of Chemistry, Central College Campus, Bangalore University, Bangalore-560001, India b Department of Chemistry, Ruhr Universität Bochum, Bochum, Germany abstractarticle info Article history: Received 16 September 2008 Accepted 13 January 2009 Available online 26 January 2009 Keywords: Hydrothermal Ethylene glycol ZnO flowers Nanorods Wurtzite structure We describe a simple route to flower like ZnO architectures, based on the decomposition of zinc acetate precursor in water-ethylene glycol solution at 140–160 °C for 1d through hydrothermal method. The PXRD pattern reveals that the ZnO crystals are of hexagonal wurtzite structure. Ethylene glycol plays a key role on the morphology control of ZnO crystals. The SEM images of ZnO products prepared at 140 °C and 160 °C mainly exhibit flower like architecture composed of many rods. Whereas, the product prepared at 180 °C shows bunches accompanying a few number of free rods. TEM results reveal that the rods resemble swords with decrease in size from one end to another. From Raman spectrum, the peaks at 437 cm − 1 , 382 cm − 1 and 41 1 cm − 1 correspond to E 2 (high), A 1 (TO) and E 1 (TO) of ZnO crystals respectively. The photoluminescence spectrum exhibits strong UV emission at ~397 nm, which comes from recombination of exciton. The possible mechanism for the formation of flower like ZnO architecture is proposed. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The controlling syntheses of inorganic materials with unusual and complex characters are of considerable interest in materials fields due to the properties of these materials that depend mainly on their shapes, sizes and structures [1]. ZnO is one of the most important multifunctional semiconductors with its wide direct energy band gap of 3.37 eV and its large exciton binding energy (about 60 meV). Out of sev eral nanomat erials studied so farc arbon nano tubes [2,3] and ZnO [4,5] exhibit the widest variety of nano structur es. ZnO is presently hotly p ursued mat erial in regard t o th e formation of my riad nano/ microstructures [6,7]. Even thoug h many meth ods h a ve been report ed i n the literature, the i nter est in the field of genesis of ZnO micr o/nanos- tructure ha s no t been diminished. Various synthe ses me thods ca pable of formation of different types of ZnO nanostructures have been demon- strated [8,9].Uptonow,well-defined ZnO nanostructures with an abundant variety of shapes, such as nanoneedles, nanocables and nano- tubes [10], nanow al ls [11] , nanobridges and nanonails [12], nanohelixes, nanosprings, nanorings [1 3,14 ], hierarchical nanostructures [1 5],and mesoporous polyhedral cages and shells [16] have been achieved through vapor-based techniq ues. Via the chemical solution r oute, tube-, tow er-, and flow er like ZnO nanostructures [17–19] andorientedhelicalZnO nanorod arrays [20] ha ve also been r ealized very recently. However, despite g rea t pr ogr ess in this field, the s hape- controll ed s ynthesis of ZnO nanocrysta ls, especially reg ard ing control o ver the comple x struct ure, still remains a remarkable challenge. The simple solution synthesis, by thermal treatment of the reactant in different solvents, ma y be the most simple and effective way to prepare sufficiently crystallized materials at relativ ely low t emper atures, while exempt ed fr om further calcinatio n. Besides this, the benefits of a utilizing solution-based method ha ve also involvedtheconsiderableinfluence of reaction species on the final size and morphology of the as-prepared samples. In this aspect, most of the previous in v estigatio ns o n flower like ZnO pr epar ed m ainl y u tilized NaOH/NH 4 OH/eth ylen ediamine as hydro xide sour ce as w ell as comple x - ing a g ents [21–23].TheNaOH/NH 4 OH/eth ylen ediamine forms com ple x and these complexes are subjected to hydrothermal treatment in presence of water/alcohol to obtain nano/microstructures [21–23].But in the present study, flo wer like ZnO arch itectur es have been prepared b y thermal decomposition of zinc acetate in presence of water-ethylene gly col mixture under h ydro thermal condition. The possible reaction mechanism for the formation of flo wer like Z nO architect ure is pr oposed. 2. Experimental Zinc acetate dihydrate (98%) and ethylene glycol (99%) were pur- chased form Merck Limited and used without further purification. Double distilled water was used throughout the experiments. In a typical hydrotherma l p rocess [24], 0.545 g zinc acetate (2.4 mmol) was dissolved in 10 ml distilled water. To this, 20 ml ethylene glycol was introduced resulting in the formation of clear solution. The clear solution was stirred for 15 min on a magnetic stirrer and transferred into Teflon-lined stainless steel autoclaves with a capacity of 50 ml, sealed and maintained at different tem- peratures (140–180 °C) for 1d under autogenously pressure. The resulting white solid products were retrieved from the solution by centrifugation, washed with distilled water followed by ethanol to remove ions possibly in the end product and finally dried in air. Materials Letters 63 (2009) 873–876 ⁎ Corresponding author. Tel.: +91 80 22961350. E-mail address: gtchandrappa@yahoo.co.in (G.T. Chandrappa). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.054 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet Powder X-ray diffraction (PXRD) data were recorded on Philips X'pert PRO X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ =1.541 Å). The Fourier tra nsform infrared spec trum of the sample was collected using Thermo Nicollet FTIR spectro- meter. Scanning electron micrograph images were taken with JEOL (JSM-840 A) scanning electron microscope (SEM). The transmission electron microscopy (TEM) was performed with a Hitachi-H-8100 instrument (accelerating voltage up to 200 kV, LaB 6 filament) equipped with EDS (Kevex Sigma TM Quasar, USA). Raman spectrum was recorded at room te mperature with a confocal la ser micro- Raman spectrometer (LABRM-HR). Photolum inescence studies were carried out on a Perkin–Elmer LS-55 luminescence sp ectrometer using Xe lamp with an excitation waveleng th of 325 nm at room temperature. 3. Results and discussion Fig. 1 shows the PXRD pattern of the flower like ZnO prepared at 160 °C for 1 day. All the diffraction peaks in the PXRD pattern can be Fig. 1. PXRD pattern of the ZnO powder prepared at 140 °C for 1 day. Fig. 2. SEM images of the ZnO crystals prepared using (i) 20 ml ethylene glycol for 1 day at (a) 140 °C, (b) 160 °C (low magnified), (c) 160 °C (high magnified), (d) 180 °C and (ii) at 160 °C for 1 day using ethylene glycol (e) 5 ml and (f) 10 ml. 874 S. Ashoka et al. / Materials Letters 63 (2009) 873–876 Fig. 3. TEM images of the ZnO rod at (a) low magnification, (b) high magnification and (c) EDS spectrum of the ZnO. Fig. 4. Growth schematic diagram of flower like ZnO architecture prepared (a) absence of ethylene glycol and (b) presence of ethylene glycol. Fig. 5. Raman spectrum of ZnO crystals. Fig. 6. Room temperature PL spectrum of ZnO crystals. 875S. Ashoka et al. / Materials Letters 63 (2009) 873–876 indexed as the hexagonal phase of ZnO (wurtzite structure) without contamination from other crystallite phases [25]. It has beenreportedthatanhydrouszinc acetate, which was used as a precursor in this work, can undergo decomposition in water-ethylene glycol mixture and formsZnO nuclei without formingany intermediates [26]. The possible reaction for the formation of ZnO is as follows. ZnðCH 3 COOÞ 2 →ZnO þ CH 3 COOCOCH 3 The formation of ZnO was also confirmed by FTIR spectrum (not shown). The broad band around 417 cm − 1 associated with the characteristic vibrational mode of Zn\O bonding [27]. Fig. 2 shows the SEM images of ZnO products prepared at 140 °C, 160 °C and 180 °C for 1 day. Fig. 2a exhibits well ordered flowers with good symmetry. These flowers made of 1D rods with typical diameter of about 300–700 nm. From the Fig. 2b –c one can observe the existence of both flowers as well as twinned crystals. These twinned crystals are the intermediates for the formation of flowers. Fig. 2d reveals that the bulk quantityof ZnO bunches exist. Every bunch is composed of closely packed submicrometer-sized rods with diameters of 350–600 nm and lengths of 4–5 µm and forms radiating structures. In addition to bunches, we can also observe some submicrometer- sized rods with sharp tips. Flower like ZnO architectures were not obtained when the quantity of ethylene glycol is reduced to less than 20 ml. On the other hand, the morphology of the ZnO is changed to hexagonal rods (Fig. 2e and f) when the quantity of ethylene glycol is reduced to 5–10 ml while carrying out the experiment at 160 °C for 1 day. Fig. 3a and b shows the TEM images of the single rod taken at different magnification. The sword like rod with size decreases from one end to another exhibit rough surface as observed. The diameter of the rod is found to be in the range of 40–70 nm. Fig. 3c presents the EDX spectrum of the ZnO rod. The quantitative data indicates that the flowers are made of Zn and O with stoichiometric ratio of ~1:1 within the experimental error and no other elements are found. This result is in good agreement with Raman spectroscopic and PL studies. In the present synthetic route, ethylene glycol is a key factor to prepare ZnO flower like architecture, which is confirmed by the results of experiments that did not use an ethylene glycol assisted hydrothermal process. As a wurtzite-structured metal oxide, z inc oxide belongs t o the p63mc space group. The (0001) and (000-1) p lanes of ZnO c rystals ar e r ich in Zn (positive polar plane) and O (negative polar plane) respectivel y . The h ydroxyl groups o f e thylene glycol can adsorb on the positive po lar plane of ZnO by coulomb interaction. The adsorption retards the growth along (0001) plane resulting in the formation of polyhedral crystals [25].The two polyhedral crystals are joined each other by the action of ethylene glycol, forms twinned crystals. Similar observation was made by Zang et al in pr esence of pol yvin y l alcohol [23]. Th ese twin ned cry stals h a ving h igh surface energy along negati ve polar plane. Zinc acetate molecules will adsorbonthenegativepolarplaneresultinginadecreaseofthesurface energy and further nucleation takes place along this direction. The adsorbed zinc acetate molecules will trigger the nucleation and promoting the formation of rods aroun d the twinned crystals. The prepared flower like architecture is different from the previous reports [22,23,28]. Randomly distributed ZnO r ods are formed in the absence of eth ylene glycol. The possible gr owth schematic diagram for the formation of flower like architecture is shown in Fig. 4 . In general, slow cry stallization is r equired t o form products with a thermodynamically stable structure because the crystall izing partn ers ha ve time t o recogn ize each ot her and follow the l ow est-en ergy path . How ev er, at 180 °C, the te mper ature is high and fast crystallization takes place. This fast crystallization often leads to kinetically controlled products, such as unstable or metastable crystal structures, and ev en defects can be formed during fast nucleation [29]. Hence, we believe that the flo wer s like ZnO archit ectures are prefer ably formed from slow nucleation and growth. Whereas, the ZnO bunches is obtained from fast nucleati on and gro wth pro cess. Raman spectroscopy was carried out to study the vibrational properties of the ZnO crystals. Fig. 5 represents the Raman spectrum of the samples at the range of 250 to 600 cm − 1 . The peaks at 437 cm − 1 , 382 cm − 1 and 411 cm − 1 corresponds to E 2 (high), A 1 (TO) and E 1 (TO) of ZnO crystals respectively [30]. The absence of E 1 (LO) peak (583 cm − 1 ) indicating good quality of the as prepared samples since the E 1 (LO) mode is assoc iated with defects such as oxygen vacancy, zinc interstitials, or their complexes [31]. ZnO exhibited a wide band gap at room temperature with a large exciton binding energy, which is suitable for effective UV emission. However, due to the poor crystal quality of the nanomaterials, i.e., high density of structural defects, the UV emission of nanoscaled ZnO is liable to be quenched and only defect emission in visible region is detected [32]. This deficiency hinders progress for the applications of ZnO in optoelectronic and lasing devices. The photoluminescence of the obtained ZnO crystals (Fig. 6) exhibits strong UV emission at ~397 nm, which comes from recombination of exciton and no defect emission is detected [17]. 4. Conclusions Flower like ZnO architectures were fabricated by hydrothermal method using zinc acetate in water-ethylene glycol solution at 140– 160 °C for 1 day. Ethylene glycol is a key factor for the preparation of flower like architectures. The ZnO products prepared by this method having good quality i.e., free from defects sites. The synthesis method discussed in the present work opens a new approach for the fabri- cation of other metal oxides nanostructure. Acknowledgements The author G.T. Chandrappa is thankful to the Department of Science and Technology, NSTI Phase-IV, New Delhi, Government of India for financial support to carryout the research. We also thank Prof. Sarala Upadhya the Department of Mechanical Engineering, UVCE, Bangalore, for recording SEM images. References [1] Taubert S. Angew Chem Int Ed Engl 2004;43:5380. [2] Iijima S. Nature 1991;354:56. [3] Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM. Nat Matters 2004;3:610. [4] Wang ZL. 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J Phys Condens Matter 2004;16:7123. [32] Kong YC, Yu DP, Zhang B, Fang W, Feng SQ. Appl Phys Lett 2001;78:407. 876 S. Ashoka et al. / Materials Letters 63 (2009) 873–876 . Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architectures S. Ashoka a , G. Nagaraju a , C.N. Tharamani b ,. 2009 Keywords: Hydrothermal Ethylene glycol ZnO flowers Nanorods Wurtzite structure We describe a simple route to flower like ZnO architectures, based on the decomposition of zinc acetate precursor in water -ethylene. synthetic route, ethylene glycol is a key factor to prepare ZnO flower like architecture, which is confirmed by the results of experiments that did not use an ethylene glycol assisted hydrothermal